Frequency reuse in GSM

Frequency reuse in GSM

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 is one of the fundamental concepts on which commercial wireless systems are based that involve the partitioning of an RF radiating area into cells.
The increased capacity in a commercial wireless network, compared with a network with a single transmitter, comes from the fact that the same radio frequency can be reused in a different area for a completely different transmission.
Frequency reuse in mobile cellular systems means that frequencies allocated to

the services are reused in a regular pattern of cells, each covered by one base station. The repeating regular pattern of cells is called cluster. Since each cell is designed to use radio frequencies only within its boundaries, the same frequencies can be reused in other cells not far away without interference, in another cluster. Such cells are called ‘co-channel’ cells. The reuse of frequencies enables a cellular system to handle a huge number of calls with a limited number of channels. Figure shows a frequency planning with cluster size of 7, showing the co-channels cells in different clusters by the same letter. The closest distance between the co-channel cells (in different clusters) is determined by the choice of the cluster size and the layout of
the cell cluster.

·        Formula for calculating frequency reuse distance is as follows:

D= [root (3*N)]*R

Possible cluster sizes are N= i² + ij + j², with integer i and j (N = 1, 3, 4, 7, 9 ...). Integers i and j determine the relative location of co-channel cells.

·       d=distance between centres of adjacent cells

                                  d=root (3*R)

o   Frequency reuse –parameters:

D=minimum distance between centres of cells that use the same band of frequencies (called co-channels)

R=radius of a cell

N=number of cells in a repetitious pattern termed reuse factor (each cell in the pattern uses a unique band of frequencies).

In this example,

The cluster size N = 7 and the frequency reuse factor is 1/7 since each cell contains one-seventh of the

Total number of available channels.

The capacity is directly proportional to M. The factor N is called the cluster size and is typically 4, 7

or 12. If the cluster size N is reduced while the cell size is kept constant, more clusters are required to

Cover a given area and hence more capacity is achieved from the design viewpoint, the smallest

Possible value of N is desirable to maximize capacity over a given coverage area. The frequency reuse

Factor of a cellular system is 1/N, since each cell within a cluster is assigned 1/N of the total available

channels in the system

In this example,

N = 19 (i.e., i = 3, j = 2).

Where,

i and j are non-negative integers.

To find the nearest co-channel neighbours of a particular cell,

a. move i cells along any chain of hexagons then,

b. turn 60 degrees counter-clockwise and move j cells

v Frequency Reuse Advantages:

·       Higher capacity, higher number of users

·       Less transmission power needed

·       More robust, decentralized

·       Base station deals with interference, transmission area etc.