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CDM-Qx Satellite Modem
Forward Error Correction Options
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.
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 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;
accordingly, 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
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.
Exceptionally bandwidth efficient
compared to QPSK
Table 6-3. 8-PSK/TCM Coding Summary
FOR
6–4
AGAINST
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
Revision 6
MN/CDMQx.IOM
-8
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