Mocomtech SLM-5650 - Manual (Page 121)

SLM-5650 Satellite Modem Revision 1
Options MN/SLM5650.IOM
A–5
Table A-3. Concatenated RS Coding Summary
FOR AGAINST
Exceptionally good BER performance -
several orders of magnitude
improvement in link BER under given
link conditions. Very pronounced threshold effect - does not fail
gracefully in poor Eb/No conditions. Additional coding
overhead actually degrades sync threshold, and reduces
link fade margin.
Very small additional bandwidth
expansion Significant processing delay (~25 kbps) - not good for
voice, or IP applications
Interoperable with legacy Intelsat
networks Adds to demod acquisition time.
A.4 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.
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 SLM-5650 fully implements the IESS-310 specification at data rates up to 51.84 Mbps.
In accordance with the specification, the R-S outer code can be disabled.