Cellular system in wireless communication

7/29/2019 Cellular system in wireless communication

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9/17/20

Advanced Wireless Communications

Instructor: Dr. Adnan A. Khan

[email protected]

_ _ .

Fall 2013

Week-2

• OHT-1 15%

• OHT-2 15%

Grading Policy

• Quiz / Homework 5%

• Research Paper 25%

• Final Exam: 40%



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(Theodore S. Rappaport)

The Cellular Concept…

System Design Fundamentals

The Cellular Concept

• Introduction

• Frequency Reuse

• Channel Assignment Strategies

• Handoff Strategies



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Early Mobile Telephone Systems

¾ One high-power transmitter was used to cover a large area---approx. 50km. Located at a very high spot.

Frequency channels.

¾ Capacity of such

systems was very

limited.

e o e n

-12 simultaneous calls

over very area

Basics¾ Early mobile telephony systems were not cellular. Coverage

over a large area was provided by a high powered transmitter

mounted on a tall tower. Frequency reuse was not employed.

at resu te n very ow capac ty.

¾ The cellular concept arose from the need to restructure the

radio telephone system with the increase in demand. The

increase in demand could not be satisfied just by additional

spectrum allocations.

smaller transmitters. Small Cells. Neighboring base stations

(BS) are assigned different sets of channels. Capacity can be

increased by rsystematically placing the BS, channel reuse

with interference in view.



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Cell – a geographical area covered by a BS

Frequency Reuse – the frequency channels allocationscheme

, .

hex pattern is the simplest pattern that can cover an area

¾ In practice, cells are not hexagonal and BS are not exactly

in the center of the cell

B

A

E

DF

An Example of a Cellular Cluster



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Capacity Computations

¾ Assume there are N cells, each allocated k different frequency

channels. These N cells are said to form a cluster. Total number

of channels per cluster is given by

S = k N

¾ Total capacity associated with M clusters:

C = M k N = M S

¾ A cluster may be replicated more times in a given area if the

accordingly).

¾ Capacity of cellular system is directly proportional to “M” ,

number of times a cluster is replicated.

¾ Decrease N for More Capacity: If Cluster Size, N is

decreased while cell size remains fixed, more clusters are

required to cover the area (M increases). Therefore, Capacity

increases.

Capacity versus interference for same size cell

¾ Increase N for Less Interference: On the other hand, if N is

increased (large cluster size) means that co-channels are now

farther than before, and hence we have will have less

interference.

a base station can tolerate.



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Frequency Reuse

The frequency reuse factor (1/N) is the fraction of channels

allocated to each cell in a cluster.

¾ Geometry of the hexagonal cells is such that to cover

adjacent areas completely, N can have only some of the values.

Frequency Reuse

,

N = i2 + i j + j2

where i and j are positive integers. Typical values of N are

4,7,12 etc.

¾ To draw the cell pattern given i and j and to find nearest co-

c anne ne g our

1. Move i cells across hexagons.

2. Turn 60º CCW and move j cells.



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B

CG

Frequency Reuse (N=7, i=2, j=1)

A

E

DF

B

CG A

B

CGi

j

A

E

DFE

DF

Frequency Reuse (N=19, i=3, j=2)



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A total of 33 MHz are allocated to a system which uses

2x25 kHz for full duplex (i.e., each channel is 50 kHz). What is

the number of channels per cell? Number of channels per system

Example:

a. For reuse N = 4:

33 000

2 25660

, kHz

kHzchannels

×=

660

4165= channels / cell

For reuse N = 7:

660

795= channels/ cell

Now assume 1 MHz of the 33 MHz is allocated to control

channels. Each control channel is still 50 kHz

Total number of voice (traffic) channels is now

For N = 4 => 640/4 = 160 voice ch.+ control ch.

For N = 7 => 640/7 = 91 voice ch. + control ch.

2 25640

, z

kHzchannels

×=



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Strategies

Channel Allocation Techniques¾ To satisfy the user, a channel needs to be available on request.

¾ Reasonable probability of call blockage (GOS) is 2%.

¾ GOS fluctuate with location and time.

¾ The goal is to keep a uniform GOS across the system.

