Introduction

Up to now we have considered only the detection (or demodulation) of signals transmitted over channels of infinite bandwidth, or at least a large enough bandwidth that any signal distortion is negligible and can be ignored. Though in some situations this assumption is reasonable, satellite communications is a common example, bandlimitation is also common. The classical example is the telephone channel where the twisted-pair wires used as the transmission medium have a bandwidth on the order of kilohertz. But even a medium such as optical fiber exhibits a phenomenon called dispersion which results in an effect very analogous to bandlimitation.

It is important to realize that bandlimitation depends not only on the channel medium but also on the source, specifically the source rate, Rs (symbols/second). One common measure of the bandwidth needed or occupied by a source is W = 1/Ts = Rs (hertz). As source rates keep increasing to accommodate more data eventually any channel starts to look bandlimited. Bandlimitation can also be imposed on a communication system by regulatory requirements. A user is usually allotted only so much bandwidth in which to transmit her/his information.

The general effect of bandlimitation on a transmitted signal of finite time duration, Ts seconds, is to disperse it or to spread it out. Therefore the signal transmitted in a particular time slot (or symbol interval) will interfere with signals in other time slots resulting in what is called intersymbol interference (each signal represents a data symbol) or ISI.