Cell repeat patterns

Date: 2015-10-07; view: 493.

The cell plan has to be chosen such that the number of channel sets (N) fit together in a regular fashion without gaps or overlaps. Only certain values of N achieve this, and typical arrangements of interest to cellular radio are N = 4, 7 and 12 as shown in Figure 47.4. The value of N has a major effect on the capacity of the cellular system.

As the number of channels sets is decreased, the number of channels per cell increases, hence the system capacity increases. For example, if there are a total of 140 channels available, a 4 cell repeat pattern would provide 35 channels per cell, whilst a 7 cell would provide 20 channels per cell.

On this basis, the smallest possible value of N seems desirable. However, as N decreases, so the distance between cells using the same channels reduces, which in turn increases the level of co-chan­nel interference.

The repeat distance D and the cell radius R are both related by the geometry of the cell pattern. These are shown in Figure 47.5 and Equation 47.1.

In practice, in a real network, it is not possible to achieve a regular cell pattern. This is because radio propagation at the frequencies used by cellular radio systems is affected by the terrain and by buildings, trees and other features of the landscape.

Figure 47.5Frequency re-use - D/R ratio