Signal processing for high-speed data modems

Abstract

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. The results show that combined equalization and TCM decading achieve better performance in the presence of severe distortion than separate conventional equalization and decoding. The author concludes with a brief discussion on the realization of modems with programmable digital signal processors.