Number and Duration of Fades at 6 and 4 GHz
01 March 1971
Microwave transmission on line-of-sight radio-relay links is affected by the lower atmosphere. When atmospheric conditions permit multipath propagation, the output from a receiving antenna can be practically zero for seconds at a time. Such deep fades are rare events. Still, they are sufficiently numerous to cause problems in highperformance communication systems. There is a certainty to fading that other causes of outages and performance degradation do not possess. For example, catastrophic equipment failure may or may not occur during the projected life of the equipment on a communications link. On the other hand, come summer and fall, one can state with some assurance that fading will occur on certain links in a particular microwave radio-relay system. The number of fades and the durations of the fades have a direct bearing on system performance. Previous investigations of frequency 815 816 T H E BELL SYSTEM TECHNICAL JOURNAL, MARCH 1971 diversity at Bell Laboratories 1 - 2 and reports on extensive British 3 and Japanese 4 experiments dwell mostly on fade-depth distributions. The available information on fade duration distributions is rudimentary. 5 - 7 As a part of a current and continuing effort, both experimental and theoretical, to describe the fundamental properties of line-of-sight microwave channels, 2,8 ' 9 we will present results on the number of fades, the average durations of fades, and the probability distributions of fade durations. Both fading of single signals and simultaneous fading of pairs of signals within a frequency band will be treated.