Digital Communications with Emphasis on Data Modems

18COMMUNICATIONS IN FADING ENVIRONMENTS

18.1 INTRODUCTION

The fading channel is characterized as a multiplicative disturbance in which the signal amplitude fluctuates with time depending upon the temporal characteristics of the channel. The term “channel” is used somewhat broadly in the sense that the amplitude fluctuations often result from many signal paths that appear as a signal with random phase variations at the receiver terminal. These multipath signals combine in such a way that the phases constructively and destructively combine resulting in random variations of the received signal amplitude. The amplitude of the received signal is often dominated by a direct or line of sight (LOS) path and the received signal amplitude fluctuations are related to the direct‐path signal amplitude. The signals arriving at the receiver over direct and indirect paths are, respectively, referred to as the specular and random components of the received signal. The specular component corresponds to the received signal over the path without fading.

An important aspect of the fading environment is the temporal characteristic of the amplitude fluctuations. For example, if all of the signal paths through the channel were fixed or unchanging, the received signal amplitude would be a constant random variable independent of time. However, real channels are seldom stationary but fluctuate with time due to natural disturbances, such as atmospheric and ionospheric turbulence, Doppler velocities, ...