Detection processes for a 9600 bit/s modem

The paper describes various detection processes for a modem that has been designed for operation at 9600 bit/s over the public switched telephone network. The modem is a synchronous serial system, using a 16-point q.a.m. signal, with near-maximum-likelihood detection at the receiver. The detection processes are modifications of a technique that has recently been developed for use with severely distorted digital signals, the modifications leading to much simpler implementation. Results of computer simulation tests are presented, comparing the tolerances of the various detectors to additive white Gaussian noise, when the data are transmitted over different telephone circuits. The effects, on the tolerance to noise of a detector, of inaccuracies in the estimates of the level and carrier phase of the received signal are also studied.