Two correction schemes for the minimization of the severe non-linear distortion introduced by an ADPCM link

Abstract

The paper is concerned with serial data transmission at a rate of 9600 bit/s over a telephone channel containing a 32 kbit/s Adaptive Differential Pulse Code Modulation (ADPCM) link. The transmitted data signal is a 2400 baud 16-level (QAM) signal. The ADPCM link introduces time varying and at times severe nonlinear distortion into the data signal. The two correction schemes described, known here as system A and system B, operate by attempting to correct the nonlinear distortion. The correction may be applied either at the transmitter or at the receiver, and in either case it may operate on the baseband or on the bandpass signal. The most promising scheme studied operates on the received bandpass signal at the receiver. Results of computer simulation tests are presented. Some tests use a decision feedback equalizer and others use a near-maximum likelihood detector at the receiver. The results show that the most significant improvement can be gained by using system A with the near-maximum likelihood detector.