Comtech EF Data CDM-570 - Manual (Page 170)

CDM-570/570L Satellite Modem with Optional IP Module Revision 12
Forward Error Correction Options MN/CDM570L.IOM
8–4
8.4 Trellis Coding (requires 8-PSK/8-QAM 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.
Now, Intelsat recognized that, as more and more high power transponders are put into 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 it is required that
the scheme be concatenated with an outer RS codec. When combined, there is a threshold value
of E
b
/N
o
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 RS 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 CDM-570/570L implements a Closed Network version of Rate 2/3 8-PSK/TCM/RS, using
either the 220, 200 or 200,180 Reed-Solomon outer codes. Although not compatible, it provides
identical performance to the Open Network IESS-310 standard.
Table 8-3. 8-PSK/TCM Coding Summary
FOR AGAINST
Exceptionally bandwidth efficient
compared to QPSK. • Needs concatenated RS 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.