In telecommunication, trellis modulation (also known as trellis coded modulation, or simply TCM) is a modulation scheme which allows highly efficient transmission of information over band-limited channels such as telephone lines. Trellis modulation was invented by Gottfried Ungerboeck .

In the early 1980s, modems operating over plain old telephone service ("POTS") typically achieved 9.6 to 14.4 kbit/s. 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 was 19.2 kbit/s. Paradoxically, 19.2 kbit/s is only 60% of the theoretical bit rate predicted by Shannon's Theorem for POTS lines (approximately 30 kbit/s).

In 1982, Ungerboeck published a paper describing the principles of trellis modulation. A flurry of research activity ensued, and by 1984, the International Telecommunication Union published modem standards utilizing trellis modulation to achieve rates of 19.2 kbit/s and higher.

Once manufacturers introduced modems with trellis modulation, transmission rates increased dramatically to the point where interactive transfer of multimedia over POTS became feasible. Thus, it can be argued that Ungerboeck's invention played a key role in the telecommunications revolution and the explosive growth of the World Wide Web.

Relevant Papers

• G. Ungerboeck, "Channel coding with multilevel/phase signals," IEEE Trans. Inform. Theory, vol. IT-28, pp. 55-67, 1982.

• G. Ungerboeck, "Trellis-coded modulation with redundant signal sets part I: introduction," IEEE Communications Magazine, vol. 25-2, pp. 5-11, 1987.