EKT 450 Mobile Communication System

Chapter 6: The Cellular Concept

Prof Dr. Sabira Khatun,

Dr. Muzammil Jusoh, Dr. Norsuhaida Ahmad

School of Computer and Communication Engineering

1

Introduction

2

• Introduction to Cellular System

• Frequency Reuse

• Channel Assignment Strategies

• Handoff Strategies

• Interference + System Capacity

• Trunking + Grade of Service

• Improving Capacity in Cellular System

Introduction to Cellular System

3

Traditional mobile service was structured in a fashion

similar to television broadcasting: One very powerful

transmitter located at the highest spot in an area would

broadcast in a radius of up to 50 kilometers.

Drawbacks:

• High power consumption

• Impossible to reuse same

frequencies throughout the

system - Low capacity

Introduction to Cellular System

4

• Solves the problem of spectral congestion and user

capacity.

• Offer very high capacity in a limited spectrum without

major technological changes.

• Reuse of radio channel in different cells.

• Enable a fix number of channels to serve an arbitrarily

large number of users by reusing the channel throughout

the coverage region.

Frequency Reuse

5

• The design process of selecting and allocating channel

groups for all the cellular base stations within a system

is called frequency reuse or frequency planning.

• Each cellular base station is allocated a group of radio

channels within a small geographic area called a cell.

• Neighboring cells are assigned different channel groups.

• By limiting the coverage area to within the boundary of

the cell, the channel groups may be reused to cover

different cells.

• Keep interference levels within tolerable limit.

Frequency Reuse

6

• Seven groups of channel (into different cells) from A to

G

• Actual radio coverage is called footprint

determined from field measurements or propagation

prediction models.

• Omni-directional antenna versus directional antenna.

Why Hexagonal Cell?

7

• Hexagonal cell shape has been universally adopted,

since it permits easy and manageable analysis of a

cellular system.

• For a given distance between the center of a polygon

and its farthest perimeter points, the hexagon has the

largest area among other sensible geometric cell

shapes.

• By using the hexagon geometry:

o The fewest number of cells can cover a geographic

region

o Closely approximates a circular radiation pattern

which would occur for an Omni-directional base

station antenna and free space propagation. What other sensible geometric cell

shapes?

Why Hexagonal Cell?

8

• When using hexagons to model coverage areas, base

station transmitters are depicted as either being:

o In the center of the cell (center-excited), or

o On three of the six cell vertices (edge-excited).

• Normally:

o Omni-directional antennas are used in center-

excited cells

o Sectored directional antennas are used in edge-

excited cells.

Concept of Frequency Reuse

9

• Consider a cellular system which has a total of S

duplex channels.

• Each cell is allocated a group of channels, k (k < S).

• The S channels are divided among N cells.

• The total number of available radio channels, S = kN

• The N cells which use the complete set of channels is

called cluster.

• The cluster can be repeated M times within the

system. The total number of channels, C, is used as

a measure of capacity

MSMkNC

Concept of Frequency Reuse

10

• The capacity is directly proportional to the number of

replication M.

• The cluster size, N, is typically equal to 4, 7, or 12.

• The frequency reuse factor is given by 1/N.

• Hexagonal geometry has:

• exactly six equidistance neighbors.

• the lines joining the centers of any cell and each

of its neighbors are separated by multiples of 60

degrees.

How to maximize

capacity?

solution

Concept of Frequency Reuse

11

• The total number of channels, C, is used as a

measure of capacity

• If k and N remain constant,

• If C and k remain constant,

MSMkNC

MC

MN

1

Concept of Frequency Reuse

12

• Only certain cluster sizes and cell layout are possible.

• The number of cells per cluster, N, can only have

values which satisfy:

where i and j are non-negative integers.

22 jijiN

Concept of Frequency Reuse

13

To find the nearest co-

channel of a neighboring

cell:

1. Move i cells along any

chain of hexagons.

2. Turn 60 degrees

counter clockwise.

3. Move j cell.

Method of locating co-channel cells in a cellular system. In this example, N = 19 (i.e., i = 3, j = 2). (Adapted from [Oet83] IEEE.)

