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Cellular Concept

Cellular telephone systems must accommodate a large number of users

over a large geographic area with limited frequency spectrum, i.e., with

limited number of channels.

Cellular concept is based on the process of substituting high power

transmitters with low power transmitters to support many users

This is achieved by dividing the coverage area into small segments, called

cells utilizing low power transmitters

Cell cluster: Refers to a group of cells adjacent to one another to provide

mobile coverage in a given area. Cells within a cluster use different radio

channels to avoid interference

This cluster can repeat itself and hence the same set of channels can be

used again and again (Frequency Reuse)

Each cell has a low power transmitter with a coverage area equal to the

area of the cell.

Cell Shape

The design of cells requires that they take regular shapes (polygons)

namely equilateral triangle, square and regular hexagon to ensure that

an entire area is covered without any overlaps or gaps.

A cell must be designed such that it is most reliable too, i.e., it supports

even the weakest mobile with occurs at the edges of the cell. For any

distance between the center and the farthest point in the cell from it, a

regular hexagon covers the maximum area.

Therefore, regular hexagonal geometry is normally adopted as the cells

in mobile communication

Macro-cellular nets, with cell radius 1 - 30 km

Micro-cellular net, with cell radius 200 - 2000 m

Pico-cellular nets, with cell radius 4 - 200 meter

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Types of cells

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Operational Channels in a cell

In each cell, there are four types of channels that take active part during a

mobile call:

Forward Voice Channel (FVC): This channel is used for the voice

transmission from the BS to the MS.

Reverse Voice Channel (RVC): This is used for the voice transmission from

the MS to the BS.

Forward Control Channel (FCC): Control channels are generally used for

controlling the activity of the call, i.e., they are used for setting up calls and

to divert the call to unused voice channels. Hence these are also called setup

channels. These channels transmit and receive call initiation and service

request messages. The FCC is used for control signalling purpose from the

BS to MS.

Reverse Control Channel (RCC): This is used for the call control purpose

from the MS to the BS. Control channels are usually monitored by mobiles.

Frequency Reuse

Frequency reuse, or, frequency planning, is a technique of reusing

frequencies and channels within a communication system to improve

capacity and spectral efficiency

Frequency reuse in mobile cellular systems means that frequencies

allocated to the service are reused in a regular pattern of cells, each

covered by one base station.

The repeating regular pattern of cells is called cluster. Two cells using

the same frequencies in different clusters is known as Co-channel cells

The reuse of frequencies enables a cellular system to handle a huge

number of calls with a limited number of channels

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Frequency Reuse/planning cont’d

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Frequency Reuse/planning cont’d

Consider a cellular system with S duplex channels available for use and

let N be the number of cells in a cluster

If each cell is allotted k duplex channels (where k<S), The total number

of available radio channels in the system is S=k N

The N cells which use the complete set of channels is called cluster.

If the cluster are repeated M times within the total area, the total

number of duplex channels, or, the total number of users in the system

(total system capacity) would be;

C = kMN = MS

If k and N remain constant, then the system capacity is directly

proportional to the number of times a cluster is repeated;

Cα M

if C and k remain constant, then

N α 𝟏 𝑴

The smaller the number of cells, the larger the larger the number of clusters

and hence the higher the capacity

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Frequency Reuse/planning cont’d

However for small N, co-channel cells are located much closer and hence

more interference. The value of N is determined by calculating the amount

of interference that can be tolerated in the system.

The frequency reuse factor is given by 1/N

The cluster size N is given by the following formula;

where i and j are integer numbers

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Assignment 2

Derive the equation of the cluster size of a mobile cellular system,

N=i2+ij+j2

Find the relationship between any two nearest co-channel cell distance

D and the cluster size N (Show that the Frequency Reuse distance

𝐷

𝑅 = √(3𝑁) where R is the cell radius

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Improving Coverage and Capacity in Cellular Systems

There are two techniques used;

Sectoring

Cell splitting

Cell Splitting

Cell splitting is used in an already functioning network where

network expansion is required in certain regions as opposed to the

entire network

A cell (or multiple cells) can be split into smaller cells and

frequencies are redistributed in a way that does not cause additional

interference

A cell area is proportional to R2 (cell radius) hence splitting the cell

radius by a half reduces the cell area by a quarter. Theoretically, 4 of

the smaller cells can fit into 1 of the large cells but since it is not

possible to fit 4 quarter‐size hexagonal cells completely into 1 full‐size

hexagonal cell, some regions will have to be covered by adjacent cells

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Cell Splitting cont’d

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Original cell that has reached its capacity

