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CDM-600L Satellite Modem
Forward Error Correction Options
7.5
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, 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
Reed-Solomon 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 Reed-Solomon 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-600L fully implements the IESS-310 specification at data rates up to 20 Mbps. In
accordance with the specification, the Reed-Solomon outer code can be disabled.
Performance curves for both cases are shown in the following Figures.
FOR
Exceptionally bandwidth efficient compared to
QPSK
Table 7-4. 8-PSK/TCM Coding Summary
7–5
-8
.
AGAINST
Needs concatenated Reed-Solomon 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 2
MN/CDM600L.IOM
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