Adaptive Equalization of Highly Dispersive Channels for Data Transmission

A common approach to data transmission is to code the amplitudes of successive pulses in a periodic pulse train with a discrete set of possible amplitude levels. The coded pulse train is then linearly modulated, transmitted through the channel, demodulated, equalized, and synchronously sampled and quantized. As a result of dispersion of the pulse shape by the channel, the number of detectable amplitude 55 56 T H E BELL SYSTEM T E C H N I C A L J O U R N A L , JANUARY 1969 levels has very often been limited by intersymbol interference rather than by additive noise. In principle, if the channel is known precisely it is virtually always possible to design an equalizer t h a t will make the intersymbol interference (at the sampling instants) arbitrarily small. However, in practice a channel is random in the sense of being one of an ensemble of possible channels. Consequently, a fixed equalizer designed on average channel characteristics may not adequately reduce intersymbol interference. An adaptive equalizer is then needed which can be "trained," with the guidance of a suitable training signal transmitted through the channel, to adjust its parameters to optimal values. If the channel is also time-varying, an adaptive equalizer operating in a tracking mode is needed which can update its parameter values by tracking the changing channel characteristics during the course of normal data transmission. In both cases the adaptation may be achieved by observing or estimating the error between actual and desired equalizer responses and using this error to estimate the direction in which the parameters should be changed to approach the optimal values.