Digital Communications with Emphasis on Data Modems

2DIGITAL SIGNAL PROCESSING AND MODEM DESIGN CONSIDERATIONS

2.1 INTRODUCTION

This chapter describes the conversion of the analog signal at the input of the demodulator to a digitally sampled version of the signal required for the application of digital signal processing techniques in the recovery of the desired information. This signal conversion involves the discrete‐time and amplitude sampling of the analog signal and represents the window into the world of digital signal processing with unprecedented capabilities in terms of algorithms for reliable and efficient waveform processing and information detection. The information is typically in the form of binary data formatted to convey text messages using, for example, the American Standard Code for Information Interchange (ASCII)¹ that uses 7‐ or 8‐bit characters to describe 128 or 256 characters. However, advances in computer technology using 32 and 64 bits provide virtually unlimited character sets in the foreseeable future.

A general description of discrete amplitude and time sampling is given in Sections 2.2 and 2.3, with a case study of discrete amplitude sampling in Section 2.2.1. Specialized applications of discrete‐time sampling involving baseband and bandpass sampling are described, respectively, in Sections 2.5 and 2.6. Subcarrier sampling is described in Section 2.6.1 and Hilbert transform sampling is described in Section 2.6.2. In Section 2.7 nonideal modulator and demodulator implementations are discussed and ...