The Number of Fades in Space-Diversity Reception

Line-of-sight microwave transmission is affected by a phenomenon called multipath propagation. When this phenomenon is present, the output from a receiving antenna can be practically zero for seconds at a time. However, such deep fades rarely occur simultaneously in the outputs of two vertically separated receiving antennas, and this is the basis for space-diversity reception on line-of-sight microwave links. The number of fades and the average durations of fades are fundamental properties of the line-of-sight microwave channel. In high performance systems, they may determine the limit of attainable performance. Previous discussions of space diversity do not cover the number of fades and their average durations. 1-3 Experimental 1513 1514 T H E HELL S Y S T E M TECHNICAL JOURNAL, SEPTEMBER 1970 data on these are difficult to obtain because long periods of time are needed to observe a number of fades sufficient for theoretical analysis. We present a set of experimental results obtained from measurements made in 1966 in Ohio. We also present, and this is a major part of the paper, theoretical results on the number of fades and their average durations. The diversity scheme for which our results are obtained is one where the diversity signal is, at all instants of time, the stronger of two received signals. This scheme is of interest because of its inherent simplicity, and because experimental values for the diversity signal can be obtained easily during computer processing of experimental observations of the received signals.