Introduction

The main objective of a communication system is the reliable transfer of information over a channel. Typically the information is represented by audio or video signals, though one may easily postulate other signals, e.g., chemical, temperature, and of course text, i.e., the written word which you are now reading. Regardless of how the message signals originate they are, by their nature, best modeled as a random signal (or process). This is due to the fact that any signal that conveys information must have some uncertainty in it. Otherwise its transmission would be of no interest to the receiver, indeed the message would be quite boring (known knowns so to speak). Further, when a message signal is transmitted through a channel it is inevitably distorted or corrupted due to channel imperfections. Again the corrupting influences such as the addition of the ever present thermal noise in electronic components, the multipath fading experienced in wireless communications, are unpredictable in nature and again best modeled as nondeterministic signals or random processes.

However, in communication systems one also utilizes signals that are deterministic, i.e., completely determined and therefore predictable or nonrandom. The simplest example is perhaps the carrier used by AM or FM analog modulation. Another common example is the use of test signals to probe a channel's characteristics. Channel imperfections can also be modeled as deterministic phenomena: these include linear and nonlinear distortion, intersymbol interference in bandlimited channels, etc.