EEE440 Lecture Slide Part 4ver2011-2012 (1)

7/31/2019 EEE440 Lecture Slide Part 4ver2011-2012 (1)

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EEE440 ModernCommunication Systems

Cellular Systems



Introduction

• The geographical area of coverage is organised

into cells

• Each cell is controlled by a base station

• A common model of cellular structure in a two-dimensional case is to consider all cells to be

hexagonal in shape and all of the same size

• In real systems, cells have complex shapesdepending on antenna directivity and location,

propagation conditions and terrain topography



Structure of a cellular system



• BS-controls multiple MS

• MSC – controls multiple BS – Responsible for intercellular handover, mobile location, paging

and mobility management

• HLR – contains reference and profile information

for all mobile users registered with the MSC as

their home location

• VLR- registers visiting MS

Structure of a cellular system



Spectral allocation

Radio spectrum allocation is made byauthorities. e.g. in Malaysia, the MCMC

allocates spectrum to mobile operators



Spectral allocation



Channel allocation

• The band is broken into a number of channels

• Channels in a wireless communication system typicallyconsist of time slots in TDMA , frequency bands inFDMA and/or CDMA pseudo noise sequences, but in an

abstract sense, they can represent any generictransmission resource

• The number of channels limit the number of simultaneous users

• To increase the capacity a given service area is dividedinto a number of cells

• The channels can be reused in different cells



Channel reuse

• Different cells can use the same frequency

channel

• However, adjacent cells cannot be

assigned the same frequency because of

inter-channel inteference

• The assignment must be spaced far

enough apart to keep interence to

tolerable levels



Channel reuse

• For example in a one dimensional cell

structure, the total number of channels can

be divided into 4 groups (4-reuse)

• There are three-cells separating cells with

the same set of frequencies



Channel reuse



Channel reuse

• The assignment strategy depends on the

tolerable interference which is quantified by

calculating the signal-to-interference ratio (SIR)

or also called carrier-to-interference ratio (CIR)

SIR = desired average signal power at a receiver

total average interference power

• The SIR should be greater than a specifiedthreshold for a proper signal operation

• For GSM the desired SIR is 7-12dB



SIR calculations

• Calculated on an average power basis

• Focus on the distance-dependent part of

the received power equation (ignores

shadow and multipath fading)

• Assume g(d)=kd -n ; n = 3 or 4



SIR calculations

• Consider 1-dimensional cell structure

– D= spacing between interfering cells

– R=the half width (center to edge) of each cell

• Consider downlink power receive at a mobilelocated at the edge of a cell (worst situation) atpoint P

• Say each base station located at the centre of its

cell transmits with the same average power, PT



SIR calculations

• The average received power at distance d

meter from a base station is given by

P T d -n ; n = 3 or 4

• The SIR at the mobile at point P is given

by

intP

PtRSIR

n

Sum of all interfering base stations



SIR calculations

• Theoretically all base stations transmitting

at the same frequency will interfere with

the home base station transmission

• However, in reality only a relatively small

number of nearby interferes need be

considered because of the rapidly

decreasing received power as thedistance, d increases



SIR calculations

• Consider the first tier interferers only

• The two interfering base stations closest to

the mobile at point P are located at (D+R)

and (D-R) respectively from the mobile• The corresponding SIR is given by

nn

n

R D R D

RSIR

)()(



SIR calculations

• Calculate the SIR in dB for different values

of n (3 or 4) and different cell reuse (3 or

4)

• What can you conclude?

• Individual assignment questions.



SIR calculations

• Consider 2-dimensional cell structure• All hexagonal cells of same size

• The number of cells for an area is given generally by,

C=i 2 + j 2 + ij ; i, j = integers 1,2,3…

• For GSM C=3 or 4



SIR calculations

• Consider a typical hexagonal cell

• The distance from the center of the cell to any vertex isthe radius R

• Each edge is of length R

• The distance across the cells = √3R



SIR calculations

• There are 6 interfering base stations

around the home base station

• The spacing between the closest

interfering base stations is given by

D3 =3R for 3-cell reuse (c=3)

D4=2√3R for 4-cell reuse (c=4)

– In general for C-cell reuse, Dc=√3C R



SIR calculations

• Consider the case when the mobile is at

the middle of the cell

• The SIR is given bySIR = PT / (6PT√3C R-n)

= 1/ (6√3C R-n)

• At the edge of the cell, the are many

proposed approximations



SIR calculations

nnn

R

D

R

D

R

DSIR

411

1

Estimate the appropriate C for GSM with minimum required SIR of 7dB.



Channel Allocation Schemes• Allocate channels to base stations and access points

and to avoid co-channel interference among nearby cells• Fixed Channel Allocation

– requires manual frequency planning to allocate specific channels to specific cells

– This allocation is static and can not be changed

– the number of channels in the cell remains constant irrespectively of the number of customers in that cell.

– For efficient operation, FCA systems typically allocate channels in a manner thatmaximizes frequency reuse.

– Thus, in a FCA system, the distance between cells using the same channel is theminimum reuse distance for that system.

– The problem with FCA systems occurs whenever the offered traffic to a networkof base stations is not uniform.

– Consider a case in which two adjacent cells are allocated N channels each.

There clearly can be situations in which one cell has a need for N+k channelswhile the adjacent cell only requires N-m channels (for positive integers k and m ).

– In such a case, k users in the first cell would be blocked from making calls whilem channels in the second cell would go unused.

– Clearly in this situation of non-uniform spatial offered traffic, the availablechannels are not being used efficiently. This result in traffic congestion and somecalls being lost when traffic gets heavy in some cells, and idle capacity in other

cells.



Channel Allocation Schemes

• Dynamic – handles bursty cell traffic and utilizes the cellular radio resources more

efficiently.

