GSM AIR INTERFACE

 

The GSM air interface uses Time Division Multiple Access (TDMA) technology in order to separate calls over a period of time as well as across a number of frequencies.

A cell comprises one or more carrier frequencies, onto each of which can be modulated up to eight separate calls, normally referred to as Traffic Channels (TCH) over a period of time known as a frame.

Each frame comprises 8 time slots, each of which contains a burst of information related to its particular call. As the frame structure is repeated for the duration of the call, the individual bursts in each time slot can be demodulated and used to “reassemble” the call information.

  

Frequency Re-use
Although the hexagon grid is essential for initial planning it is theoretical only because the site acquisition process tends to drive the system configuration, not the other way round. The Network Operator will then analysis the frequency requirement and produces a plan with the total number of channels required. The total number of channels allocated is split into a number of channel groups. These channel groups are then assigned on a per cell basis in a regular pattern that repeats across the network. The most common re-use methods are:

·          12-Cell

·          7-Cell

·          3-Cell

The 12 and 7 cell methods are usually deployed in areas that use an omni configuration and do not require a high traffic density, e.g. rural.

The type of re-use pattern employed is a choice between quantity (capacity) and quality. As the number of channel groups increases the number of channels per cell decreases, thereby reducing capacity. On the other hand the distance between co-channel cells increases therefore reducing the risk of co-channel interference

 

Sectorisation

Where support for higher traffic capacity is required the existing cells can be divided into sectors using higher gain directional antennas. Sectorisation is a very cost effective means of increasing capacity and also a good method for reducing co-channel interference as the design facilitates the re-use of the same channels in close proximity to another cell site.