Mocomtech SLM-5650 - Manual (Page 122)

SLM-5650 Satellite Modem Revision 1
Options MN/SLM5650.IOM
A–6
Table A-4. 8-PSK/TCM Coding Summary
FOR AGAINST
Exceptionally bandwidth efficient compared to
QPSK Needs concatenated RS outer codec to give
acceptable coding gain performance
Interoperable with legacy Intelsat networks Demodulator acquisition threshold much higher
than for QPSK
8-PSK is more sensitive to phase noise and group
delay distortion than QPSK
A.5 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 a 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 – 3 dB below the Viterbi/Reed-Solomon or TCM cases
.
Table A-5. Available TPC Modes
TPC Code Rate/Modulation Data Rate Range
Rate 5/16 BPSK 64 kbps to 9.375 Mbps
Rate 21/44 BPSK 64 kbps to 14.3 Mbps
Rate 21/44 QPSK, OQPSK 64 kbps to 28.636 Mbps
Rate 3/4 QPSK, OQPSK 64 kbps to 45.0 Mbps
Rate 3/4 8-PSK 256 kbps to 51.84 Mbps
Rate 3/4 16-QAM 256 kbps to 51.84 Mbps
Rate 7/8 QPSK, OQPSK 64 kbps to 51.84 Mbps
Rate 7/8 8-PSK 256 kbps to 51.84 Mbps
Rate 7/8 16-QAM 256 kbps to 51.84 Mbps
Rate 17/18 QPSK, OQPSK 64 kbps to 51.84 Mbps
Rate 17/18 8-PSK 256 kbps to 51.84 Mbps