– DCA allows the number of channels in a cell to vary with the traffic load,hence increasing channel capacity with little costs.

– In DCA systems, no set relationship exists between channels and cells.Instead, channels are part of a pool of resources.

– Whenever a channel is needed by a cell, the channel is allocated under the constraint that frequency reuse requirements can not be violated.

– There are two problems that typically occur with DCA based systems.

– First, DCA methods typically have a degree of randomness associatedwith them and this leads to the fact that frequency reuse is often not

maximized unlike the case for FCA systems in which cells using thesame channel are separated by the minimum reuse distance.

– Secondly, DCA methods often involve complex algorithms for decidingwhich available channel is most efficient. These algorithms can be verycomputationally intensive and may require large computing resources inorder to be real-time.




• Hybrid – Combined FCA and DCA

– Channel Borrowing is one of the most straightforward hybrid allocationschemes.

– Here, channels are assigned to cells just as in fixed allocation schemes.

– If a cell needs a channel in excess of the channels previously assignedto it, that cell may borrow a channel from one of its neighboring cellsgiven that a channel is available and use of this channel won't violatefrequency reuse requirements.

– Note that since every channel has a predetermined relationship with aspecific cell, channel borrowing is often categorized as a subclass of fixed allocation schemes.

– The major problem with channel borrowing is that when a cell borrows achannel from a neighboring cell, other nearby cells are prohibited fromusing the borrowed channel because of co-channel interference.

– This can lead to increased call blocking over time. To reduce this callblocking penalty, algorithms are necessary to ensure that the channelsare borrowed from the most available neighboring cells; i.e., theneighboring cells with the most unassigned channels.




• Two extensions of the channel borrowing approach are Borrowing withChannel Ordering (BCO) and Borrowing with Directional ChannelLocking (BDCL).

• Borrowing with Channel Ordering was designed as an improvement over the simpler Channel Borrowing approach

• BCO systems have two distinctive characteristics

– The ratio of fixed to dynamic channels varies with traffic load. – Nominal channels are ordered such that the first nominal channel of a cell hasthe highest priority of being applied to a call within the cell.

• The last nominal channel is most likely to be borrowed by neighboringchannels.

• Once a channel is borrowed, that channel is locked in the co-channel cellswithin the reuse distance of the cell in question.

• To be "locked" means that a channel can not be used or borrowed.• From a frequency reuse standpoint, in a BCO system, a channel may be

borrowed only if it is free in the neighboring cochannel cells.




• In Borrowing with Directional Channel Locking, borrowed channels are only locked innearby cells that are affected by the borrowing.

• This differs from the BCO scheme in which a borrowed channel is locked in every cell

within the reuse distance.• The benefit of BDCL is that more channels are available in the presence of borrowingand subsequent call blocking is reduced.

• A disadvantage of BDCL is that the statement "borrowed channels are only locked innearby cells that are affected by the borrowing" requires a clear understanding of theterm "affected.“

• This may require microscopic analysis of the area in which the cellular system will belocated. Ideally, a system can be general enough that detailed analysis of specific

propagation measurements is not necessary for implementation.




• A natural extension of channel borrowing is to set aside a portion of thechannels in a system as dynamic channels with the remaining (nominal)channels being fixed to specified cells.

• If a cell requires an extra channel, instead of borrowing the channel from aneighboring cell, the channel is borrowed from the common "bank" of dynamic channels.

• An important consideration in hybrid systems of this type is the ratio of dynamic channels to fixed channels.

• The optimum ratio depends upon the traffic load

• Locally Optimized Dynamic Assignment Strategy (LODA), this method isbest described as a purely dynamic channel allocation procedure asopposed to a hybrid method.

• In this strategy there are no nominal channels; all channels are dynamic.

• When a given cell needs to accommodate a call, it chooses from among thebank of available channels according to some cost criteria.

• The channel with minimum cost is assigned.

• In a general sense, the cost is a measure of the future blocking probabilityin the vicinity of the cell given that the candidate channel is assigned.



Traffic handling capacity

• The number of channels available per cell is given by thetotal number of channels divided by the cell reuse

parameter, C

• System performance is measured by the probability of

call blocking which describes the chance that a user attempting to place a call receives a busy signal.

• The measure depends on the number of channels

available to handle simultaneous calls and the traffic

expected to utilise the system

• With a specified call blocking probability (e.g. 1% or 5%)

a limit must be put on the amount of traffic expected to

use the cell




• Traffic intensity or traffic load is commonly

defined as the product of the average number of

call attempts per unit time(λ) and the average

call length (1/µ)• Traffic intensity, A = λ/µ in unit Erlangs

• The statistical model assumes that the pattern of

call attempts or arrival obeys a Poisson

distribution with average rate of arrival λ and the

call lengths are exponentially distributed with

average length 1/µ




• With N channels available, the cell

blocking probability, PB is given by the

Erlang-B formula

• A table or plot of PB

vs A (Erlang-B

function) is used to find the number of

channels required for a given traffic load

and PB



Cell size

• Asssume that users are uniformly distributed

over the cell

• The area of the hexagonal cell of radius R is

(3√3R2)/2• Say there is 1 call every 15 minutes and a

typical call last for 200 seconds on average

• The load for 1 user is given by• For a total cell load, A=101 the number of users

is about 450 users



Cell size

• The user density for 450 users is given by

450 / (3√3R2)/2 = 173/R2 mobiles per unit

area

• Consider a rural area with density of

mobile = 2 terminals per km2. What is the

cell radius

• For suburban = 100 mobiles per km2?

• For urban = 1000 mobiles per km2?

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EEE440 Lecture Slide Part 4ver2011-2012 (1)