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Modems Glossary

Modems — TCM

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One of the difficulties of QAM-based modems is their relative intolerance to transmission impairments.
     Their dense constellation makes it relatively easy for a slight degree of phase jitter or amplitude spike to result in a received constellation point offset a sufficient amount from its ideal position to be misinterpreted, resulting in a multiple-bit error (each symbol in error results in a number of bits in error).
     The solution to this problem was to incorporate forward error correction (FEC), which in modems is called trellis coding, since the receiver uses the trellis decision tree associated with Viterbi decoding.
     Using trellis coding, the error rate can be reduced by three orders of magnitude (from 1 in 10 to 1 in 10⁴).

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