Automatic Equalization for Digital Communication

Present data rates 011 voice telephone channels are limited to about 2400 bits per second. Although the noise margin on these facilities is sufficient tp permit much higher rates, the nonuniform transmission characteristics of the channel cause what might be termed a distortion barrier, prohibiting faster transmission. The distortion of data pulses by the channel results in these pulses being smeared out in time so as to overlap other transmitted pulses. This intersymbol interference is one of the chief degrading factors in present systems and becomes the determining factor in the design of higher-rate systems. To alleviate the effects of intersymbol interference it is necessary to equalize the channel. In the past equalization has generally been accomplished by flattening the amplitude characteristic and linearizing the phase characteristic using fixed amplitude-frequency and phase-frequency networks. Although this type of equalization is adequate for speech transmission requirements, it does not provide the precise control over the channel time response which is necessary for high-speed data transmission. Thus to realize the full transmission capability of the channel there is a need for automatic, time-domain equalization. Among the basic philosophies for automatic equalization of data systems arc pre-equalization at the transmitter and post-channel equalization at the receiver. Since the former technique requires a feedback channel, we will concentrate our efforts here on equalization at the receiver.