Signal processing for high-speed data modems

Modems for leased telephone lines have in the last few years reached data rates which come close to the theoretical limit given by Shannon´s capacity theorem. In this paper an overview of the key signal-processing algorithms used in leased-line modems is presented. The transmission of several bits per symbol with uncoded modulation is reviewed, and the use of trellis-coded modulation (TCM) with two-dimensional signal constellations is discussed. It is shown that the coding gains offered by TCM are vital to reliable operation at 14.4 kb/s and above. Algorithms for compensating intersymbol interference and carrier-phase errors are then presented, and the limitations of conventional equalizers addressed. The adaptation of the receiver to the unknown characteristics of a telephone line is discussed, and rapid-training algorithms allowing very short startup times are given. Attention then focuses on recent work which combines the functions of equalization and symbol decoding. This leads to a modem receiver comprising a whitened-matched filter and a reduced-state maximum-likelihood sequence decoder. The performance of such a receiver in the presence of severe channel distortion is compared with that of a conventional receiver using separate equalization and TCM decoding