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SLM-7650 Satellite Modem
Theory of Operation
Care should be taken not to operate the demodulator near its sync threshold. Small
fluctuations in Eb/No may cause total loss of the link, with the subsequent need for
the demodulator to re-acquire the signal.
CAUTION
It cannot be emphasized strongly enough that the purpose of the concatenated Reed-
Solomon is to dramatically improve the BER performance of a link under given
noise conditions. It should NOT be considered as a method to reduce the link EIRP
IMPORTANT
requirement to produce a given BER. Factors such as rain-fade margin, particularly
at Ku-band, are extremely important, and reducing link Effective Isotropic Radiated
Power (EIRP) can seriously degrade the availability of such a link.
Exceptionally good BER performance
- several orders of magnitude
improvement in link BER under given
link conditions.
Very small additional bandwidth
expansion
4.3.3.3
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 8PSK, 16-PSK, 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
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
Table 4-2. Concatenated RS Coding Summary
FOR
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.
Significant processing delay (~25 kbit/s) - not good for
voice, or IP applications
Adds to demod acquisition time.
4–6
Revision 4
MN/SLM7650.IOM
AGAINST
-8
.
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