A Passband Data-Driven Echo Canceller for Full-Duplex Transmission on Two-Wire Circuits

A digitally-implemented echo canceller operating at a rate greater than twice the highest passband frequency is proposed for fullduplex data transmission on a two-wire circuit. Located at each end of the communication circuit, the cancellers operate independently of the local receivers and do not require synchronization of the two data stations. Economies in storage and A/D conversion, in comparison with voice-type cancellers described in the literature, are achieved by accepting data symbols as reference input instead of samples of the transmitted line signal. Convergence of transversal filter tap weights is demonstrated under a mean-squared error criterion, and use of the real passband error rather than the complex analytic error is found to lead to the same residual error at the expense of a doubled convergence time. An operational protocol and adaptation algorithm are proposed which make possible both rapid start-up and slower adaptation during double talking. Provision is made for limiting the number and location of active taps on the transversal filter to those actually necessary for replicating the echo channel, resulting in two transversal filter sections of moderate length separated by a bulk delay. Results from a computer simulation of the proposed canceller are offered to demonstrate that convergence of mean-squared tap-weight error follows the predicted exponential characteristic, that the length of the bulk delay can be determined from a single channel sounding under typical channel noise conditions, and that the use of an averaged-gradient algorithm will allow the canceller to adapt, although slowly, to a change in the echo channel during full-duplex operation.