Comtech EF Data SLM-5650A - Manual (Page 400)

SLM-5650A Satellite Modem Revision 8
Appendix D MN-SLM5650A
D–6
D.6 Turbo Product Codec (FAST 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 that is a 2- or 3-dimensional array of block codes.
Comtech EF Data has chosen to implement an FEC codec based on TPC. 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, TPC is an
entirely standalone 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, 8PSK and 16QAM
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 Data Rate Range
Rate 5/16 BPSK 64 kbps to 20 Mbps
Rate 21/44 BPSK 64 kbps to 30.545454 Mbps
Rate 21/44 QPSK, OQPSK 64 kbps to 61.090909 Mbps
Rate 3/4 QPSK, OQPSK 64 kbps to 96.0 Mbps
Rate 3/4 8PSK 256 kbps to 144 Mbps
Rate 3/4 16QAM 256 kbps to 155.52 Mbps
Rate 7/8 QPSK, OQPSK 64 kbps to 112 Mbps
Rate 7/8 8-PSK 256 kbps to 155.52 Mbps
Rate 7/8 16QAM 256 kbps to 155.52 Mbps
Rate 17/18 QPSK, OQPSK 64 kbps to 120.888888 Mbps
Rate 17/18 8PSK 256 kbps to 155.52 Mbps