Cells & Clusters

Cells

A cell can be defined as an area of radio coverage from one BTS antenna system. It is the smallest building block in a mobile network and a cell can be represented by a hexagon.

There are two types of cells;

1. Omni directional cell:

An omni-directional cell (or omnicell) is served by a BTS with an antenna which transmits equally in all directions (360 degrees).

2. Sector cell:

A sector cell is the area of coverage from an antenna, which transmits, in a given direction only. For example, this may be equal to 120 degrees or 180 degrees of an equivalent omni- directional cell. One BTS can serve one of these sector cells with a collection of BTS’s at a site serving more than one, leading to terms such as two-sectored sites and more commonly, three-sectored sites.

Clusters

Groups of frequencies can be placed together into patterns of cells called clusters. A cluster is a group of cells in which all available frequencies have been used once and only once.

Since the same frequencies can be used in neighboring clusters, interference may become a problem. Therefore, the frequency re-use distance must be kept as large as possible. However, to maximize capacity the frequency re-use distance should be kept as low as possible.

The re-use patterns recommended for GSM are the 4/12 and the 3/9 pattern. 4/12 means that there are four three-sector sites supporting twelve cells using twelve frequency groups.

In the 3/9 cell pattern there are always 9 channels separating each frequency in a cell. However, when compared with the 4/12 pattern, cells A1 and C3 are neighbors and use adjacent frequencies. Therefore, the C/A interference will increase. In this case, an operator may use frequency hopping which, if planned correctly, could reduce the possibility of such adjacent channel interference