Cellular Systems--Cellular Concepts The cellular concept was a major breakthrough in solving

the problem of spectral congestion and user capacity It offered very high capacity in a limited spectrum allocation without any major technological changes

The cellular concept has the following system level ideas Replacing a single high power transmitter with many low power

transmitters each providing coverage to only a small area Neighboring cells are assigned different groups of channels in

order to minimize interference The same set of channels is then reused at different

geographical locations

Cellular Concepts

When designing a cellular mobile communication system it is important to provide good coverage and services in a high user-density area

Reuse can be done once the total interference from all users in the cells using the same frequency (co-channel cell) for transmission suffers from sufficient attenuation Factors need to be considered include Geographical separation (path loss) Shadowing effect

Cell Footprint

The actual radio coverage of a cell is known as the cell footprint Irregular cell structure and irregular placing of the

transmitter may be acceptable in the initial system design However as traffic grows where new cells and channels need to be added it may lead to inability to reuse frequencies because of co-channel interference

For systematic cell planning a regular shape is assumed for the footprint

Cell Footprint

Coverage contour should be circular However it is impractical because it provides ambiguous areas with either multiple or no coverage

Due to economic reasons the hexagon has been chosen due to its maximum area coverage

Hence a conventional cellular layout is often defined by a uniform grid of regular hexagons

Cell Footprint

Frequency reuse

A cellular system which has a total of S duplex channels

S channels are divided among N cells with each cell uses unique and disjoint channels

If each cell is allocated a group of k channels then

S = k N

Terminology

Cluster size The N cells which collectively use the complete set of available frequency is called the cluster size

Co-channel cell The set of cells using the same set of frequencies as the target cell

Interference tier A set of co-channel cells at the same distance from the reference cell is called an interference tier The set of closest co-channel cells is call the first tier There is always 6 co-channel cells in the first tier

Co-ordinates for hexagonal cellular geometry With these co-

ordinates an array of cells can be laid out so that the center of every cell falls on a point specified by a pair of integer co-ordinates

Co-ordinates for hexagonal cellular geometry

Designing a cellular system

N=19 (i=3 j=2)

Designing a cellular system

The cluster size must satisfy N = i2 + ij + j2 where i j are non-negative integers

Designing a cellular system

Designing a cellular system

Can also verify that

where Q is the co-channel reuse ratio

Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another It must be user transparent successful and not

too frequent Not only involves identifying a new BS but also

requires that the voice and control signals be allocated to channels associated with the new BS

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Cellular Concepts

When designing a cellular mobile communication system it is important to provide good coverage and services in a high user-density area

Reuse can be done once the total interference from all users in the cells using the same frequency (co-channel cell) for transmission suffers from sufficient attenuation Factors need to be considered include Geographical separation (path loss) Shadowing effect

Cell Footprint

The actual radio coverage of a cell is known as the cell footprint Irregular cell structure and irregular placing of the

transmitter may be acceptable in the initial system design However as traffic grows where new cells and channels need to be added it may lead to inability to reuse frequencies because of co-channel interference

For systematic cell planning a regular shape is assumed for the footprint

Cell Footprint

Coverage contour should be circular However it is impractical because it provides ambiguous areas with either multiple or no coverage

Due to economic reasons the hexagon has been chosen due to its maximum area coverage

Hence a conventional cellular layout is often defined by a uniform grid of regular hexagons

Cell Footprint

Frequency reuse

A cellular system which has a total of S duplex channels

S channels are divided among N cells with each cell uses unique and disjoint channels

If each cell is allocated a group of k channels then

S = k N

Terminology

Cluster size The N cells which collectively use the complete set of available frequency is called the cluster size

Co-channel cell The set of cells using the same set of frequencies as the target cell

Interference tier A set of co-channel cells at the same distance from the reference cell is called an interference tier The set of closest co-channel cells is call the first tier There is always 6 co-channel cells in the first tier

