TABLE OF CONTENTS Present-day digital radio systems frequently employ either 8-phase PSK modulation or a duobinary technique sometimes termed quadrature partial response signal (QPRS). It merely represents two 3-level duobinary signals of the (1+ ?) type, phase modulated in quadrature. Here again ?= e ?j ?T , where 1/ T is speed in b/s. At present, at least four types of digital radios, at 2 GHz and 8 GHz, employ duobinary 3-level techniques [7.13, 7.14, 7.15]. There is one digital radio, at 2 GHz, that uses a 7-level modified duobinary method [7.16].
Why are correlative techniques employed in digital radios? Perhaps the most important reason is the simplicity of implementation which is comparable to 4-phase radios, yet less complex than the implementation of 8-phase digital radio [7.14]. At the same time, correlative-QPRS, for example, easily accommodates 2 bits per hertz in carrier modulation, or even 2.25 bits/Hz [7.15]. From a performance point of view, the 3-level QPRS requires 3 dB more signal power than the 4-phase. On the other hand, the 8-phase signal is 5.3 dB poorer than the 4-phase (see the performance curves in Chapter 3).
Another reason for using correlative, modified duobinary [7.16], for example, is that a service channel and other supervisory signals can be made completely independent of the main traffic bit stream. That is, should the data stream be lost, the service (order) channel will still exist; this will be described later. In...
