Frequency Reuse Patterns in GSM

Proper frequency planning is essential in the development of a quality cellular system. Due to the limited amount of available RF spectrum, the scarcity of channels available to each operator, and the fact that most available frequencies are non-contiguous for most operators, frequencies must be reused throughout the system to increase network capacity. The frequencies are assigned such that there is minimal cochannel and adjacent channel interference between sites. Frequency reuse is based on hexagonal cell groupings called clusters. The size of the cluster will determine how the cluster is repeated throughout the network, i.e. the reuse pattern. The frequency reuse patterns are designated as N/F. Where N is the number of cell sites in a cluster and F is the number of frequency groups within a cluster. Ericsson uses 7/21 and 4/12 reuse patterns. The cluster patterns

 In a 7/21 plan, there are 7 cell sites (A, B, . . . , G) and 21 frequency groups (A1, A2, A3, . . . , G1, G2, G3). Because A1 and G3 are adjacent frequency groups, G2 and G3 are switched in the pattern so as to prevent interference. In a 4/12 pattern, there are 4 cell sites and 12 frequency groups. The frequency groups for these patterns are described in this document. There is no frequency reuse within a cluster. The voice channel group is used to assign a frequency group to a cell.