Trellis coded modulation

Trellis coded modulation (TCM) is a <a href="/facts/Modulation/aAXPjINv">modulation</a> scheme that transmits information with high efficiency over band-limited channels such as <a href="/facts/Telephone_line/YdKUAka9">telephone lines</a>. <a href="/facts/Gottfried_Ungerboeck/YRUVamyJ">Gottfried Ungerboeck</a> 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, <a href="/facts/Modem/9H0Xwtq0">modems</a> operating over <a href="/facts/Plain_old_telephone_service/7uEc2wFY">plain old telephone service</a> (POTS) typically achieved 9.6 <a href="/facts/Kbit%2fs/Nv9VoT7T">kbit/s</a> by employing four bits per symbol <a href="/facts/Quadrature_amplitude_modulation/02j52SWG">QAM</a> 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).
14 kbit/s is only 40% of the theoretical maximum bit rate predicted by <a href="/facts/Shannon%2527s_theorem/vvr8u4Gg">Shannon's theorem</a> for POTS lines (approximately 35 kbit/s). 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.

Trellis coded modulation open-in-new

Trellis coded modulation