Also known as: trellis-coded modulation, TCM

Trellis-coded modulation (TCM) integrates convolutional coding with the modulation symbol mapping, so coding gain is achieved without extra bandwidth.¹

Trellis-coded modulation combines error coding with the symbol mapping; P25 uses it on Phase 1 traffic.

How it works

The encoder constrains which symbol sequences are valid; the receiver uses the Viterbi algorithm over the resulting trellis to pick the most likely sequence. P25 applies a trellis code to parts of its data.

Relevance to SDR

TCM improves decoding robustness on the protocols that use it, recovering data at lower SNR than uncoded modulation.

Sources

1. Trellis modulation — Wikipedia, for combining convolutional coding with the constellation mapping for coding gain. ↩

See also

• Convolutional code
• Viterbi algorithm
• Forward error correction (FEC)
• Project 25 (P25)

Learn it step by step

• The demodulation pipeline

From the blog

• SDR Internals, Part 9: Framing & forward error correction