Digital Communications 2

2Baseband Transmissions

2.1. Introduction

Two types of digital communications exist: baseband transmissions and transmissions on a sine waveform, also called transmissions on a carrier frequency. In baseband transmissions, the symbols to be transmitted in the channel do not have their spectrum relocated around a carrier frequency. For transmissions on sine waveforms, the symbols to be transmitted are carried by a frequency f_c > 0. In order to separate both transmission types, the baseband modulator is called a line coder. Although both types of communications share their main principles, we will first study baseband transmissions in this chapter, and then study transmissions on a carrier frequency in Chapter 3.

This chapter first presents the most common line codes, explaining for each of them which signals are chosen to represent a bit or a group of bits, and the transition constraints between these signals, if relevant. Section 2.5.2 then details the transmission channel, and especially the additive white Gaussian noise channel, which constitutes the reference case. Section 2.4 deals with the processes at the receiver to obtain an optimum reception on the additive white Gaussian noise channel. Demodulation can be performed with a correlator of the matched filter, which is shown to be equivalent. Then the optimum detector, based on maximum a priori likelihood criterion, is detailed. Its performances for a reference line code are evaluated. Finally, the last section concerns ...