Example 1

14

If a total of 33 MHz of bandwidth is allocated to

a particular FDD cellular telephone system

which uses two 25 kHz simplex channels to

provide full duplex voice and control channels,

compute the number of channels available per

cell if a system uses:

a. Four-cell reuse

b. Seven-cell reuse

c. Twelve-cell reuse

solution

Channel Assignment Strategies

15

• The choice of channel assignment strategy impacts

the performance of the system, particularly as to how

cells are managed when a mobile user is handed off

from one cell to another.

• Fixed channel assignment:

– Each cell is allocated a predetermined set of voice

channels.

– Any call attempt within the cells can only be

served by unused channels in that particular cell.

– If all the channels in the cell are occupied, the call

is blocked and the subscriber does not receive

service.

Channel Assignment Strategies

16

• Dynamic channel assignment: - Voice channels are not allocated to the cells permanently

- Each time a call request is made, the serving base station

requests a channel from the MSC.

- It reduces the likelihood of blocking, which increases the

trunking capacity of the system, since all the available

channels in a market are accessible to all of the calls.

- But, it requires the MSC to collect real-time data on

channel occupancy, traffic distribution, and radio signal

strength indications (RSSI) of all channels on a

continuous basis.

- This increases the storage and computational load on the

system but provides the advantage of increased channel

utilization and decreased probability of a blocked call.

Handoff Strategies

17

• When a mobile moves into a different cell while a

conversation is in progress, the MSC automatically

transfers the call to a new channel belonging to the

new base station.

• Handoff operation:

– Identifying a new base station

– Re-allocating the voice and control channels with

the new base station.

Handoff Strategies

18

• Handoff Threshold:

– Specify minimum usable signal for acceptable

voice quality (normally -90 dBm to -100 dBm) as

– A slightly stronger signal level is used at which

handoff is made, as

– Handoff margin, cannot be too

large or too small.

o If Δ is too large, unnecessary handoffs burden

the MSC.

o If Δ is too small, there may be insufficient time

to complete handoff before a call is lost.

usablemin ,, rhandoffr PP

usablemin ,rP

handoffrP ,

Handoff Strategies

19

Handoff Strategies

20

• Handoff must ensure that the drop in the

measured signal is not due to momentary fading

and that the mobile is actually moving away from the

serving base station.

• Running average measurement of signal strength

should be optimized so that unnecessary handoffs

are avoided.

– Depends on the speed at which the vehicle is

moving.

– Steep short term average the hand off should

be made quickly.

– The speed can be estimated from the statistics of

the received short-term fading signal at the base

station.

Handoff Strategies

21

• Dwell time: the time over which a call may be maintained

within a cell without handoff, depends on:

– propagation

– interference

– distance

– speed

Handoff Measurement

22

• In the first generation (1G) analog cellular systems:

o Signal strength measurements are made by the

base station to determine the relative location of

each mobile user with respect to the base station.

o Additionally, a spare receiver in each base station,

called the location receiver, is used to determine

signal strengths of mobile users which are in

neighboring cells (and appear to be in need of

handoff.)

Handoff Measurement

23

• In second generation systems (TDMA technology):

o Handoff decisions are made mobile assisted

handoff (MAHO).

o Every mobile station measures the received

power from surrounding base stations and

continually reports the results of these

measurements to the serving base station.

o A handoff is initiated when the power received

from the base station of a neighboring cell

begins to exceed the power received from the

current base station by a certain level or for a

certain period of time.

o The MAHO performs at a much faster rate, and is

particularly suited for micro cellular environments.

Handoff Measurement

24

• Intersystem handoff:

o Moves from one cellular system to a

different cellular system controlled by a

different MSC.

o It may become a long-distance call and a

roamer.

o Compatibility between the two MSCs need

to be determined.

• Handoff requests is much important than

handling a new call.

Prioritizing Handoff

25

• Guard channel concept:

o A fraction of total available channels in a

cell is reserved exclusively for handoff

requests from ongoing calls which may

be handed off into the cell.

• Queuing of handoff requests

o To decrease the probability of forced

termination of a call due to lack of

available channels.

Practical Handoff Consideration

26

• Different type of users:

– High speed users need frequent handoff during a

call.

– Low speed users may never need a handoff

during a call.

• Microcells to provide capacity, the MSC can become

burdened if high speed users are constantly being

passed between very small cells.

• Minimize handoff intervention:

– Handle the simultaneous traffic of high speed and

low speed users.

