Comtech EF Data CDM-625, Инструкция по установке и эксплуатации

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CDM-625 Advanced Satellite Modem Revision 1
Forward Error Correction Options MN-0000036 (Ref MN/CDM625.IOM)
7.3.2 Open Network Modes
Code Rate Mode
219, 201 Standard IESS-308 E1, and IESS-310 mode
225, 205 Standard IESS-308 T1
194, 178 Standard IESS-308 T2/E2
126, 112 Standard IESS-309 modes
A characteristic of concatenated R-S coding is the very pronounced threshold effect. For any
given modem design, there will be a threshold value of Eb/No below which the demodulator
cannot stay synchronized. This may be due to the carrier-recovery circuits, or the synchronization
threshold of the primary FEC device, or both. In the CDM-625, and Rate 1/2 operation, this
threshold is around 4 dB Eb/No. Below this value, operation is not possible, but above this value,
the error performance of the concatenated R-S system produces exceptionally low error rates for a
very small increase in Eb/No.
CAUTION
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.
Table 7-2. 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
Adds to demod acquisition time
7.4 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
.
7–3