Co-ordinates for hexagonal cellular geometry With these co-

ordinates an array of cells can be laid out so that the center of every cell falls on a point specified by a pair of integer co-ordinates

Co-ordinates for hexagonal cellular geometry

Designing a cellular system

N=19 (i=3 j=2)

Designing a cellular system

The cluster size must satisfy N = i2 + ij + j2 where i j are non-negative integers

Designing a cellular system

Designing a cellular system

Can also verify that

where Q is the co-channel reuse ratio

Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another It must be user transparent successful and not

too frequent Not only involves identifying a new BS but also

requires that the voice and control signals be allocated to channels associated with the new BS

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Cell Footprint

The actual radio coverage of a cell is known as the cell footprint Irregular cell structure and irregular placing of the

transmitter may be acceptable in the initial system design However as traffic grows where new cells and channels need to be added it may lead to inability to reuse frequencies because of co-channel interference

For systematic cell planning a regular shape is assumed for the footprint

Cell Footprint

Coverage contour should be circular However it is impractical because it provides ambiguous areas with either multiple or no coverage

Due to economic reasons the hexagon has been chosen due to its maximum area coverage

Hence a conventional cellular layout is often defined by a uniform grid of regular hexagons

Cell Footprint

Frequency reuse

A cellular system which has a total of S duplex channels

S channels are divided among N cells with each cell uses unique and disjoint channels

If each cell is allocated a group of k channels then

S = k N

Terminology

Cluster size The N cells which collectively use the complete set of available frequency is called the cluster size

Co-channel cell The set of cells using the same set of frequencies as the target cell

Interference tier A set of co-channel cells at the same distance from the reference cell is called an interference tier The set of closest co-channel cells is call the first tier There is always 6 co-channel cells in the first tier

Co-ordinates for hexagonal cellular geometry With these co-

ordinates an array of cells can be laid out so that the center of every cell falls on a point specified by a pair of integer co-ordinates

Co-ordinates for hexagonal cellular geometry

Designing a cellular system

N=19 (i=3 j=2)

Designing a cellular system

The cluster size must satisfy N = i2 + ij + j2 where i j are non-negative integers

Designing a cellular system

Designing a cellular system

Can also verify that

where Q is the co-channel reuse ratio

Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another It must be user transparent successful and not

too frequent Not only involves identifying a new BS but also

requires that the voice and control signals be allocated to channels associated with the new BS

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Cell Footprint

Coverage contour should be circular However it is impractical because it provides ambiguous areas with either multiple or no coverage

Due to economic reasons the hexagon has been chosen due to its maximum area coverage

Hence a conventional cellular layout is often defined by a uniform grid of regular hexagons

Cell Footprint

Frequency reuse

A cellular system which has a total of S duplex channels

S channels are divided among N cells with each cell uses unique and disjoint channels

If each cell is allocated a group of k channels then

S = k N

Terminology

Cluster size The N cells which collectively use the complete set of available frequency is called the cluster size

Co-channel cell The set of cells using the same set of frequencies as the target cell

Interference tier A set of co-channel cells at the same distance from the reference cell is called an interference tier The set of closest co-channel cells is call the first tier There is always 6 co-channel cells in the first tier

Co-ordinates for hexagonal cellular geometry With these co-

ordinates an array of cells can be laid out so that the center of every cell falls on a point specified by a pair of integer co-ordinates

Co-ordinates for hexagonal cellular geometry

Designing a cellular system

N=19 (i=3 j=2)

Designing a cellular system

The cluster size must satisfy N = i2 + ij + j2 where i j are non-negative integers

Designing a cellular system

Designing a cellular system

Can also verify that

where Q is the co-channel reuse ratio

Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another It must be user transparent successful and not

too frequent Not only involves identifying a new BS but also

requires that the voice and control signals be allocated to channels associated with the new BS

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Cell Footprint

Frequency reuse

A cellular system which has a total of S duplex channels

S channels are divided among N cells with each cell uses unique and disjoint channels

If each cell is allocated a group of k channels then

S = k N

Terminology

Cluster size The N cells which collectively use the complete set of available frequency is called the cluster size

