Chapter 1 - Fundamentals of Time-Varying Communication Channels

Abstract

This chapter discusses the fundamentals of wireless channels from a signal processing and communications perspective. In many of wireless systems, the transmitter or the receiver is mobile. These may move with significant velocities. These situations give rise to time variations of the wireless channel due to the Doppler effect. Because of their practical relevance, linear time-varying (LTV) channels, also referred to as time-frequency (TF) dispersive or doubly dispersive, as well as TF selective or doubly selective, have attracted considerable interest in the fields of signal processing, communications, propagation, information theory, and mathematics. The chapter provides a survey of many of the concepts and tools that have been developed during the past six decades for characterizing, modeling, and measuring time-frequency dispersive channels. Some of the important physical aspects of LTV channels, and basic tools for a deterministic description of LTV channels are discussed with the statistical description of random LTV channels. Several techniques for measurement of LTV channels and their statistics are also discussed. Based on considerations of the physical propagation mechanisms, various deterministic and stochastic channel descriptions, including both wide-sense stationary uncorrelated scattering (WSSUS) and non-WSSUS channels are described.