Combining Echo Cancellation and Decision Feedback Equalization

In a previous publication, a new approach to adaptive echo cancelling for full-duplex data transmission over two-wire facilities was presented.1 Its novelty was that the compensation signal is synthesized directly from the data symbols, rather than from the transmitter output signal, and canceller adjustments are controlled by the receiver's estimated error signal, rather than the receiver's input signal, as has been done in previous echo cancellers.2"' This approach can be applied as long as the underlying modulation concept is linear; it allows for considerable economies in circuit implementation and also eliminates the double talker problem. The number of taps can be kept 491 minimal if echo compensation is done at the baud rate, in synchronism with the receiver sampling operation. For this case, it has been shown in Ref. 1 that rapid convergence (time proportional to the number of echo taps) can be achieved, and that this convergence does not depend on channel response, echo response, timing phase, carrier phase, or the energy ratio of the echo signal to the distant received signal. Further studies dealing with this scheme are presented in Refs. 6 and 7. On many real channels, the echo canceller alone would solve only part of the problem, since intersymbol interference (ISI) is severe and must be properly dealt with. The well-known adaptive equalizer is the proper cure for this, and during the past decade its art has been refined to a level of high sophistication. However, for two-wire full-duplex communication, one now must in general deal both with an adaptable echo canceller and an equalizer.