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CDM-625 Advanced Satellite Modem
Forward Error Correction Options
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 RS 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
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-625 fully implements the IESS-310 specification at data rates up to 20 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.
FOR
Exceptionally bandwidth efficient compared to
QPSK
7.5
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 R-S 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 R-S approach, and consequently, decoding delays are significantly reduced.
Furthermore, the traditional concatenated R-S schemes exhibit a very pronounced threshold effect
– a small reduction in Eb/No 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. Typically, in QPSK, 8-PSK and 16-QAM
TPC modes the demod and decoder can remain synchronized 2 – 3 dB below the Viterbi/Reed-
Solomon or TCM cases.
-8
.
Table 7-3. 8-PSK/TCM Coding Summary
7–4
MN-0000036 (Ref MN/CDM625.IOM)
AGAINST
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
Revision 1
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