Comtech EF Data SLM-5650B, Installation And Operation Manual

Page: 399 / 412

SLM-5650B Satellite Modem
Revision 2
Appendix D D–5 MN-SLM-5650B
D.6 Turbo Product Codec
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 R-S 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 R-S approach, and consequently, decoding delays are significantly reduced.
Furthermore, the traditional concatenated R-S 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 demodulator 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
2-3dB below the Viterbi/Reed-
Solomon or TCM cases
.
Table D-5. Available TPC Modes
TPC Code Rate/Modulation
Rate 5/16 BPSK
Rate 21/44 BPSK
Rate 21/44 QPSK, OQPSK
Rate 3/4 QPSK, OQPSK
Rate 3/4 8-PSK
Rate 3/4 16-QAM
Rate 7/8 QPSK, OQPSK
Rate 7/8 8-PSK
Rate 7/8 16-QAM
Rate 17/18 QPSK, OQPSK
Rate 17/18 8-PSK