Co-channel cell The set of cells using the same set of frequencies as the target cell

Interference tier A set of co-channel cells at the same distance from the reference cell is called an interference tier The set of closest co-channel cells is call the first tier There is always 6 co-channel cells in the first tier

Co-ordinates for hexagonal cellular geometry With these co-

ordinates an array of cells can be laid out so that the center of every cell falls on a point specified by a pair of integer co-ordinates

Co-ordinates for hexagonal cellular geometry

Designing a cellular system

N=19 (i=3 j=2)

Designing a cellular system

The cluster size must satisfy N = i2 + ij + j2 where i j are non-negative integers

Designing a cellular system

Designing a cellular system

Can also verify that

where Q is the co-channel reuse ratio

Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another It must be user transparent successful and not

too frequent Not only involves identifying a new BS but also

requires that the voice and control signals be allocated to channels associated with the new BS

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Frequency reuse

A cellular system which has a total of S duplex channels

S channels are divided among N cells with each cell uses unique and disjoint channels

If each cell is allocated a group of k channels then

S = k N

Terminology

Cluster size The N cells which collectively use the complete set of available frequency is called the cluster size

Co-channel cell The set of cells using the same set of frequencies as the target cell

Interference tier A set of co-channel cells at the same distance from the reference cell is called an interference tier The set of closest co-channel cells is call the first tier There is always 6 co-channel cells in the first tier

Co-ordinates for hexagonal cellular geometry With these co-

ordinates an array of cells can be laid out so that the center of every cell falls on a point specified by a pair of integer co-ordinates

Co-ordinates for hexagonal cellular geometry

Designing a cellular system

N=19 (i=3 j=2)

Designing a cellular system

The cluster size must satisfy N = i2 + ij + j2 where i j are non-negative integers

Designing a cellular system

Designing a cellular system

Can also verify that

where Q is the co-channel reuse ratio

Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another It must be user transparent successful and not

too frequent Not only involves identifying a new BS but also

requires that the voice and control signals be allocated to channels associated with the new BS

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Terminology

Cluster size The N cells which collectively use the complete set of available frequency is called the cluster size

Co-channel cell The set of cells using the same set of frequencies as the target cell

Interference tier A set of co-channel cells at the same distance from the reference cell is called an interference tier The set of closest co-channel cells is call the first tier There is always 6 co-channel cells in the first tier

Co-ordinates for hexagonal cellular geometry With these co-

ordinates an array of cells can be laid out so that the center of every cell falls on a point specified by a pair of integer co-ordinates

Co-ordinates for hexagonal cellular geometry

Designing a cellular system

N=19 (i=3 j=2)

Designing a cellular system

The cluster size must satisfy N = i2 + ij + j2 where i j are non-negative integers

Designing a cellular system

Designing a cellular system

Can also verify that

where Q is the co-channel reuse ratio

Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another It must be user transparent successful and not

too frequent Not only involves identifying a new BS but also

requires that the voice and control signals be allocated to channels associated with the new BS

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Co-ordinates for hexagonal cellular geometry With these co-

ordinates an array of cells can be laid out so that the center of every cell falls on a point specified by a pair of integer co-ordinates

Co-ordinates for hexagonal cellular geometry

Designing a cellular system

N=19 (i=3 j=2)

Designing a cellular system

The cluster size must satisfy N = i2 + ij + j2 where i j are non-negative integers

Designing a cellular system

Designing a cellular system

Can also verify that

where Q is the co-channel reuse ratio

Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another It must be user transparent successful and not

too frequent Not only involves identifying a new BS but also

requires that the voice and control signals be allocated to channels associated with the new BS

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Co-ordinates for hexagonal cellular geometry

Designing a cellular system

N=19 (i=3 j=2)

Designing a cellular system

The cluster size must satisfy N = i2 + ij + j2 where i j are non-negative integers