Cell A has been split into 3 smaller cells

Cell Splitting cont’d

After cell splitting, the new small cells are reassigned new frequencies

that do not cause co‐channel interference with adjacent cells

The power transmitted in the small cells is reduced compared to the

power transmitted in the large cells as it would require much less power

to cover the cell compared to the large cells

The power is reduced by a factor of;

n = path loss exponent

Advantages of cell splitting;

Increased system capacity

Reduced transmitted power which increases battery life of mobile phones

Disadvantages;

Requires the construction of new network towers which is very costly

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Sectoring

Sectoring

Involves the dividing of a cell into sectors and using 3 or 6 directional

antennas to provide coverage to a sector of the hexagon

When 3 directional antennas are used, 120° sectoring is achieved (each

antenna covers 120°), and when 6 directional antennas are used, 60° sectoring is achieved (each antenna covers 60°).

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Sectoring Cont’d

Dividing the cells into sectors actually reduces the network capacity

because the channels allocated to a cell are now divided among the

different sectors

The gain in network capacity is achieved by reducing the number of

interfering co‐channel cells

Example

A cellular system uses a frequency reuse factor N = 4 ( i = 0, j = 2 ). If the

path loss exponent n = 4 , and cell radius R = 5 km. Find in dB the following

quantities:

The SIR for the system with no cell sectoring.

The SIR for the system when 120° cell sectoring is used (note that

worst case occurs when mobile phone is at the furthest point from the

interfering towers).

The SIR for the system when 60° cell sectoring is used (note that worst

case occurs when mobile phone is at the furthest point from the

interfering towers). 14

Channel Assignment strategies

With the rapid increase in number of mobile users, the mobile service

providers had to follow strategies which ensure the effective utilization

of the limited radio spectrum

In addition to Frequency Reuse, a variety of Channel Assignment

strategies are adopted to improve capacity and lower interference

Channel assignment strategies are classified into two types: Fixed and

Dynamic

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Channel Assignment strategies cont’d

Fixed Channel Assignment (FCA)

Each cell is allocated a fixed number of voice channels. Any

communication within the cell can only be made with the designated

unused channels of that particular cell

The call will be blocked if all channels in that cell are occupied and the

user has to wait for channels to be released by other users

FCA is simplest of the channel assignment strategies as it requires very

simple circuitry but provides worst channel utilization

Borrowing strategy; Another approach in which the channels were

borrowed from adjacent cell if all of its own designated channels were

occupied. In such cases the MSC supervises the borrowing process and

ensures that none of the calls in progress are interrupted

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Channel Assignment strategies cont’d

Dynamic Channel Assignment (DCA)

Channels are temporarily assigned for use in cells for the duration of the

call. Each time a call attempt is made from a cell the corresponding BTS

requests a channel from MSC. The MSC then allocates a channel to the

requesting the BTS. After the call is over the channel is returned and kept

in a central pool.

To avoid co-channel interference any channel that in use in one cell can

only be reassigned simultaneously to another cell in the system if the

distance between the two cells is larger than minimum reuse distance

DCA reduces the likelihood of blocking and increases the capacity of the

network as all of the channels are available to all cells.

However, it results in heavy load on the MSC during heavy traffic

conditions

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Handover Strategies

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

The handover process involves identifying a new base station and re-

allocating the voice and control channels with the new base station.

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Once a signal level is set as the minimum

acceptable for good voice quality Pr(min),

then a slightly stronger level is chosen as

the threshold PrH at which handover has to

be made

A handover margin, ∆= PrH − Pr(min) is

defined and it cannot be too large or too

small

If ∆ is too large, unnecessary handovers will

burden the MSC

If ∆ is too small, there may be insufficient

time to complete handover before a call is

lost.

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Factors affecting Handover process

Transmitted power: The transmission power is different for different

cells and therefore, the handover threshold or the power margin varies

from cell to cell.

Received power: The received power mostly depends on the Line of

Sight (LoS) path between the user and the BS. Especially when the user

is on the boundary of the two cells, the LoS path plays a critical role in

the handover procedure and therefore the power margin depends on the

minimum received power value from cell to cell.

Area and shape of the cell: Apart from the power levels, the cell

structure also a plays an important role in the handover process

Mobility of users: The number of mobile users entering or going out of

a particular cell, also fixes the handover strategy of a cell

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