Comtech EF Data CDM-625A - Manual (Page 543)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix B Revision 3
B–7
B.6 Turbo Product Codec (Hardware Option)
Turbo coding is an FEC technique developed within the last few years, which delivers significant
performance improvements compared to more traditional techniques. Two general classes of
Turbo Codes have been developed, Turbo Convolutional Codes (TCC), and Turbo Product Codes
(TPC, a block coding technique). Comtech EF Data has chosen to implement an FEC codec based
on TPC. A Turbo Product Code is a 2 or 3 dimensional array of block codes. Encoding is relatively
straightforward, but decoding is a very complex process requiring multiple iterations of
processing for maximum performance to be achieved.
Unlike the popular method of concatenating an RS codec with a primary FEC codec, Turbo
Product Coding is an entirely stand-alone method. It does not require the complex interleaving/
de-interleaving of the RS approach, and consequently, decoding delays are significantly reduced.
Furthermore, the traditional concatenated RS schemes exhibit a very pronounced threshold
effect – a small reduction in Eb/No can result in total loss of demod and decoder
synchronization. TPC does not suffer from this problem – the demod and decoder remain
synchronized down to the point where the output error rate becomes unusable. This is
considered to be a particularly advantageous characteristic in a fading environment. Typically, in
QPSK, 8-PSK and 16-QAM TPC modes the demod and decoder can remain synchronized 2 – 3 dB
below the Viterbi/Reed-Solomon or TCM cases.