Passband echo cancellation algorithms for high-speed modems

Passband algorithms for the cancellation of data echoes in high-speed modems are described. The conventional, all-real single-filter canceller is discussed, along with the hybrid passband/baseband Weinstein canceller. In addition, a novel passband echo canceller which exceeds the performance of known fully-passband echo cancellers is introduced. This canceller uses the concept of adding out-of-band noise to the adaptive filter input signal, thereby increasing the degrees of freedom available to the echo canceller during convergence and increasing the level of echo cancellation achieved. The basics of high-speed modem connections are explored and the need for a high degree of echo cancellation on such connections is indicated. The performance of each type of passband echo canceller is examined, and comparisons are made. Performance results for a 9600 b/s data modem utilizing the noise-enhanced echo canceller indicate the advantages of this technique