Designing a cellular system

Designing a cellular system

Can also verify that

where Q is the co-channel reuse ratio

Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another It must be user transparent successful and not

too frequent Not only involves identifying a new BS but also

requires that the voice and control signals be allocated to channels associated with the new BS

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Designing a cellular system

N=19 (i=3 j=2)

Designing a cellular system

The cluster size must satisfy N = i2 + ij + j2 where i j are non-negative integers

Designing a cellular system

Designing a cellular system

Can also verify that

where Q is the co-channel reuse ratio

Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another It must be user transparent successful and not

too frequent Not only involves identifying a new BS but also

requires that the voice and control signals be allocated to channels associated with the new BS

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Designing a cellular system

The cluster size must satisfy N = i2 + ij + j2 where i j are non-negative integers

Designing a cellular system

Designing a cellular system

Can also verify that

where Q is the co-channel reuse ratio

Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another It must be user transparent successful and not

too frequent Not only involves identifying a new BS but also

requires that the voice and control signals be allocated to channels associated with the new BS

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Designing a cellular system

Designing a cellular system

Can also verify that

where Q is the co-channel reuse ratio

Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another It must be user transparent successful and not

too frequent Not only involves identifying a new BS but also

requires that the voice and control signals be allocated to channels associated with the new BS

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Designing a cellular system

Can also verify that

where Q is the co-channel reuse ratio

Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another It must be user transparent successful and not

too frequent Not only involves identifying a new BS but also

requires that the voice and control signals be allocated to channels associated with the new BS

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another It must be user transparent successful and not

too frequent Not only involves identifying a new BS but also

requires that the voice and control signals be allocated to channels associated with the new BS

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Handover Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Handover Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small

Insufficient time

to complete handoff

before call is lost More call losses

too large Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Prioritizing Handover Dropped call is considered a more serious event

than call blocking Channel assignment schemes therefore must give priority to handover requests

A fraction of the total available channels in a cell is reserved only for handover requests However this reduces the total carried traffic Dynamic allocation can improve this

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel The time span over which a handover is usually required leaves room for queuing handover request

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users This will result in interference and traffic management problem

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Practical handover

The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Practical handover

A hard handover does ldquobreak before makerdquo ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Interference and System Capacity In a given coverage area there are several cells

that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (Dprime)

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Interference and System Capacity

Drsquo is normally approximated by the base station separation between the two cells D unless when accuracy is needed Hence

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Interference and System Capacity For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Interference and System Capacity

If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Outage probability

The probability that a mobile station does not receive a usable signal

For GSM this is 12 dB and for AMPS this is 18 dB If there is 6 co-channel cells then

Exercise please verify this For n=4 a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS For n=4 a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Outage probability

Approximation in distance has been made on the 2nd tier onwards

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Outage probability

More accurate SIR can be obtained by computing the actual distance

Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Coverage Problems Revision

Recall that the mean measured value

Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Boundary coverage

There will be a proportion of locations at distance R (cell radius) where a terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

where Q(x) is the standard normal distribution

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Cell coverage Proportion of locations within the area defined by the cell r

adius R receiving a signal above the threshold γ

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Cell coverage

Solution can be found using the graph provided (n path loss exponent)

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Cell coverage Example if n=4 σ=8 dB and if the boundary is to h

ave 75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Cell coverage The mean signal level at any distance is determined by p

ath loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Cell coverage --Cellular Traffic

The basic consideration in the design of a cellular system is the sizing of the system Sizing has two components to be considered Coverage area Traffic handling capability

After the system is sized channels are assigned to cells using the assignment schemes mentioned before

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Cell coverage --Terminology in traffic theory Trunking exploits the statistical characteristics of the u

sers calling behaviour Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels

Grade of service (GoS) A user is allocated a channel on a per call basis GoS is a measure of the ability of a user to access a trunked system during the busiest hour It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before)

Trunking theory is used to determine the number of channels required to service a certain offered traffic at a specific GoS

Call holding time (H) the average duration of a call Request rate (λ) average number of call requests peru

nit time

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Cell coverage --Traffic flow or intensity A

