QAM Receiver II — The Passband Adaptive Equalizer and Carrier Recovery System

An important milestone in high speed data transmission over narrow band channels like the voice band telephone channel was the invention and commercialization of the FIR adaptive equalizer by R.W. Lucky at AT&T Bell Laboratories in the early 1960's [II.D.28]. The purpose of the adaptive equalizer is to remove the intersymbol interference caused by the amplitude and phase distortions of the channel. Adaptive filters are used because the frequency response of the channel is not known accurately in many situations. Lucky's original equalizers used the zero forcing algorithm. Other people soon replaced this algorithm by Widrow's [II.D.41] more powerful least-mean-square (LMS) algorithm. Another major influence has been the remarkable advances in VLSI technology. This has led to ever more powerful DSP's which allow complex algorithms to be implemented very inexpensively. for example, modems that include data rates of 300 bps, 1200 bps, and 2400 up to 56000 bps, as well as error correction, data compression capabilities, and FAX modes can be bought for less the $100.