ts or time division multiple access (TDMA). A third multiple access scheme used code division multiple access (CDMA) where spread spectrum techniques are used. Here different orthogonal spreading codes are used to distinguish between the different users and allow them access to the system. Basic concepts As the name implies the basic concept around which cellular tele- communications operates is that the coverage area is split up into small areas called cells. Each one is covered by a relatively low power base station. This has advantages over the earliest concepts used by relatively high power mobile phones where a single high power station was used to provide coverage over a large area. The cell system enables local area coverage using low power handsets SO that at a certain distance away the frequencies can be reused. Interference is kept to a minimum by the fact that adjacent cells use different frequencies or channels, although a different technique is used for a CDMA system as we shall see later. By arranging that the mobile handset is able to change from one of the channels supported by one cell to one of the channels supported by the adjacent one, virtually seamless coverage can be achieved. While this creates additional complexity in the base station and handset, it enables much greater use to be made of the available channels. Different arrangements of cells may be used dependent upon the requirements. These may vary according to the terrain and the level of usage. The cells are arranged in groups that are termed clusters. The most typical is a seven cell cluster like that shown in Figure 10.1. This is very much an idealized view because coverage never stops exactly at the edge of the cell and the shapes vary according to the terrain. Accordingly other formats including four, 12 and even 21 cell clusters may be found. As there are only a limited number of channels available, the balance between the number of channels used by each base station and the number of cells in a cluster has to be made. If few c