¾ Reduction of variations in GOS allow more users – an increase

in capacity.

¾ Three types of algorithms for channel allocation:

. xe c anne a oca on2. Channel Borrowing

3. Dynamic channel allocation (DCA)



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Fixed Channel Allocation Techniques

¾ Each cell is aloc a predetermined set of voice channels

¾ Any call attempt can be served only once unused channel is aval

otherwise Blocked

¾ Available spectrum is W Hz and each channel is B Hz. Total

number of channels:

Nc = W/B

¾ For a cluster size N, the number of channels per cell:

=

¾ To minimize interference, assign adjacent channels to different

cells.

Channel Borrowing¾ Borrow frequencies from low traffic cells to high traffic cells.

¾ Temporary channel borrowing: channel is returned after call is

completed.

¾ If all the channels in a cell are occupied, channels are borrowed

from neighboring cells.

¾ The MSC supervises such borrowing procedures and ensures

disruption free service.



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Dynamic Channel Allocation

¾ No permanent channel aloc, all channels are placed in a pool

¾ Channel returned to the ool when call is com leted

¾ Once a call is originated the BS requests MSC

¾ The switch than aloc a channel following an algo that takes into

acct:-

¾ Likelihood of future blocking within the cell

¾ Candidate channel freq

¾ Reuse distance, etc

¾Issues related to channel allocation are still under research

Comparison of Channel Allocation

TechniquesFixed Channel Allocation

¾ Advantages:

--- Less load on MSC

--- Simple

¾ Disadvantages:

Blocking may happen

Dynamic Channel Allocation

¾ Advantages:

Voice channels are not allocated permanently. That is, resource

is shared on need-basis

¾ Disadvantages:

--- Requires more processing at MSC (burden on MSC)

--- May be very complicated



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an ra eg es

¾ HAND OFF: “When a mobile moves into a different

cell while a conversation is in progress, MSC transfers

the call to a new channel belonging to the new base

”s a on . ar an o

¾ In modern wireless systems, there may be no change in

the actual channel; only the base station changes (Soft

Hand off).

¾ Mobile Switching Center needs to:

Identify the new base station

ocate new o ce an ontro c anne s assoc atewith the new base station.



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¾ Desired Behavior:

Prioritize Hand off to new call initiation

Successful transfer of call

Hand off should be as infre uent as ossible

Hand off should be imperceptible

¾ Hand off Threshold Power Level:

There is a minimum power level of the signal at BS for

. - –

Hand off Threshold is defined D dBm above this level, so

that system has time to process hand off.

Pr (Hand off Threshold) = Pr min. usable + D



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¾ When to Hand off ? (MSC needs to ensure)

Power drop is not temporary (FADING)

MS is actually moving away from current BS

¾ MSC measures average received signal power to:

Avoid unnecessary hand off

Complete necessary hand off before call drops

¾ Time available to decide hand off depends on

subscriber’s speed.

¾ Speed of user can be estimated from slope of short-

term average Power

Steep curveÆ Quick Hand off

1st Generation Systems¾ Signal Strength Measurements are made by base stations

and supervised by MSC.

¾ Base station constantl monitors ower levels of its reverse voice channels, which tells relative location of user.

¾ Reverse Signal Strength Indicators (RSSI): Power levelsof all calls in a cell are maintained.

¾ Locator Receiver (a spare receiver at each BS): Power levels of Users of neighboring cells are also measured.

¾ Both these ieces of information are assed to MSC for

hand off decision.¾ In 1G: Hand off processing takes almost 10sec



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2nd Generation Systems

Digital TDMA¾ Concept of Mobile Assisted Hand Off (MAHO)

¾ Mobile station continuall measures ower levels of

surrounding Base Stations and reports these measurements

to serving Base Station.

¾ Hand off is initiated when power level from BS of

neighboring cell exceeds that of current cell, for certain

time, or by a certain level.

MSC is shared by Mobile Stations.

¾ MAHO is particularly suited for Micro-Cellular Systems.

¾ In GSM: The time reduces to 1-2 seconds

Inter-System Hand Off

– “-----

one cellular system to another, with different

MSC”.

