Comtech EF Data CDM-625 - Manual (Page 276)

CDM-625 Advanced Satellite Modem Revision 15
Forward Error Correction Options MN-CDM625
7–4
7.4.2 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 with 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.
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 7-3. Concatenated RS Coding Summary
FOR AGAINST
• Exceptionally good BER performance – several
orders of magnitude improvement in link BER
under given link conditions
•
Very small additional bandwidth expansion.
• 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 kbps) – not
good for voice or IP applications.
• Adds to demod acquisition time.
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 that uses a Viterbi decoder at the core,
surrounded by adjunct processing. The scheme, called pragmatic Trellis Coded Modulation, is able
to achieve performance very close to the optimum Ungerboeck method, but with far less
complexity.