Effects of channel narrowing in small zone FM mobile communication systems

In small zone (cellular) FM mobile communication systems, greater geographical reuse is achieved when the modulation index is larger. A large index, however, imposes a small number of channels. The spectrum utilization factor in total thus is determined as a synthesis of available channel number and geographical frequency reuse. From this viewpoint, relationship between the modulation index and spectrum utilization factor is discussed under the Rayleigh fading condition, with and without diversity. To take into account the actual traffic condition not only uniform but also a concentrated traffic distribution condition is considered. It is shown that the optimum peak deviation that realizes the most efficient spectrum utilization is from 3 to 4 kHz for uniform traffic distribution, and 2 kHz for concentrated traffic distribution. These results show that narrow channel spacing, such as 12.5 kHz, is better than the conventional 25 kHz or wider channel spacing to achieve efficient spectrum utilization.