Frequency Reuse In Cellular System

i. The design process of selecting and allocating channel groups for all cellular base stations within a system is called as frequency reuse or frequency planning.

ii. Cellular radio systems rely on an intelligent allocation and reuse of channels throughout a coverage region. Each cellular base station is allocated a group of radio channels to be used with the small geographic area called a cell. Base stations in adjacent cells are assigned channel group which contains completely different channels than neighboring cells.

iii. By limiting the coverage area to boundaries of a cell, same group of channels may be used to cover different cells that are separated from each other by distances large enough to keep interference levels within tolerance limits.

iv. If each cell is allotted K sets of channels and if S channels are divided among N cells in unique and disjoint groups which have the same number of channels, then a total number of available channels can be S=KN.

If the cluster is replicated M times then capacity C=MKN=MS.

v. Frequency reuse factor = 1Cluster size (N)$\frac{1}{Clustersize(N)}$

vi. The cells which have been allotted the same group of channels are called co-channels.

The co-channel reuse factor Q is defined as Q=DR=3N−−−√$Q=\frac{D}{R}=\sqrt{3N}$

Where, D=Minimum safe distance after which group of frequencies can be repeated

R=Cell radius

Thus a small value of Q means larger capacity as cluster size N is small whereas large value of Q improves transmission quality. To improve capacity N is usually 4, 7 or 12.

vii. To improve capacity and efficiency of frequency reuse plan, cell sectoring is employed with Omnidirectional antenna.