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

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SLM-5650B Satellite Modem
Revision 2
Appendix D D–2 MN-SLM-5650B
Table D-1. Viterbi Decoding Summary
FOR AGAINST
Good BER performance - very useful coding gain.
Higher coding gain possible with other methods
Almost universally used, with de facto standards for
constraint length and coding polynomials
Shortest decoding delay (~100 bits) of any FEC scheme
- good for coded voice, VOIP, etc
Short constraint length produce small error bursts; good
for coded voice.
No pronounced threshold effect - fails gracefully.
Coding gain independent of data rate.
D.3 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, which uses a Viterbi decoder at the core,
surrounded by adjunct processing. The scheme is able to achieve performance very close to the
optimum Ungerboeck method, but with far less complexity, and is called
pragmatic Trellis Coded
Modulation .
Intelsat recognized that, as more and more high-power transponders are put in to service, the
transponders are no longer
power limited , but bandwidth limited. In order to maximize transponder
capacity, they looked at 8-PSK as a method of reducing the occupied bandwidth of a carrier, and
adopted Qualcomm’s pragmatic TCM, at Rate 2/3.
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, and 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.