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Chapter 2

The Cellular Concept

By Capt. Samuel Amde (Lecturer)

Wireless Cellular and Telecommunication Networks

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

the problem of spectral congestion and user capacity.

• 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 inorder to minimize interference.

– The same set of channels is then reused at different geographical

locations.

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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 initialsystem 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.

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Cell Footprint

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Cellular Systems

Advanced Mobile Phone Service(AMPS) system components

and layout

•Radio base stations

•Communications links

•Mobile switching office

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AMPS COMPONENTS FUNCTION

Cellular telephony is designed to provide communications between

two moving units, called Mobile Stations (MSs), or between one

mobile unit and one stationary unit, often called a land unit.

Each cellular service area is divided into small regions called cells.

Each cell contains an antenna & is controlled by a small office,

called the base station (BS). Base Station is controlled by a switching office, called a Mobile

Switching Centre (MSC).

MSC coordinates communications between all the base stations and

the telephone central office. It is a computerized centre that is

responsible for connecting calls, recording call information, and

billing.

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Terminology• Cell :A cell is the basic geographic unit of a cellular

system. The term cellular comes from thehoneycomb shape of the areas into which a coverageregion is divided.

• Cluster :A cluster is a group of cells. No channels

are reused within a cluster.• Co-channel cell : The set of cells using the same set

of frequencies as the target cell.

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• Consider a cellular system which has a total of S duplex

channels.

• Each cell is allocated a group of k channels, .

• The S channels are divided among N cells.

• The total number of available radio channels

• 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

• The capacity is directly proportional to the number ofreplication M .

• The cluster size, N , is typically equal to 1,3, 4, 7, or 12.

• The frequency reuse factor is given by

S k

kN S

MS MkN C

N /110


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Cellular Design Reuse Pattern

• Example: cell cluster size N = 7,

frequency reuse factor = 1/7, assume S=

490 total channels, K = S/N = 70 channels

per cell

• Assume S = 490 total channels, N = 7, K =70 channels/cell

• Clusters are replicated M=3 times

• System capacity = 3x490 = 1470 total

channels

• Only certain cluster sizes and cell layout

are possible.

• The number of cells per cluster, N , can

only have values which satisfy 22 jiji N 11


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EX. Consider a cellular system in which there are a total of 1001

radio channels available for handling traffic. Suppose the area of a cell

is 6 km2 and the area of the entire system is 2100 km2.

(a) Calculate the system capacity if the cluster size is 7

(b) How many times would the cluster of size 4 have to be replicatedin order to approximately cover the entire cellular area?

(c) Calculate the system capacity if the cluster size is 4.

(d) Does decreasing the cluster size increase the system capacity?

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Handoff Strategies

• handoff = moving a call from one zone to another zone due to

subscriber’s mobility

• 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 Threshold

– Minimum usable signal for acceptable voice quality (-90dBm to -100dBm)

– Handoff margin cannot be too large or too

small.

– If is too large, unnecessary handoffs burden the MSC

– If is too small, there may be insufficient time to complete handoff

before a call is lost.

usableminimum,, r handoff r PP

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A hard handoff does “break before make”, ie. The old channel

connection is broken before the new allocated channel connection is

setup. This obviously can cause call dropping.

In soft handoff , we do “make before break”, 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.

Two types of handoff

MSC

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

maintained within a cell without handoff.

• Dwell time depends on

– propagation

– interference

– distance – speed

• Different type of users

– High speed users need frequent handoff during a call.

– Low speed users may never need a handoff during a

call.

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Interference and System Capacity

• Sources of interference

– another mobile in the same cell

– a call in progress in the neighboring cell

– other base stations operating in the samefrequency band

• Two major cellular interference

– co-channel interference

– adjacent channel interference

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Co-channel Interference and System Capacity

• Frequency reuse - there are several cells that use the same set of

frequencies

– co-channel cells

– co-channel interference

• To reduce co-channel interference, co-channel cell must be

separated by a minimum distance.• 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

• Increasing the ratio Q=D/R, the interference is reduced.

