Time- and Frequency-Domain Representation of Multipath Fading on Line-of- Sight Microwave Paths

The use of digital radio in the common carrier, line-of-sight, microwave radio bands has rekindled an interest in propagation modeling.1"1 Recently, a statistical model was developed4 ' for a 26.4-mile path in 763 the 6-GHz band (30-MHz bandwidth) at Palmetto, Georgia, using a multipath channel model. It was found, in the modeling study, 1 that for appreciable periods of time the channel was well characterized with modeled delay spreads exceeding 25 ns. Such a spread is considerably larger than the 8.5 ns that would be predicted on the basis of (deterministic) ray modeling,'' or expected on the basis of previous propagation studies.7"9 One of the objectives of the present study is to verify that these results are not inconsistent for multipath (three or more paths) channels. The analysis provides the basis for determining the extent to which one can predict the transmission characteristics in a narrowband (20to 40-MHz bandwidth) channel from knowledge of the physical delays present in the radio path and, conversely, the extent to which one can determine the physical delays present in this path from observation of the channel. These developments clarify the limitations of existing statistical models for channel characterization in the frequency domain and indicate directions for developing extended models. While a number of authors have placed bounds on the attenuation and group delay produced by finite-delay-spread channels, 3 ' 1011 the present work is the first attempt to describe the overall structure of the frequency response of a multipath channel.