– The situation occurs when

• “Signal in the resident cell becomes weak, and no

other cell within the s stem can take the call”.

– Issues:

• Local Call becomes long-distance call

• Compatibility between two MSCs.



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Prioritizing Hand off

– Systems differ in methods and policies of hand

off:

• Some systems take hand off like a new call

initiation: user will be more annoyed in case of call

drop than call blocking for some time.

– Various Methods of Prioritizing Hand off have

been devised and implemented

Guard Channel Concept

–

• do not use them for call initiation.

– Advantage

• Increased probability of successful hand off

– Disadvantage

• Lower capacity (less number of channels for call

initiation) – Suits Dynamic Channel Allocation case



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Queuing Strategy

– Queuing of Hand off Requests:• Decreases probability of forced termination of a call due to

lack of available channels.

– Trade off between “probability of forced termination”and “total carried traffic” is,

• “Probability of forced termination” decreases at the cost of reduced Total Carried Traffic.

– Queuing is possible because finite time available between the time the rec sig drops below the handoff threshold and time call is terminated due to low sistrength.

– Delay Time and Queue Size are related to tfc pattern in aservice area.

– Queuing doesn’t guarantee zero probability of call drop.

Practical Hand Off Considerations – High Speed Vehicle vs Pedestrians

• In micro-cells for more capacity, MSC may be over- burdened in case hi h s eed users chan in the micro-cells quickly

• Obtaining a new Cell Site is not always practical due tosome non tech reasons

• Service providers install addl base stas at same loc

– Umbrella Cell

• By using diff antenna heights and diff power levels it possible to provide large and small cells ay same locknown as Umbrella Cell



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1. Umbrella Cell Approach – Minimizes number of hand offs for high speed

users

• Provides additional micro-cell for pedestrians users.

– A High speed user converting into a low speed

user may be switched to smaller micro-cell by

Base Station, without MSC intervention.

– S eed Estimation can be erformed b slo e of

short term average calculations of the received

power, or by some more sophisticated

algorithms.



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2. Cell Dragging –

strong signal to base station.

• In Line Of Sight (LOS) case, power does not drop to

hand off threshold level even when user has moved

deep into the neighboring cell.

– Solution: Hand Off Threshold and Radio

Coverage Parameters need to be adjusted.

3. Soft Hand Off

• .

MSC decides from which BS to take the signal and pass to

PSTN. (Soft Hand Off)



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Interference and System Capacity

Interference

• Major performance limiting factor in cellular radio.

• In comparison with wired Comm. Systems, theamount and sources of interferences in WirelessSystems are greater.

• Creates bottleneck in increasing capacity

• Sources of interference are:

– – A Call in progress in the Neighboring Cells

– The same frequency cells

– Non-cellular signals in the same spectrum



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Interference

• Interference in Voice Channels: Cross-Talk

• Interference in Control Channels:missed/blocked calls

• Urban areas usually have more interference,ecause o

– Greater RF Noise Floor – More Number of Mobiles

Types of Interference

. -

2. Adjacent Channel Interference (ACI)

3. Other services: like a competitor cellular

service in the same area



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Co-Channel Interference and System

Capacity

• -

channel cells.

• Interference between signals from these cells is

called Co-Channel Interference (CCI).

• Cannot be controlled by increasing RF power.

• epen s on m n mum s ance e ween co-

channels.

Co-Channel Interference and SystemCapacity

• Yellow cells use same set of requency c anne s, an ence,

interfere with each other

• In cellular system there are 6 first-layer co-channels

• In constant cell size and RF power,CCI is a function of Distance between co-channel cells D andsize of each cell (R).

• Increasing ratio D/R, CCI decreases.

• Define Channel Reuse Ratio – Q = D/R



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Capacity• For hexagonal geometry, D/R can be calculated:

• Smaller Q provides larger capacity, since that would

mean smaller N.

• Larger Q improves quality, owing to less CCI. For

– N = 3, Q = 3

– N = 7 = 4.58

N R

DQ 3==

, .

– N = 12, Q = 6

– N = 13, Q= 6.24

• Proof of is given N DQ 3==e ow

The Geometry of the Hexagons is suchthat the number of cells per cluster, N,can only have values such that

N = i*i +i*j+j*j, i and j are non-negative integers.