Practical Handoff Consideration

27

• Using different antenna heights and different power

levels it is possible to provide large and small cells which

are co-located at a single location. This technique is called

umbrella cell approach and is used to provide large area

coverage to high speed users while providing small area

coverage to users traveling at low speeds.

• The umbrella cell approach ensures that the number of

handoffs in minimized for high speed users and provides

additional microcell channels for pedestrian users.

Hard Handoff and Soft Handoff

28

• Hard handoff: when the signal strength of a neighboring cell exceeds that of the current cell, plus a threshold, the mobile station is instructed to switch to a new frequency band that is within the allocation of the new cell assign different radio channels during a handoff.

• For 1st generation analog systems, if takes about 10

seconds and the value for Δ is on the order of 6 dB

to 12 dB.

• For 2nd generation digital systems, typically requires

only 1 or 2 seconds, and Δ usually is between 0 dB

and 6 dB.

Hard Handoff and Soft Handoff

29

• Soft handoff: a mobile station is temporarily connected to more than one base station simultaneously. A mobile unit may start out assigned to a single cell. If the unit enters a region in which the transmissions from two base stations are comparable (within some threshold of each other), the mobile unit enters the soft handoff state in which it is connected to the two base stations.

• Consequently, handoff does not mean a physical change in the assigned channel, rather than a different base station handles the radio communication task.

• By simultaneously evaluating the receiver signals from a single subscriber at several neighboring base stations, the MSC may actually decide which version of the user’s signal is best at any moment in time.

Interference and System Capacity

30

• Sources of interference:

– another mobile in the same cell

– a call in progress in the neighboring cell

– other base stations operating in the same

frequency band

– non-cellular system leaks energy into the cellular

frequency band

• Two major cellular interference:

– co-channel interference

– adjacent channel interference

Co- and Adjacent-Channel Cells

31

Co-channel

cells

Adjacent-

channel

cells

Co-channel

interference

Adjacent-

channel

interference

Interference and System Capacity

32

• Interference on voice channels causes cross talk, where

the subscriber hears interference in the background due to

an undesired transmission.

• On control channels, interference leads to missed and

blocked calls due to errors in the digital signaling.

• Interference is more severe in urban areas, due to the

greater RF noise floor and the large number of base

stations and mobiles.

• The interference are difficult to control in practice largely

due to random propagation effects.

• Even more difficult to control is out-of-band interference

mainly from the base stations of competing cellular

carriers (locating their base stations in close proximity.)

Co-Channel Interference and System

Capacity

33

• Frequency reuse - there are several cells that

use the same set of frequencies:

– co-channel cells

– co-channel interference

• To reduce co-channel interference, it cannot

be combated by simply increasing the

carrier power of a transmitter. Instead, co-

channel cell must be separated by a

minimum distance.

Co-Channel Interference and System

Capacity

34

• When the size of the cell is approximately the same

– co-channel interference is independent of the

transmitted power

– co-channel interference is a function of

• R : Radius of the cell

• D : distance to the center of the nearest co-

channel cell

• Q is called the co-channel reuse ratio

NR

DQ 3

Co-Channel Interference and System

Capacity

35

• A small value of Q (small N) provides large capacity

• A large value of Q (large N) improves the

transmission quality - smaller level of co-channel

interference

• A tradeoff must be made between these two

objectives

Co-Channel Interference and System

Capacity

36

Example 2

37

You are trying to design a cellular network that

will cover an area of at least 2800 km2. There are

300 available voice channels. Your design is

required to support at least 100 concurrent calls

in each cell.

If the co-channel cell centre distance is required

to be 9 km, how many base stations will you

need in this network?

solution

Co-Channel Interference and System

Capacity

38

• Let i0 be the number of co-channel interfering cells. The signal-

to-interference ratio (SIR) for a mobile receiver can be

expressed as:

S : the desired signal power

Ii : interference power caused by the i-th interfering co-channel

cell base station

• The average received power at a distance d from the

transmitting antenna is approximated by

or

n is the path loss exponent which ranges between 2 and 4.

0

1

i

i

iI

S

I

S

n

rd

dPP

0

0

0

0 log10)dBm()dBm(d

dnPPr Receive

r

d0

P0 : Measured power

Co-Channel Interference and System

Capacity

39

• When the transmission power of each base station

is equal and the path loss exponent same

throughout the coverage area, SIR for a mobile can

be approximated as

Example: for N=7, the first layer of interfering cell, i0 = 6.