• Q is called the co-channel reuse ratio19


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• For a hexagonal geometry

• A small value of Q provides large capacity

• A large value of Q improves the transmission quality - smaller

level of co-channel interference• A tradeoff must be made between these two objectives

N R DQ 3

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• Let 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

: interference power caused by the ith 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.

0i

0

1

i

i

i I

S

I

S

i I

n

r d

d PP

0

0

0

0 log10)dBm()dBm(d

d nPPr

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• When the transmission power of each base station is equal, SIR for a

mobile can be approximated as

• Consider only the first layer of interfering cells

0

1

i

i

n

i

n

D

R

I

S

00

3)/(i N

i R D

I S

nn

• Example: AMPS requires that SIR be

greater than 18dB

– N should be at least 6.49 for n=4.

– Minimum cluster size is 7

60 i

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Consider only the first layer of interfering cells

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Adjacent Channel Interference

• Adjacent channel interference: interference from adjacent in frequency to thedesired 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

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• Adjacent channel interference can be minimized through

careful filtering and channel assignment .

• Keep the frequency separation between each channel in a

given cell as large as possible

• A channel separation greater than six is needed to bring

the adjacent channel interference to an acceptable level.

…cont

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Power Control for Reducing Interference

• Ensure each mobile transmits the smallest power

necessary to maintain a good quality link on the

reverse channel

– long battery life

– increase SIR

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Traffic Intensity

Is a measure of the average occupancy of a resource duringa specified period of time, normally a busy hour.

The traffic intensity offered by each user is:

Erlangs H A where

H is the average holding time of a call

is the average number of call requested/hour

If there are U users and an unspecified number of channels.

The total offered traffic intensity is:

UA AT Erlangs

Busy hours traffic: Calls/busy hours *Mean call hold time 29


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Traffic Intensity - contd.In a trunks system of C channels and equally distributedtraffic among the channels, the traffic intensity per

channel is:

C UA Ac / Erlangs/channels

The traffic volume

is a measure of the total work done by a resource or facility,normally over 24 hours

VT = A * T Erlangs-Hours

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Offered Traffic

The offered traffic: Volume of traffic offered to a switch

that are all processed is defined as:

Offered traffic = carried traffic + overflow

The carried traffic: The actual traffic carried by a switch.

Overflow (blocked) traffic : Portion of the traffic notprocessed.

Busy Hour Call Attempts (BHCA)

• Used to evaluate and plan capacity for telephone networks

• Is the number of telephone calls made at the peak hour • The higher the BHCA, the higher the stress on the network

processors.

• Not to be confused with Busy Hour Call Completion (BHCC),

which truly measures the throughput capacity of the network.31


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Example I

A call established at 1am between a mobile and MSC. Assuming a

continuous connection and data transfer rate at 30 kbit/s,determine the traffic intensity if the call is terminated at 1.50am.

Solution:

Traffic intensity = (1 call)*(50 mins)*(1 hour/60 min) = 0.833 Er

Note, traffic intensity has nothing to do with the data rate, only the

holding time is taken into account.

Note:

• If the traffic intensity > 1 Erlang: The incoming call rate exceeds theoutgoing calls, thus resulting in queuing delay which will grow without

bound (if the traffic intensity stays the same).

• If the traffic intensity is < 1 Erlang, then the network can handle more

average traffic. 32


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Example II

• Consider a PSTN which receives 240 calls/hr. Each call lasts an averageof 5 minutes. What is the outgoing traffic intensity to the publicnetwork.

