Flexible pseudobinary and pseudoquaternary detectors for 9600 bit/s modem

The authors describe two new 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 QAM signal, with near-maximum-likelihood detection at the receiver. The detector is preceded by an adaptive filter that is adjusted to make the sampled impulse response of the channel and filter minimum phase. Results of computer simulation tests are presented, comparing the tolerance to additive white Gaussian noise of the new detectors with the most recent near-maximum-likelihood detector, when operating over models of two different telephone circuits. An estimate of the relative complexities of the different near-maximum-likelihood detectors suggests that the developed detection process achieves a much better compromise between performance and complexity than the others.<>