Echo cancellation of FXO line card expansion

In this paper, we show that the whole echo path in an FXO varies sharply and suddenly because multiple switches involved in a call could not be active at the same time. An echo canceller adapting the echo path, therefore, will have poor performance. We then present an echo cancellation system consisting of two echo cancellers: one is a fixed echo canceller canceling the echo from the switch inside FXO and another is an adaptive echo canceller canceling the echo from the switches inside analog line network. The fixed echo canceller coefficients are obtained during system booting or rebooting via applying white noise to an LMS algorithm.