• For simplification, assume all interferers have

equidistance,

0

1

i

i

n

i

n

D

R

I

S

00

3)/(

i

N

i

RD

I

Sn

n

which relates S/I to the cluster size,

and in turn determines the overall

capacity of the system

Example 3

40

If a signal-to-interference ratio of 15 dB is

required for satisfactory forward channel

performance of a cellular system, what is the

frequency reuse factor and cluster size that

should be used for maximum capacity if the path

loss exponent is (a) n = 4, (b) n = 3 ?

Assume that there are six co-channel cells in the

first tier, and all of them are at the same distance

from the mobile. Use suitable approximations.

solution

Adjacent-Channel Interference

41

• Adjacent channel interference: interference from

adjacent in frequency to the desired signal.

– Imperfect receiver filters allow nearby frequencies to

leak into the pass-band

– Performance degrade seriously due to near-far

effect.

desired signal

receiving filter response

desired signalinterference

interference

signal on adjacent channelsignal on adjacent channel

FILTER

Adjacent-Channel Interference

42

• Adjacent channel interference can be minimized

through :

o careful filtering and channel assignment.

o Keep the frequency separation between each

channel in a given cell as large as possible

o A channel separation greater than six is

needed to bring the adjacent channel interference

to an acceptable level.

Power Control for Reducing

Interference

43

• In practical cellular radio and personal communication systems, the power levels transmitted by every mobile unit are under constant control by the serving base stations.

• This is done to ensure that each mobile transmits the smallest power necessary on the reverse channel.

• Power control not only helps prolong battery life, also reduces the interference on the reverse channel.

• It is especially important for CDMA systems, because every user in every cell share the same radio channel. (to reduce the co-channel interference.)

Power Control for Reducing

Interference

44

Need for power control ?

Trunking and Grade of Service

45

• Trunking allows a large number of users to share the

relatively small number of channels in a cell by

providing access to each user, on demand, from a

pool of available channels.

• It exploits the statistical behavior of users so that a

fixed number of channels or circuits may

accommodate a large, random user community.

• The measure of traffic intensity, namely Erlang.

For example:

0.5 Erlangs of traffic =

a radio channel that is occupied for 30 minutes

during an hour.

Trunking and Grade of Service

46

Trunking and Grade of Service

47

The Grade of Service (GOS):

• 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, or the likelihood of a call

experiencing a delay greater than a certain

queuing time.

Trunking and Grade of Service

48

• The traffic intensity generated by each user

Where H is the average duration of a call, λ is the average number of

call requests per unit time for each user.

• The total offered traffic intensity for U users

• In a C channel trunked system, if the traffic is equally

distributed among the channels, the traffic intensity per

channel is

HAu

C

UAA u

C

uUAA

Trunking and Grade of Service

49

• Example:

o AMPS cellular system is designed for a

GOS of 2% blocking.

o This implies that the channel allocations

for cell sites are designed so that 2 out of

100 calls will be blocked due to channel

occupancy during the busiest hour.

Trunking and Grade of Service

50

Two types of trunked systems:

• Blocked Calls Cleared trunking :

– Offers no queuing for call requests.

– Calls arrive as determined by a Poisson distribution.

– There are an infinite number of users.

– There are memoryless arrivals of requests.

– The probability of a user occupying a channel is

exponentially distributed.

– A finite number of channels available.

– This is known as an M/M/m queue.

a memoryless poisson arrivals an exponential service time

the number of trunked channels

Trunking and Grade of Service

51

This leads to the derivation of the Erlang B

formula:

where C is the number of trunked channels, A

is the total offered traffic.

GOS

!

!][

0

C

k

k

c

r

k

A

C

A

blockingP

Trunking and Grade of Service

52

Trunking and Grade of Service

53

• Blocked Calls Delayed trunking :

– Queuing is provided to hold calls which are blocked.

– If no channel available, the call request may be

delayed until a channel becomes available.

– Measure of GOS : probability that a call is blocked

after waiting a specific length of time.

– A call not having immediate access to a channel is

determined by Erlang C:

1

0 !)1(!

