Trellis coded modulation

Trellis coded modulation (TCM) is a modulation scheme that transmits information with high efficiency over band-limited channels such as telephone lines. Gottfried Ungerboeck invented trellis modulation while working for IBM in the 1970s, and first described it in a conference paper in 1976. It went largely unnoticed, however, until he published a new, detailed exposition in 1982 that achieved sudden and widespread recognition.

In the late 1980s, modems operating over plain old telephone service (POTS) typically achieved 9.6 kbit/s by employing four bits per symbol QAM modulation at 2,400 baud (symbols/second). This bit rate ceiling existed despite the best efforts of many researchers, and some engineers predicted that without a major upgrade of the public phone infrastructure, the maximum achievable rate for a POTS modem might be 14 kbit/s for two-way communication (3,429 baud × 4 bits/symbol, using QAM).^{[citation needed]}

14 kbit/s is only 40% of the theoretical maximum bit rate predicted by Shannon's theorem for POTS lines (approximately 35 kbit/s).^[1] Ungerboeck's theories demonstrated that there was considerable untapped potential in the system, and by applying the concept to new modem standards, speed rapidly increased to 14.4, 28.8 and ultimately 33.6 kbit/s.

^ Forney, G. David; et al. (September 1984). "Efficient modulation for band-limited channels". IEEE Journal on Selected Areas in Communications. 2 (5): 632–647. doi:10.1109/jsac.1984.1146101. S2CID 13818684.

[1] Forney, G. David; et al. (September 1984). "Efficient modulation for band-limited channels". IEEE Journal on Selected Areas in Communications. 2 (5): 632–647. doi:10.1109/jsac.1984.1146101. S2CID 13818684.

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