Fading Loss in Diversity Systems

In a communication system using optimum diversity combining, the mean signal-to-noise ratio of the combined signal is the sum of the mean signal-to-noise ratios of the separate signals. However, the effective signal-to-noise ratio of the combined signal is less than this. The ratio of the mean to effective signal-to-noise ratios of the combined signal is called the fading loss. The effective signal-to-noise ratio, and hence the fading loss, depends upon the measure of effectiveness used, which in turn depends upon the form of information conveyed by the signal. In a high-quality speech system, the fading loss is shown to depend principally upon the order of diversity (the number of signals combined). In a binary communication system, the fading loss is shown to depend principally upon the mean signal-to-noise ratio of the separate signals. The system design considerations in these two cases are therefore different.