A Study of Frequency Selective Fading for a Microwave Line-of-Sight Narrowband Radio Channel

Unusual atmospheric conditions may support microwave propagation over two or more distinct paths between two line-of-sight radio antennas. The various signal paths will typically differ in their propagation delay, thereby permitting constructive and destructive interference at the receiving antenna. When the relative delay is significant with respect to the radio frequency signal period, the interference can be quite selective, with deep nulls in parts of the radio band, and smaller variations at adjacent frequencies. The variation in received power is called fading, and the variation in the amount of fading with radio frequency is known as frequency selective fading. 731 732 T H E BELL SYSTEM T E C H N I C A L J O U R N A L , MARCH 1972 Experimental data on selective fading are difficult to obtain because of the long time periods (millions of seconds) of continuous measurement required to obtain a sufficient sampling of the fading events for a meaningful characterization. As a result, the experimental literature on the subject generally has been sparse, has been incomplete, or has tended to de-emphasize the magnitude of the propagational effects. W. T. Barnett 1 has explored these frequency effects for discrete microwave signals separated by 20 to 500 MHz, and in an important early study R. L. Kaylor 2 observed maximum amplitude deviations as large as several tens of decibels for the same bandwidths. Because of the ever-increasing emphasis on performance and the need for more efficient use of the microwave frequency spectrum, as well as for other reasons, an extensive experimental program was undertaken in 1970 to more precisely characterize the spectral and temporal effects of frequency selective fading within a narrowband microwave radio channel during a fading season.