Method to Find the Nearest Co-Channel Neighbor

Move i Cells along any chain of

A

B

CG

F

E

D

A

B

CG

F

E

D

A

B

CG B

CG

I =2

J =1

, ,

Turn 60 degree counter-clockwiseand

Move j Cells.

• From the figure, using cosine law

EXAMPLE:Finding Co-Channel Neighbor,

N= 7, I = 2, j=1

F

E

DA

F

E

D



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2 ' 2 2 '

'

2 2 2 2 2

[ .(2 )] [ .(2 ')] 2 (2 '). .(2 ).cos120

( 3/2) ,

3 . 3 . . .3

3 .

D i R j R i R j R

R R

D i R j R i j R

D N R

= + −

=

= + +

=

/ 3Q D R N = =


• - -S

• Number of Co-channel interfering

cells io= 6

• S is the power of the signal of interest and I k is

power of k th interference.

∑=

0

1

i

i

i I

• That is, received signal power is inversely related

to nth power of distance

– where n = Path Loss Exponent

nd S

−∝



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Capacity• Then we can express the SIR in terms of distance

−n

• where the denominator represents the users in neighboring

clusters using the same channel and

– Transmit power and path loss exponent are same

• Let D =D be the distance between cell centers. Then

∑=

−

==0

1

/i

i

n

i D

SIR I S

• Note how S/I improves with N .

00

)3()/(

i

N

i

R D

I

S nn

==

−


• -

the link performance which then decides

Frequency Reuse Plan, and System Capacity.



9/17/20

Channel Planning for Wireless

Systems•

channels a more difficult problem in practice.

• Theoretical analysis provides a guideline.

• In general 5% of the available spectrum isreserved for control channels

• Frequency reuse of control channels moreconservative.

• In CDMA N=1, however, practical difficultiesforces some kind of frequency planning.

3.5.3 Adjacent Channel Interference

¾ Interference from channels that are adjacent in frequency,

¾ The primary reason for that is Imperfect Receive Filters which

cause the ad acent channel ener to leak into our s ectrum.

¾ Problem is severer if the user of adjacent channel is in close

proximity.Î Near-Far Effect

¾ Near-Far Effect: The other transmitter(who may or may not be

of the same type) captures the receiver of the subscriber.

,

on a channel close to the one being used by a weaker mobile:

The BS faces difficulty in discriminating the desired mobile

user from the “bleed over” of the adjacent channel mobile.



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Unintended

Tx

Near-Far Effect: Case 1

Mobile User

Rx

BS as Tx

Weaker signal

Strong “bleed

over”

¾ The Mobile receiver is captured by the unintended, unknown

transmitter, instead of the desired base station

BS as Rx

Near-Far Effect: Case 2

Adjacent

Channel

Desired

Mobile Tx

ea er s gna

Strong “bleed

over”

¾ The Base Station faces difficulty in recognizing the actual

mobile user, when the adjacent channel bleed over is too

high.



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Minimization of ACI

(1) Careful Filtering ---- min. leakage or sharp transition

(2) Better Channel Assignment Strategy

,

large as possible.

¾ Also, secondary level of interference can be reduced by not

assigning adjacent channels to neighboring cells.

¾ For tolerable ACI, we either need to increase the frequency

.

. run ng an ra e o erv ce

¾ 3.7 Improving Coverage and Capacity in

Cellular Systems



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Trunking¾ A means for providing access to users on demand from

Trunking and Grade of Service (GOS)

available pool of channels.

¾ With trunking, a small number of channels canaccommodate large number of random users.

Telephone companies use trunking theory to determinenumber of circuits required.

Trunking theory is about how a population can be

handled by a limited number of servers.

Terminology

1. Traffic intensity is measured in Erlangs:One Erlang: traffic in a channel completely occupied

One call-hour per hour or one call-min per min

.

2. Grade of Service (GOS): probability that a call isblocked (or delayed).

3. Set-Up Time: time to allocate a channel

4. Blocked Call: Call that cannot be completed at time of

.5. Holding Time: (H) average duration of typical call

6. Load: Traffic intensity across the whole system

7. Request Rate: (λ ) average number of call requests per unit time.



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Traffic per user Au = λ H where λ is the request rate andH is the holding time.

