Fading Phenomena in the Wireless Environment

Abstract

The personal radio communication industry has spawned much research into propagation phenomena in the 1 to 3 GHz frequency spectrum, and has provided technological advances that create opportunities to capitalize on the use of these higher frequencies for military tactical communications and civilian wireless uses. However, the interests of the personal communication industry has focused upon propagation path configurations that can be considered “high-low,” that is, paths in which one end is close to the ground (an individual user) and the other end is at a higher elevation and utilizes an antenna support structure such as a tower or a building, aircraft or orbiting satellite. Tactical military communication and other emerging wireless applications, on the other hand, usually employ “low-low” path configurations—paths between individual users where the antenna heights at both ends of each link will be 1.5 meters above the ground or less. The difference in geometry of a “high-low” and a “low-low” path is rather obvious, but what is not so obvious is the difference in propagation phenomena, especially fading characteristics, of the two types of paths. A “high-low” path in an urban area is usually characterized by Rayleigh propagation in which no direct line-of-sight propagation path exists and all of the energy from radio transmitter to receiver is by forward scatter and reflections. A “low-low” path between two users is usually via a direct line-of-sight propagation path, but with insufficient terrain clearance to support Gaussian propagation; therefore, the path is characterized as Rician. This paper reviews the Rayleigh, Gaussian and Rician propagation phenomena; describes the different factors that are associated with each of these three types of propagation; and discusses research and experimental work to enable more accurate prediction of Rician propagation loss, expected fading and bit-error rates under different environmental (terrain, foliage, weather, etc.) conditions.