ABSTRACT
Most of the concepts presented in the preceding sections can be readily extended to echo cancellation for communications systems employing quadrature amplitude modulation; the case of full-duplex transmission over a voiceband data channel requires a specific discussion. Several techniques have been proposed to increase the speed of convergence of the least mean square algorithm. In particular, for echo cancellation in data transmission, the speed of adaptation is reduced by the presence of the signal from the remote transmitter in the cancellation error. A different model is obtained if echo cancellation is accomplished before demodulation. Digital signal processing techniques for echo cancellation provide large echo attenuation, and eliminate the need for additional line interfaces and digital-to-analog and analog-to-digital converters that are required by echo cancellation in the analog signal domain. In voiceband modems for data transmission over the telephone network, digital techniques for echo cancellation allow a precise tracking of the carrier phase and frequency shift of far-end echos.
