Comtech EF Data SLM-5650A - Manual (Page 399)

SLM-5650A Satellite Modem Revision 8
Appendix D MN-SLM5650A
D–5
D.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 8PSK,
16PSK, 16QAM, 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 8PSK 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 8PSK/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 SLM-5650A fully implements the IESS-310 specification at data rates up to 51.84 Mbps. In
accordance with the specification, the R-S outer code cannot be disabled.
Table D-4. 8PSK/TCM Coding Summary
FOR AGAINST
Exceptionally bandwidth efficient compared to
QPSK
Needs concatenated RS outer codec to give
acceptable coding gain performance
Interoperable with legacy Intelsat networks Demodulator acquisition threshold much higher
than for QPSK
8PSK is more sensitive to phase noise and group
delay distortion than QPSK