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256 CHAPTER NINE

FIGURE 9-14 32 QAM and 64 QAM

If the 16 QAM system is still going to transmit the equivalent of 4 bits per symbol,
the encoder can pack in redundant data by restricting the 32 or 64 different symbol loca-
tions that will be permitted transitions. This effectively puts an identifiable pattern into
the data without expanding the bandwidth. The Viterbi decoder can still be used, but it
uses the distance between symbols as a metric as it looks for suspicious data transitions.

Turbo Coding

Some advances have been made since Viterbi brought out his codes in the late ‘60s.
Viterbi decoder complexity tends to grow exponentially for stronger coding gains.
Classical turbo codes have been out for a while, with much better results, but the clas-
sical turbo codes have some limits, reaching a limit short of the Shannon capacity limit.
In addition, the classical turbo codes are complex to compute and use expensive hard-
ware. Turbo product coding (an improvement on classic turbo codes), is more promis-
ing. The technique allows a determined communications link designer to get arbitrarily
close to Shannon’s capacity limit, sending as much data through a channel as the S/N
ratio will allow. A good deal of computation is required, but the computations tend to
be iterative and lend themselves to an implementation in silicon. Performance is largely
bounded by memory limits.
Turbo product codes replace the entire RS, Viterbi concatenated chain. The per-
formance delivered can bring the BER curve within an arbitrarily small number of dB