Digital Communications with Emphasis on Data Modems

17COMMUNICATIONS THROUGH BANDLIMITED TIME‐INVARIANT LINEAR CHANNELS

17.1 INTRODUCTION

In this chapter, the analysis and simulation of bandlimited time‐invariant linear communication channels is discussed. The channel is viewed in broad terms and may include the transmitter and receiver filtering as well as the transmission medium. The linear time‐invariant channel is frequently encountered in practice and is relatively easy to analyze and simulate. Since these channels are often related to classical filter theory, the analysis and application of linear filters draw on the abundance of time‐tested work related to filter theory and design.

In communication systems, the channel distortion manifests itself in two distinct ways: the transmitted waveform is distorted in such a way that the received symbol energy within the symbol interval is reduced leading to a loss in signal‐to‐noise ratio (SNR); the channel distortion results in dispersion of the symbol energy into adjacent symbols resulting in noise‐like terms referred to as intersymbol interference (ISI). The channel distortion results from the amplitude and phase responses of the channel that becomes increasingly severe as the waveform bandwidth approaches the channel bandwidth. The ISI results in the closing of the demodulator eye‐opening at the sampled matched filter output that reduces the probability of a correct decision. It also impacts the symbol timing and carrier phase tracking functions of the demodulator that often ...