Mocomtech CDM-QX operation manual Trellis Coding, PSK/TCM Coding Summary

CDM-Qx Satellite Modem	Revision 5
Forward Error Correction Options	MN/CDMQx.IOM

6.4Trellis 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 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.

Now, 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 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 10-8.

The detractions of the concatenated R-S 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 modem fully implements the FEC, but not the framing of the IESS-310 specification at data rates up to 18 Mbps. In accordance with the specification, the R-S outer code can be disabled. Performance curves for both cases are shown in the following Figures.

Table 6-3. 8-PSK/TCM Coding Summary

FOR

AGAINST

Exceptionally bandwidth efficient	Needs concatenated R-S outer codec to give
compared to QPSK	acceptable coding gain performance
	Demod acquisition threshold much higher than for
	QPSK
	8-PSK is more sensitive to phase noise and
	group delay distortion than QPSK

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