Multiple-Path Fading on Line-of-Sight Microwave Radio Systems as a Function of Path Length and Frequency

Terrestrial and satellite radio systems for common carrier applications at frequencies above 10 GHz have been proposed recently. 1 - 2 For terrestrial systems the lengths of transmission paths may be limited to a few kilometers by the attenuation caused by rain. 3-5 A similar limitation holds for satellite systems--for that part of the transmission path which is contained in earth's atmosphere. For both terrestrial and satellite systems, it is anticipated that economies can be achieved if the information can be transmitted as sequences of short pulses. But 0. E. DeLange has shown that pulses with durations of a few nanoseconds can suffer serious degradation 2375 2376 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1971 during transmission over a 22.8-mile path at a frequency of 4 GHz; in his experiment, the degradation occurred during conditions of frequency-selective fading. 0,7 For the proposed systems, this result raises an important question--for the path lengths of interest, can pulses of a few nanoseconds duration be transmitted without suffering intolerable degradation due to frequency-selective fading? No answer to this question has been found in the literature; one of the purposes of this paper is to answer it. In pioneering work on the propagation of microwaves, A. B. Crawford, DeLange, W. C. Jakes, and W. M. Sharpless have shown that severe frequency-selective fading on line-of-sight transmission paths can be explained in terms of multiple-path transmission. 0-9 Their results have been confirmed by others and it is generally accepted that the mechanism of multiple-path transmission is responsible for most severe frequency-selective fading.