]0[C

k

kc

c

r

k

A

C

ACA

AdelayP

Trunking and Grade of Service

54

The GOS of a trunked system where blocked

calls are delayed :

The average delay for all calls in a queued

system

)/)(exp(]0[

]0|[]0[][

HtACdelayP

delaytdelayPdelayPtdelayP

r

rrr

AC

HdelayPD r

]0[

Trunking and Grade of Service

55

The Erlang B formula plotted in graphical form

Trunking and Grade of Service

56

The Erlang C formula plotted in graphical form

Example 4

57

How many users can be supported for 0.5%

blocking probability for the following number of

trunked channels in a blocked calls cleared

system?

(a) 1, (b) 5, (c) 10, (d) 20 and (e) 100

Assume each user generates 0.1 Erlangs of

traffic.

solution

Example 5

58

Pauh has an area of 1300 square miles and is covered by CELCOM

using a seven-cell reuse pattern. Each cell has a radius of four miles

and the city is allocated 40 MHz of spectrum with a full duplex

channel bandwidth of 60 kHz. Assume a GOS of 2% for an Erlang B

system is specified. If the offered traffic per user is 0.03 Erlangs,

compute:

(a) The number of cells in the service area,

(b) The number of channels per cell,

(c) Traffic intensity of each cell,

(d) The maximum carried traffic,

(e) The total number of users that can be served for 2% GOS

(f) The number of mobiles per unique channel (where it is

understood that channels are reused)

(g) The theoretical maximum number of users that could be served

at one time by CELCOM.

solution

Example 6

59

An urban area has a population of two million residents.

Three competing trunked mobile networks (CELCOM,

MAXIS and DIGI) provide cellular service in this area.

CELCOM has 394 cells with 19 channels each, MAXIS has

98 cells with 57 channels each, and DIGI has 49 cells, each

with 100 channels.

Find number of users that can be supported at 2%

blocking if each user averages two calls per hour at an

average call duration of three minutes. Assume that all three

trunked systems are operated at maximum capacity,

compute the percentage market penetration of each

cellular provider.

solution

Improving Capacity in Cellular System

60

• Methods for improving capacity in cellular systems:

– Cell Splitting :

o Subdividing a congested cell into smaller cells.

o Allows an orderly growth of the cellular system

– Sectoring :

o Directional antennas to control the interference and

frequency reuse of channels.

– Microcell Zone Concept :

o Distributing the coverage of a cell and extends the cell

boundary to hard-to-reach place.

– Repeaters for range extension.

– More bandwidth – standards, country regulation etc.

– Borrow channel from nearby cells.

Improving Capacity in Cellular System

61

• Cell Splitting : Split congested cell into smaller cells.

– Preserve frequency reuse plan.

– Reduce transmission power.

Improving Capacity in Cellular System

62

microcell

Reduce R to R/2

Improving Capacity in Cellular System

63

Improving Capacity in Cellular System

64

• Transmission power reduction from to

• Examining the receiving power at the new and old cell boundary

• If we take n = 4 and set the received power equal to each other

• The transmit power must be reduced by 12 dB in order to fill in

the original coverage area.

• Problem: if only part of the cells are splitted:

– Different cell sizes will exist simultaneously

• Handoff issues - high speed and low speed traffic can be

simultaneously accommodated

1tP 2tP

n

tr RPP 1]boundary cell oldat [

n

tr RPP )2/(]boundary cellnew at [ 2

16

12

tt

PP

Improving Capacity in Cellular System

65

• Sectoring : Decrease the co-channel interference and

keep the cell radius R unchanged

– Replacing single omni-directional antenna by

several directional antennas.

– Radiating within a specified sector.

Improving Capacity in Cellular System

66

67

Base Station Antennas

Omnidirectional : broadcast

3600

Sector : broadcasts 600 /

900 / 1200

Panel / Dish : Point to point

68

600 Sectoring

69

1200 Sectoring

Improving Capacity in Cellular System

70

Microcell Zone Concept :

• Antennas are placed at the outer edges of the cell

• Any channel may be assigned to any zone by the base station

• Mobile is served by the zone with the strongest signal.

• Handoff within a cell

– No channel re-

assignment

– Switch the channel

to a different zone

site

• Reduce interference

– Low power

transmitters are

employed

Improving Capacity in Cellular System

71

• Repeaters for range extension: