Optimum Mean-Square Decision Feedback Equalization

A great deal of research, particularly in the past decade, has been expended on the problem of linear equalization. This has yielded a considerable body of theory and technology making possible the design of apparatus for successfully combating intersymbol interference in PAM data transmission systems operating over noisy linear channels where delay distortion predominates. Since linear equalizers must compensate for the channel characteristics in the presence of noise, they cannot be expected to perform well over severely frequency-attenuating channels or channels possessing nulls in the amplitude characteristic. Interest in the high data rates over voiceband and cable channels inevitably leads to the search for more effective equalization methods. Faster pulse rates place signal energy well within the badly attenuated portion of the transmission spectrum, resulting in severe intersymbol 1341 1342 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1 9 7 3 interference correctable by linear methods only at the expense of a significant enhancement of the noise. A "bootstrap" technique, commonly referred to as "decision feedback," when combined with linear equalization can yield significant performance improvement. 1,2 In this method the samples of the pulse tails (postcursors) interfering with subsequent or future data symbols are subtracted without incurring a significant noise penalty. The effect of pulse tails (precursors) which occur prior to detection and interfere with past symbols is minimized by a conventional linear equalizer.