There are a variety of FEC codes that can be effective but the Maximum
likelyhood (Viterbi) decoder is very effective especially if supplies with soft
input and when interleaving is used. There are many efficient hardware and
software implementations of the Viterbi decoder.
Soft input means of course that not one or zero values are sent to the decoder
but some higher resolution measurement (phase value, amplitude etc) over some
range ... usually 4-8 bits of resolution is sufficient In the WINMOR and V4 HF
protocols I used the soft Viterbi decoder by Phil Karn (originally in C which I
re wrote to VB.NET ). Phil’s decoder is based on a length 7 rate 1/2
convolutional code (commonly referred to as NASA Voyager code) with a 32 sample
“look back”. For optimum use the code should be interleaved (reduced
sensitivity to burst errors which convolutional codes to not handle as well.)
Also Viterbi codes should be “flushed” with a few (up to 6 for a length 7 code)
“flush” bits at the end of each frame for optimum performance.
For those not familiar with FEC the performance of the code is usually referred
to as coding gain (dB). This is the equivalent performance improvement in bit
error rate (BER) that would be from increasing the S/N ratio by the same ratio
in dB. Of course most coding gain measurements are done as a comparison to
white Gaussian Noise which is not the typical HF environment. None the less a
well implemented Rate 1/2 code can achieve performance gains in the 2-3 dB
range about the same as doubling the transmitter power...a significant
improvement.
Rick
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