Comtech EF Data CDM-625 - Manual (Page 277)

CDM-625 Advanced Satellite Modem Revision 15
Forward Error Correction Options MN-CDM625
7–5
As more and more high power transponders are put in to service, Intelsat recognized that the
transponders are no longer power limited, but bandwidth limited. In order to maximize
transponder capacity, 8-PSK was looked at as a method of reducing the occupied bandwidth of a
carrier, and Qualcomm’s pragmatic TCM, at Rate 2/3, was adopted.
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, 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 -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 unit 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.
Table 7-4. 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.
7.6 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 an 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.