tem capacity increases. For at the base station's receiver. Most cellular standards therefore example, if there are a total of 140 channels available, a 4 cell repeat incorporate mobile power control, the base station commanding the pattern would provide 35 channels per cell, whilst a 7 cell would mobile to transmit at one of a number of power levels. As the mobile provide 20 channels per cell. moves closer to or further from the base station, further commands On this basis, the smallest possible value of N seems desirable. are issued to keep the received signal level to prescribed limits. By However, as N decreases, so the distance between cells using the reducing the average mobile power level, co-channel interference is same channels reduces, which in turn increases the level of co-chan- reduced, improving overall system quality. nel interference. The repeat distance D and the cell radius R are both related by the geometry of the cell pattern. These are shown in Figure 47.5 and Radio planning Equation 47.1. As previously described, cellular radio re-uses the same radio chan- Re-use ratio D/R = V3 N (47.1) nels in different cells and because of this re-use, two mobiles using the same channel in different cells may interfere with each other, a In practice, in a real network, it is not possible to achieve a regular phenomenon known as co-channel interference. The key objective cell pattern. This is because radio propagation at the frequencies Figure 47.4 Cell repeat patterns: (a) four cell repeat; (b) seven cell repeat; (c) twelve cell repeat 47/6 Cellular radio systems Signal Strength Fast Fading Slow Fading Distance from base station Figure 47.6 Fading effects fading is mainly caused by terrain features and occurs over large distances of hundreds of metres. In addition, the path loss is also dependent upon the type of terrain, for example, urban with dense buildings, or rural with trees, or even Figure 47.5 Frequency re-use - D/R ratio over water. The height of the mobile and base station above ground level als