Comtech EF Data CDM-625A - Manual (Page 542)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix B Revision 3
B–6
B.5 Trellis Coding (FAST Option)
In the other FEC methods described here, the processes of coding and modulation are
independent – the FEC codec has no knowledge of, or interaction with the modulator. However,
there are schemes in which the coding and modulation are combined together, where the
encoder places FEC symbols in a precise manner into the signal constellation. This can yield an
overall improvement in performance, and is used in higher-order modulation schemes, such as
8-PSK, 16-PSK, 16-QAM, etc.
When convolution coding is used, the overall coded modulation approach is referred to as Trellis
Coded Modulation (TCM). Ungerboeck was an early pioneer and developed optimum mapping
and decoding schemes. However, the decoding scheme was seen as complex and expensive, and
Qualcomm Inc. developed a variation on the theme that uses a Viterbi decoder at the core,
surrounded by adjunct processing. The scheme, called pragmatic Trellis Coded Modulation, is able
to achieve performance very close to the optimum Ungerboeck method, but with far less
complexity.
As more and more high power transponders are put in to service, Intelsat recognized that the
transponders are no longer power limited, but bandwidth limited. In order to maximize
transponder capacity, 8-PSK was looked at as a method of reducing the occupied bandwidth of a
carrier, and Qualcomm’s pragmatic TCM, at Rate 2/3, was adopted.
A Rate 2/3 8-PSK/TCM carrier occupies only 50% of the bandwidth of a Rate 1/2 QPSK carrier.
However, the overall coding gain of the scheme is not adequate by itself, so Intelsat’s IESS-310
specification requires that the scheme be concatenated with an outer RS codec. When
combined, there is a threshold value of Eb/No of around 6 dB and, above approximately 7 dB,
the bit error rate is better than 1 x 10 -8 .
The detractions of the concatenated RS approach apply here also, along with more stringent
requirements for phase noise and group delay distortion – the natural consequences of the
higher-order modulation.
The CDM-625A fully implements the IESS-310 specification at data rates up to 20 Mbps. In
accordance with the specification, the RS outer code can be disabled. Performance curves for
both cases are shown in the following figures.
Table B-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.
• Demod acquisition threshold much higher than for QPSK.
• 8-PSK is more sensitive to phase noise and group delay
distortion than QPSK.
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