The basic frequency reuse mode of GSM is 4*3 frequency reuse. It is the basic of other frequency reuse modes, we also call it regular frequency reuse model. “4” represents 4 sites, “3” represents 3 cells in each site.
Totally 12 cells become a basic frequency reuse cluster. Different cells in the same cluster have different frequencies. The above figure shows a cell cluster of 4*3 frequency reuse mode, while those inside the bold black line is a basic cell cluster model, including 4 BTS which have 3 frequency reuse group, there are 12 cells totally.
In a specific allocation, all frequencies are allocated to each cell according to certain principle and same as other cell clusters. In this way, each frequency carrier is “reused” in different cell cluster time and time again.
Certainly, other reuse model n*m means that each basic reuse cell cluster contains n BTS, and each BTS includes m frequency reuse group. All frequency carriers in this cell cluster are allocated to respective cells according to certain principle, and by analogy for other surrounding cells.
Illustration of Frequency Allocation of 4*3 Frequency Reuse
Suppose that available bandwidth is 12.2MHz, channel number from 34 to 95, the above table illustrates the frequency carrier distribution of 4*3 frequency reuse of 12 cells in a basic cell cluster.
From which cell to start the allocation of beginning frequency carrier 34 is not restricted. As seen from the table, 5 frequency carriers can be allocated to most cells, some cells even have 6 frequency carriers. Therefore, average largest site mode is S5/5/5 under 12.2MHz condition. Under the above regular allocation mode, it’s impossible to have co-channel or adjacent channel in the same cell or adjacent cells.