A Compromise Equalizer Design Incorporating Performance Invariance

A fixed compromise equalizer is frequently employed in data transmission systems to compensate for linear distortion introduced by a channel drawn from a random ensemble. 1 Typically, compromise equalizers find application in systems which, because of economic or other considerations, do not use an adaptive equalizer. It is possible that, even when adaptive equalization is used to compensate for a particular channel characteristic, one might use a compromise equalizer to provide a good initial channel and thereby reduce the receiver 1077 1078 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , S E P T E M B E R 1 9 7 3 adaptation time. As its name suggests, the equalizer is a fixed linear filter that effects a compromise by compensating for an "average" channel. We propose a procedure that uses the statistics of the channel ensemble, the modulated signal, and the additive noise at the demodulator to design a filter that minimizes a performance measure appropriate for most transmission systems. The performance measure is an adaptively scaled mean-square error. For example, it is particularly well suited for use in a data transmission system and results in a filter that is significantly different from that obtained by directly minimizing the mean-square error. 2 In order to avoid being restricted by nonlinear demodulation techniques (e.g., those used in FSK or DPSK data systems) and to be able to accommodate asynchronous signalling, the equalizer operates directly on the received passband signal and is thus a channel, rather than a synchronous, equalizer.