Filter
Top Products
CDM-600 Satellite Modem Revision 7
Forward Error Correction Options MN/CDM600.IOM
7–5
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 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-600 fully implements the IESS-310 specification at data rates up to 20 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 7-4. 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