Digital Communications 2

3Digital Modulations on Sine Waveforms

3.1. Introduction

In Chapter 2, baseband digital modulations were studied. In this case, the line codes can transmit bits or symbols representing groups of bits, as for M-ary non return to zero (NRZ) transmissions with M = 2ⁿ and n > 1. These transmissions are well adapted to wired cable media where the bandwidth constraint is not strong. In other systems, and in particular in wireless mobile communication systems, large bandwidth cannot be used for several reasons. First, the radio spectrum available for the systems is limited. Moreover, it is important to transmit in spectrum bands where propagation has the required characteristics. Finally, interferences between competing systems using the same bandwidth generate major problems, since propagation may lead to interferences between far-away transmitter-receiver pairs, contrary to wired transmissions that are, in principle, isolated from external interferences.

Consequently, digital transmissions on sine waveforms with high spectral efficiencies must be defined in limited bandwidth, around potentially high carrier frequencies. For instance, in civilian cellular wireless communication systems, the carrier frequencies currently used lie between 800 MHz and 2.6 GHz and their bandwidth lies between 200 kHz and 5 MHz. They are called narrowband and bandpass signals.

These signals can be transposed to equivalent baseband signals. In section 3.2, we will demonstrate that this property always ...