31.2 CELLULAR SYSTEM DESIGN ISSUES

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While designing a cellular communication system, there are many issues to be addressed because designing these systems is a complicated task. Some important issues are discussed in the following:

Radio engineering: Based on the terrain of the service area (keeping in view hills, lakes, high-rise buildings, and so on) and the likely subscriber density in different areas, the service area has to be divided into different cells. The hexagonal cell is only a theoretical representation, in practice, there may be overlaps of the cells, and some small regions may not be covered by the radio. A radio survey is to be carried out to find out the best location for base station installations and to ensure maximum possible coverage.

Frequency allocation: For each base station located in a cell, a number of channels (pairs of frequencies) have to be assigned. The number of channels is based on the subscriber capacity in that locality and the maximum number of simultaneous calls that should be supported. The allocation of the frequencies for different cells should keep in mind the frequency reuse distance. A mobile operator has to obtain the frequency allocation from a national authority.

PSTN connectivity: The mobile network is connected to the PSTN to enable normal telephone subscribers to be contacted by mobile subscribers and vice versa. The trunk capacity between the mobile switching system and the PSTN switch has to be decided based on the traffic considerations.

Cell splitting: As the subscriber density increases in a cell, that cell has to be split, an additional base station has to be installed, and new frequencies have to be allocated. This has to be done keeping in view the frequency reuse patterns.

Shadow regions: Regions that do not receive the radio signals are called shadow regions. Because of the terrain, some areas may not have radio coverage, as a result of which mobile calls will be dropped when the subscriber is in the shadow region. The cellular operator has to ensure that there are no shadow regions or at least that their area is minimized. Generally, in hilly areas there will be many shadow regions. The basements of multistoried apartments are also shadow regions. To provide coverage for such areas, repeaters are used.

Note

GSM phones do not receive strong signals in indoor environments and in basements. In such places, GSM repeaters are installed that will enhance the signal strength.

Traffic analysis: Once the mobile communication system is operational, the operator has to monitor the traffic continuously to check whether there is any network congestion. If there is congestion, users will get network busy signals on their mobile phones. In case of network congestion, capacities have to be enhanced.

In designing multicell systems, the important considerations are a radio survey of the service area, frequency allocation for different cells, ensuring that shadow regions are minimized, and a traffic analysis to identify the number of cells.

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