Measured in Erlang which is defined as the call minute per minute

Total offered traffic for such a system is given as

A = λ sdotH

Exercise There are 3000 calls per hour in a cell each lasting an average of 176 min Offered traffic A = (300060)(176) = 88 Erlangs

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Cell coverage If the offered traffic exceeds the maximum possible

carried traffic blocking occurs There are two different strategies to be used Blocked calls cleared Blocked calls delayed

Trunking efficiency is defined as the carried traffic intensity in Erlangs per channel which is a value between zero and one It is a function of the number of channels per cell and the specific GoS parameters

Call arrival process it is widely accepted that calls have a Poisson arrival

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system Fixed Channel Allocation (FCA)

Channels are divided in sets A set of channels is permanently allocated to each cell

in the network Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference

Any call attempt within the cell can only be served by the unused channels in that particular cell The service is blocked if all channels have used up

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Channel Assignment Strategies (FCA)

Most easiest to implement but least flexibility An modification to this is lsquoborrowing schemersquo Cell (acc

eptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls

Borrowing can be done in a few ways borrowing from the adjacent cell which has largest number of free channels select the first free channel found etc

To be available for borrowing the channel must not interfere with existing calls The borrowed channel should be returned once the channel becomes free

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA)

Voice channels are not allocated to any cell permanently All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system

Each time a call request is made the serving BS requests a channel from the MSC It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell the reuse distance of the channel and other cost functions

increase in complexityrArr Centralized DCA scheme involves a single controller selecting a chann

el for each cell Distributed DCA scheme involves a number of controllers scattered across the network

For a new call a free channel from central pool is selected based on either the co-channel distance signal strength or signal to noise interference ratio

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

Channel Assignment Strategies

Flexible channel assignment Divide the total number of channels into two groups one of

which is used for fixed allocation to the cells while the other is kept as a central poor to be shared by all users

Mix the advantages the FCA and DCA available schemes are scheduled and predictive

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing prob

ability of a blocked call

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

System Expansion Techniques

As demand for wireless services increases the number of channels assigned to a cell eventually becomes insufficient to support the required number of users More channels must therefore be made available per unit area This can be accomplished by dividing each initial cell area i

nto a number of smaller cells a technique known as cell-splitting

It can also be accomplished by having more channels per cell ie by having a smaller reuse factor However to have a smaller reuse factor the co-channel interference must be reduced This can be done by using antenna sectorization

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order t

o increase capacity There will be a corresponding reduction in antenna height and transmitter power

Cell splitting accommodates a modular growth capability This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning

Small cells lead to more cellsarea which in turn leads to increased traffic capacity

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

System Expansion Techniques--Cell splitting For new cells to be smaller in size the transmit pow

er must be reduced If n=4 then with a reduction of cell radius by a factor of 2 the transmit power should be reduced by a factor of 24 (why)

In theory cell splitting could be repeated indefinitely In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time practical problems o

f BS sites such as co-channel interference exist Innovative channel assignment schemes must be develope

d to address this problem for practical systems

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

System Expansion Techniques --Sectorization Keep the cell radius but decrease the DR rati

o In order to do this we must reduce the relative interference without increasing the transmit power

Sectorization relies on antenna placement and directivity to reduce co-channel interference Beams are kept within either a 60deg or a 120deg sector

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

System Expansion Techniques --Sectorization If we partition a cell into three 120deg sectors the num

ber of co-channel cells are reduced from 6 to 2 in the first tier

Using six sectors of 60deg we have only one co-channel cell in the first tier

Each sector is limited to only using 13 or 16 of the available channels We therefore have a decrease in trunking efficiency and an increase in the number of required antennas

But how can the increase in system capacity be achieved

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate cap

acity problems caused by ldquohotspotsrdquo By clever channel assignment the reuse fact

or is unchanged As for cell splitting there will occur interference problems when macro and micro cells must co-exist

System Expansion Techniques --Micro cells

System Expansion Techniques --Micro cells