Performance evaluation of M-branch selection diversity with continuous phase modulation systems in Rayleigh and log-normal fading

It is well known that mobile radio signals generally exhibit both fast (Rayleigh) and slow (log-normal) fading. This will severely affect the performance of digital transmissions by reducing the signal-to-noise ratio as well as the carrier-to-interference ratio. In this paper, closed form analytical solutions are determined for the performance of M-branch microscopic selection diversity when the signal is subjected to both Rayleigh and log-normal fading. It is shown that, although microscopic (antennas separated by small distances, so only the fast fading components become uncorrelated) diversity is a superb technique to mitigate the effects of fast fading, it is certainly, ineffective against the slow fading. The microscopic diversity improvement is related to the standard deviation of the log-normal (slow) fading. When the standard deviation increases the improvement decreases. Although the authors still believe that diversity is the most important technique to minimise the effect of fading, the conclusion drawn from this paper is that it is advantageous, whenever possible, to separate the receiver antennas by a distance large enough to ensure some degree of decorrelation between the slow fading components. One possibility is base station diversity