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CDM-Qx Satellite Modem Revision 5
Forward Error Correction Options MN/CDMQx.IOM
6–5
6.4 Trellis Coding
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.
Now, 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
R-S 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 R-S 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 modem fully implements the FEC, but not the framing of the IESS-310 specification
at data rates up to 18 Mbps. In accordance with the specification, the R-S outer code can
be disabled. Performance curves for both cases are shown in the following Figures.
Table 6-3. 8-PSK/TCM Coding Summary
FOR AGAINST
Exceptionally bandwidth efficient
compared to QPSK
Needs concatenated R-S 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