Satellite Communications Payload and System

Chapter 10

Principles of Digital Communications Theory

10.1 Introduction

This chapter explains the basic concepts of digital communications theory for the following purposes:

• To show why the payload communications parameters of Chapter 7 matter
• To serve as background for the rest of the book, which treats the payload set in its place in the end-to-end communications system.

The chapter contains more drawings than equations, so it can be as useful as possible to working engineers. For equations not all the validity prerequisites are stated, since in every normal case of satellite communications the assumptions do apply (but references are provided for the curious reader). Some items, such as the encoder and decoder, are only described in words. The chapter's first section on fundamentals may be enough of an introduction to communications theory for those who only want to know what “I” and “Q” signal components are, how spectrum and filtering are defined, and how bandwidth is defined. The reader is assumed to know what the Fourier transform is.

The topics covered in this chapter are the following:

• Section 10.2 Fundamentals of digital communications theory: signal representation, spectrum, filtering, and the end-to-end communications system
• Section 10.3 Modulating transmitter, including coding
• Section 10.4 Filtering
• Section 10.5 Demodulating receiver, including decoding
• Section 10.6 SNR, E_s/N₀, and E_b/N₀
• Section 10.7 Summary of signal distortion sources in end-to-end communications ...