21-07-2012, 02:48 PM
CHANNEL PLANNING
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However, such a solution has severe limitations. It is seldom that coverage can be maintained in the entire area desired. In addition, even though the channel utilization may be very high, limited capacity soon becomes a problem due to the limited number of carriers available to any operator.
A cellular system is based upon re-use of the same set of carriers, which is obtained by dividing the area needing coverage into many smaller areas (cells) which together form clusters (Figure 7-2).
A cluster is a group of cells in which all available carriers have been used once (and only once). Since the same carriers are used in cells in neighbouring clusters, interference may become a problem. The frequency re-use distance (i.e. the distance between two sites using the same carrier) must be kept as large as possible to help prevent interference. At the same time, the distance must be kept as small as possible from a capacity point of view. Cellular systems are often interference-limited rather than signal-strength-limited.
Re-using the carrier frequencies according to well-proven re-use patterns (Figure 7-3 and Figure 7-4), neither co-channel interference nor adjacent channel interference should become a problem. This is true if the cells have homogenous propagation properties for the radio waves, and if frequency hopping is implemented.
The re-use patterns recommended for GSM are the 4/12- and the 3/9-patterns. 4/12 means that there are four three-sector sites supporting twelve cells (Figure 7-3).
TRANSITION REGIONS
A uniform re-use pattern implies a constant traffic density over the network’s coverage area. In practice, however, traffic density varies considerably over the area (and during the day). This means it is common that cells of different sizes are used in different parts of a system, small cells in high-traffic areas (normally urban) and large cells in areas with lower traffic.
Figure 7-6 shows a case where cells have assorted sizes in a coverage area. This poses special problems in channel planning, as the re-use distance will vary for different cell sizes. To avoid having a smaller cell size that has half the re-use distance of the larger cell size interfering in the larger cell, other RF channels must be used in these cells. We need a buffer zone where the same RF channels are not used in the smaller and larger cells, respectively. This is sometimes a costly but unavoidable arrangement. Another possibility that parallels this type of varied coverage is in the use of overlaid and underlaid cells or a hierarchical cell structure. There, again, exists the need to have separate channel plans.