ells are used in each cluster, then the channels have to be reused more frequently, i.e. fewer cells in a cluster mean that the distances between the two base stations that use the same channels are smaller and interference may be a problem. However, it has the advantage that greater numbers of channels are available in each cell, and hence there is the capacity to handle a greater number of calls. However, if the number of cells in a cluster is increased to reduce the level of interference, the larger number of cells has to share the same number of channels and each base station has fewer channels and the capacity is reduced. Cellular telecommunications 255 Figure 10.1 An idealized cell system The different types of cells are given different names. Macrocells are large cells that are usually used for remote or sparsely populated areas. These may be 10 km or possibly more in diameter. Microcells are those that are normally found in densely populated areas which may have a diameter of around 1 km. Picocells may also be used for covering very small areas such as particular areas of buildings, or possibly tunnels where coverage from a larger cell is not possible. Obviously for the small cells, the power levels used by the base stations are much lower and the antennas are not positioned to cover wide areas. In this way the coverage is minimized and the interference to adjacent cells is reduced. Other types of cell may be used for some specialist applications. Sometimes cells termed selective cells may be used where full 360 degree coverage is not required. They may be used to fill in a hole in the coverage, or to address a problem such as the entrance to a tunnel, etc. Another type of cell known as an umbrella cell is sometimes used in instances such as those where a heavily used road crosses an area where there are microcells. Under normal circumstances this would result in a large number of handovers as people driving along the road would quickly cross the microcells. An umbrella cell would take in the coverage of 