Solution

A = *H

= 240 calls/hr and H = 5 minutes

A = (240 calls /hr) x (5 min/call) = 1200 min/hr

Erlang cannot have any unit so

A= 1200 min/hr * (1 hour/60 minutes) = 20 Erlangs

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Erlangs - Example

• For example, if a group of user made 30 calls inone hour, and each call had an average callduration of 5 minutes, then the number of Erlangsthis represents is worked out as follows:

Minutes of traffic in the hour = number of calls xduration

Minutes of traffic in the hour = 30 x 5

Minutes of traffic in the hour = 150

Hours of traffic in the hour = 150 / 60Hours of traffic in the hour = 2.5

Traffic figure = 2.5 Erlangs

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Improving Capacity in Cellular Systems

Aim: To provide more channels per unit coverage areaTechniques: Three techniques are used to improve capacity

• Techniques for improving capacity in cellular systems

– Cell Splitting: subdividing a congested cell into

smaller cells.

– Sectoring: directional antennas to control the

interference and frequency reuse.

– Coverage zone : Distributing the coverage of a celland extends the cell boundary to hard-to-reach place.

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Cell Splitting

• Split congested cell into smaller cells.

– Preserve frequency reuse plan.

– Reduce transmission power.

microcellReduce

Rto

R/2

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Cell Splitting

• Cell splitting is the process of splitting a mobile cell

into several smaller cells. This is usually done to

make more voice channels available to accommodate

traffic growth in the area covered by the original cell

• If the radius of a cell is reduced from R to R/2, the

area of the cell is reduced from Area to Area/4. The

number of available channels is also increased.

• Cell splitting is usually done on demand; when in a

certain cell there is too much traffic which causes too

much blocking of calls. The cell is split into smaller

microcells.

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SECTORING

• :

– Use directional antennas to further control the

interference and frequency reuse of channels.

– Examples: Omni, 120O, 60O and 90O

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Sectoring

• The sectoring is done by replacing a single omni-directionalantenna with 3 directional antennas (120O sectoring) or with 6

directional antennas (60O sectoring)

• In this scheme, each cell is divided into 3 or 6 sectors. Each sector

uses a directional antenna at the BS and is assigned a set of

channels.• The number of channels in each sector is the number of channels

in a cell divided by the number of sectors. The amount of co-

channel interferer is also reduced by the number of sectors.

• Drawbacks:

• Increase the number of antennas at each BS

• The number of handoffs increases when the mobile moves from

one sector to another.

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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

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Cellular systems generations

• 1G (first generation) – voice-oriented systems based on analogtechnology; ex.: Advanced Mobile Phone Systems (AMPS)and cordless systems

• 2G (second generation) - voice-oriented systems based on

digital technology; more efficient and used less spectrum than1G; ex.: Global System for Mobile (GSM) and US TimeDivision Multiple Access (US-TDMA)

• 3G (third generation) – high-speed voice-oriented systemsintegrated with data services; ex.: General Packet Radio

Service (GPRS), Code Division Multiple Access (CDMA)• 4G (fourth generation) – still experimental, not deployed yet;

based on Internet protocol networks and will provide voice,data and multimedia service to subscribers

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Evolution of cellular networks• First-generation: Analog cellular systems (450-900 MHz)

– Frequency shift keying; FDMA for spectrum sharing

– NMT (Europe), AMPS (US)

• Second-generation: Digital cellular systems (900, 1800 MHz)

– TDMA/CDMA for spectrum sharing; Circuit switching

– GSM (Europe), IS-136 (US), PDC (Japan) –


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Channel Assignment Strategies

Aim: To increase the number of available channels without

compromising the quality of service e.g.1) Efficient Utilization of Spectrum2) Increase Capacity3) Minimize Interference

Channel assignment strategy two types

1. Fixed channel assignment (FCA)

The number of traffic channels is fixed. If all channels are busy a new call to

or from a mobile will be blocked (rejected by BS)

2. Dynamic channel assignment (DCA)

– channels are not allocated to cells permanently.

– allocate channels based on request.

– reduce the likelihood of blocking, increase capacity.

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