=

Traffic Measurement (Erlangs)

u

If traffic is trunked in C channels, then the traffic intensityper channel is Ac= U Au /C

Erlang B: Determines the probability that a call is blocked. Its a measure o or a run e sys em w c prov esno Queuing for blocked calls

Erlang B

• If blocked calls are cleared (i.e. not queued), thenunder some model assumptions, the probability of a

GOS

k

A

C

A

blockingC

k

k

c

==

∑=0 !

!]Pr[

• Assumptions – Any user can req a ch any time

– Pr of user occupying a ch is exp

– Finite trunk chs C aval



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Table 3.4: Capacity of Erlang B System

Figure 3.6: The Erlang B chart showing the probability of blocking vs. traffic intensity



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The required GOS = 0.5%. Each user generates 0.1 Erlangsof traffic. How many users in a blocked channels clearedsystem for C =5 channels?

Example

From the chart, with GOS=0.005 and the number of channels(C) = 5:

A = 1.13

u . . ~ .

An urban area has 2 million residents. Three competingcellular systems provide service:System A 394 cells x 19 channels.System B 98 cells x 57 channels.S stem C 49 cells x 100 channels.

For each user λ = 2 calls/hr, H = 3min, GOS = 2%blocking. Find the number of users that can be supportedby each system.

System A:

Au= λ H = 2 x 3/60 = 0.1 Erlangs.

From the curve for GOS = 0.02 and C = 19 => A = 12 Er.

Users per cell (U) = A/ Au = 12/0.1 = 120 users120 users/cell x 394 cells = 47,280 users can

be served



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An urban area has 2 million residents. Three competingcellular systems provide service:

System A 394 cells x 19 channels.System B 98 cells x 57 channels.S stem C 49 cells x 100 channels.

System B:

Prob Blocking = 2% = 0.02

For each user λ = 2 calls/hr, H = 3min, GOS = 2%blocking. Find the number of users that can be supportedby each system

C =57

Au =λ

H = 2 x 3/60 = 0.1 Erlangs.From table, A = 45 ErlangsUsers per cell U = A/ Au = 45/0.1 = 450 users450 users/cell x 98 cells = 44,100

An urban area has 2 million residents. Three competingcellular systems provide service:System A 394 cells x 19 channels.System B 98 cells x 57 channels.S stem C 49 cells x 100 channels.

System C:

For each user λ = 2 calls/hr, H = 3min, GOS = 2%blocking. Find the number of users that can be supportedby each system



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Erlang C Model – Blocked calls delayed

A different type of trunked system queues blocked calls – Blocked Calls Delayed. This is known as an Erlang Cmodel.

Procedure:

1) Determine Pr[delay > 0] = probability of a delay fromthe chart.

2) Pr[delay>t | delay>0] = probability that the delay islonger than t, given that there is a delay

Pr dela >t dela > 0 =ex - C- A t /H

3) Unconditional Probability of delay > t :

Pr[delay>t ] = Pr[delay > 0] Pr[delay > t | delay >0 ]

4) Average delay time D = Pr[delay > 0] H / (C- A)

Erlang C Formula

¾ The likelihood of a call not having immediate access to achannel is determined by Erlang C formula:

∑−

=

−+

=>1

0 !)1(!

]0Pr[C

k

k C

k

A

C

AC A

Adelay



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Figure 3.7: The Erlang C chart showing the probability of delay vs. traffic intensity

Example

A hexagonal cell within a 4-cell system has radius =1.387km. Total # of channels in system = 60. Load per user ( Au) = 0.029 Erlangs, λ = 1call/hour. Erlang C systemwith GOS (prob of delay) = 5%. The area of a hexagon isgiven by 2.5981R2.

system support?



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Area of cell = 2.598 x (1.387)2 = 5 sq km.

# of cells/cluster = 4.

# of channels = 60.

=> # of channels /cell = 60/4 = 15 .

(a) From Erlang C chart, GOS =0.05, C = 15 =>

A = 8.8 Er.

