Comtech EF Data CDM-600/600L - Manual (Page 125)

CDM-600/600L Open Network Satellite Modem Revision 3
Forward Error Correction Options MN/CDM600L.IOM
6–5
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 Reed-Solomon 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 Reed-Solomon 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-600/600L fully implements the IESS-310 specification at data rates up to 20 Mbps. In
accordance with the specification, the Reed-Solomon outer code can be disabled. Performance
curves for both cases are shown in the figures that follow later in this chapter.
Table 6-4. 8-PSK/TCM Coding Summary
FOR AGAINST
Exceptionally bandwidth efficient compared to
QPSK Needs concatenated Reed-Solomon 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
6.6 Turbo Product Codec (Hardware Option)
Turbo coding is an FEC technique developed within the last few years, which delivers significant
performance improvements compared to more traditional techniques. Two general classes of
Turbo Codes have been developed, Turbo Convolutional Codes (TCC), and Turbo Product Codes
(TPC, a block coding technique). Comtech EF Data has chosen to implement an FEC codec based
on TPC. A Turbo Product Code is a 2 or 3 dimensional array of block codes. Encoding is
relatively straightforward, but decoding is a very complex process requiring multiple iterations of
processing for maximum performance to be achieved.
Unlike the popular method of concatenating a Reed-Solomon codec with a primary FEC codec,
Turbo Product Coding is an entirely stand-alone method. It does not require the complex
interleaving/de-interleaving of the Reed-Solomon approach, and consequently, decoding delays
are significantly reduced. Furthermore, the traditional concatenated Reed-Solomon schemes
exhibit a very pronounced threshold effect – a small reduction in E
b
/N
o
can result in total loss of
demod and decoder synchronization. TPC does not suffer from this problem – the demod and
decoder remain synchronized down to the point where the output error rate becomes unusable.
This is considered to be a particularly advantageous characteristic in a fading environment.