Comtech EF Data CDM-Qx Specifications download pdf (Page 137)

CDM-Qx Satellite Modem Revision 5

Forward Error Correction Options MN/CDMQx.IOM

6–5

6.4 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 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

compared to QPSK

Needs concatenated R-S outer codec to give

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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