# of users = total traffic/ traffic per user

=> = = = . . .

Users per sq km = 303/5 = 60 users/sq km.

(b) What is the probability that a delayed call will have to waitfor more than 10s ?

H = Au /λ = 0.029 hour = 104.4 seconds.Pr (delay > 10 sec | delay) = exp[-(C- A)t /H ]

- - . .= 52.22%

(c) What is the probability that a call will have to wait for morethat 10 seconds ?

> = > > = 0.05 x 0.5522 = 2.76 %.



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The way in which channels are grouped affects the

.

For example:

For GOS=0.01 and C=10 => A = 4.46 Erlangs.

Let 2 groups of C = 5 => A = 1.36 x 2 = 2.72

Erlangs.

Thus how allocation is done is critical to

erformance.

¾ Cost of a cellular network is proportional to the number of Base Stations. The income is proportional to the number

Improving Capacity in Cellular Systems

of users.

¾ Ways to increase capacity:

New spectrum – expensive. PCS bands were sold for $20B.

Architectural a roaches: cell s littin cell sectorin reuse partitioning, micro-cell zones.

Dynamic allocation of channels according to load inthe cell (non-uniform distribution of channels).

Improve access technologies.



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Cell SplittingCell Splitting is the process of subdividing the

congested cell into smaller cells (microcells), Each

with its own base station and a corresponding

reduction in antenna height and transmitter power.

Cell Splitting increases the capacity since it

increases the number of times the channels are

reused.

An Example• The area covered by a circle with radius R is four

times area covered b the circle with radius R/2

• The number of cells is increased four times

• The number of clusters the number of channels

and the capacity in the coverage area are increased

• Cell Splitting does not change the co-channel re-

=



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Transmit Power

• New cells are smaller, so the transmit power of thenew cells must be reduced

• ow o e erm ne e ransm power

• The transmit power of the new cells can be found byexamining the received power at the new and old cell

boundaries and setting them equal

• Pr (at the old cell boundary) is proportional to

* -nt1

• Pr (at the new cell boundary) is proportional to

Pt2 * (R/2)-n

● Take n=4, we get

=

Transmit Power

t t

●We find that the transmit power must be reduced

by 16 times or 12 dB in order to use the microcells

to cover the original area. While maintaining the

same S/I.



9/17/20

Cell sectoring

Another way to reduce the number of cells in a cluster andhence, to reduce Interference is sectoring. Sectoring

.Three (3) 120 degrees sectors are shown as an example.

1

2

3

Analysis: mobile in center cell will experienceinterference from only 2 cells (not 6). Improvement of 6dBin S/I. Alternatively, try to reduce the reuse factor.Sectoring entails reduced trunking efficiency.

Figure 3.10 (a) 120o sectoring; (b) 60o sectoiring



9/17/20

Example of Cell Sectoring

¾ With omindirectional antennas

29S n n

Where we assumed that the power attenuation n = 4.

¾ For N = 4, we obtain S = 13.8 dB.

¾ For N = 4 and with 3 sectors, we get S = 18.4 dB:

6 I =S/I

6 6= =

29

2

S N

I =



9/17/20

Microzones

● Multiple zones and a base station make up a cell●As a mobile travels within the cell, it is served by

● This technique is superior to sectoring becauseantennas are placed at the outer edges of the cell,and any base station channel can be assigned toany zone by the base station

Microzoning



9/17/20

-

Repeaters for Range Extension

coverage for hard-to-reach areas,such as within buildings

or in valleys or tunnels

● Repeaters are bidirectional. Upon receiving signals from

base station,then amplifies and reradiates the base station

signals to the specific coverage region.

.

●The repeaters do not add capacity to the system-it

simply serves to reradiate the base station signal

into specific locations

Repeaters for Range Extension



9/17/20

Summary of the Cellular Concept

● Concepts of handoff, frequency reuse, trunking

e c ency an requency p ann ng ave een

presented

● The capacity of a cellular system depends on

several factors and the methods to increase the

capacity

● e overr ng o ect ve o t ese met o s s to

increase the number of users in the system

Thanks

Documents

Cellular system in wireless communication