In Chapter 7 we focused attention on signaling over a channel that is assumed to be distortionless except for the AWGN at the channel output. In other words, there was no limitation imposed on the channel bandwidth, with the energy per bit to noise spectral density ratio Eb/N0 being the only factor to affect the performance of the receiver. In reality, however, every physical channel is not only noisy, but also limited to some finite bandwidth. Hence the title of this chapter: signaling over band-limited channels.
The important point to note here is that if, for example, a rectangular pulse, representing one bit of information, is applied to the channel input, the shape of the pulse will be distorted at the channel output. Typically, the distorted pulse may consist of a main lobe representing the original bit of information surrounded by a long sequence of sidelobes on each side of the main lobe. The sidelobes represent a new source of channel distortion, referred to as intersymbol interference, so called because of its degrading influence on the adjacent bits of information.
There is a fundamental difference between intersymbol interference and channel noise that could be summarized as follows: