Patent ID: 6072782
Filing Date: 2000-06-06
Classification: H04B

Abstract:
An echo cancellation method for a DMT digital subscriber loop modem telecommunications system using N-sample length FFT and L-sample length cyclic prefix framing, wherein incoming data signal samples y.sub.n from a DSL channel are mixed with echo signal samples s.sub.n from an echo channel of transmitted data signal samples u.sub.n from a transmitter, and the mixed data stream of incoming data signal samples and echo signal samples is passed in a combined channel through a time domain equalizer to a fast Fourier transform time domain-to-frequency domain transformer, the method comprising:aligning frames of the echo signal samples with frames of the incoming data signal samples in the combined channel prior to passing the mixed data stream through the time domain equalizer;adapting the time domain equalizer to jointly shorten the combined impulse responses for both incoming data signal and echo signal intersymbol interference; andperforming echo cancellation in the frequency domain within the fast Fourier transform, asX.sub.k =W.sub.k .multidot.Z.sub.k -E.sub.k .multidot.U.sub.k,where W.sub.k is a tap of a frequency domain equalizer, Z.sub.k is the received mixed echo and incoming data signal samples transformed into frequency domain, E.sub.k is a tap of a frequency domain echo canceler, and U.sub.k is the transmitted signal;the frames being aligned by estimating an echo delay and offsetting transmitter timing to compensate for the delay, with alignment being done using a training sequence implemented in time domain using a block of random data ##EQU15## -L.ltoreq.n<N, in a sequence arranged so that the random block repeats in time with the sign of the data block alternating every two blocks; and, for a combined channel impulse response e.sub.k, the received signal corresponding to the transmitted signal is, ##EQU16## where p.sub.n is a pilot tone superimposed on the training sequence.