Describing Random Sequences

Abstract

The signals we have already met in this book — including the impulses, steps and sinusoids described in chapter 1 — have precisely defined sample values and are generally referred to as deterministic. By contrast, the individual values of a random signal are not defined, nor can its future fluctuations be predicted with certainty. It is a central concept of modern communications that a signal can only convey useful information if it is, to some extent, unpredictable. For example, it would be pointless to transmit a periodic sinusoid through a communication channel, since once its frequency, amplitude and phase had been determined at the receiving end, no further information could be conveyed. But if we switch a sinusoidal tone on and off as in a Morse Code message, the person at the receiving end does not know what dot-dash sequence is to be sent next — and it is this very uncertainty which results in useful information flow.