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CDD-562AL/564A/L Multi-Channel Demodulator
Forward Error Correction
Appendix A. Forward Error
A.1
Overview
The CDD-56xA/L Satellite Demodulator is equipped with VersaFEC (short-block LDPC) and Turbo
Product Code (TPC) Forward Error Correction encoders/decoders.
Turbo Product 0.95 Code Rate (TPC) is available via an optional plug-in card.
While the performance of LDPC is exceptional in terms of coding gain, its higher latency is considered
disadvantageous in some applications. For this reason, Comtech EF Data and Comtech AHA Enterprise
Products Group have jointly developed a family of short-block LDPC codes called VersaFEC®. (The
name VersaFEC is a trademark registered to Comtech AHA Enterprise Products Group.)
VersaFEC FEC is ideal for lower data rates that demand the shortest possible latency, and is available as a
FAST option. It represents the best Forward Error Correction technology currently available, and is
offered with a sufficient range of code rates and modulation types that link performance can be optimized
under almost any conditions.
A.2
Turbo Product Codec (FAST 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 RS 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 RS approach, and consequently, decoding delays are significantly reduced. Furthermore, the
traditional concatenated RS 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.
A.2.1 TPC Overview
In the past few years there has been an unprecedented resurgence in interest in Forward Error Correction
(FEC) technology. The start of this new interest has its origins in the work done by Claude Berrou et al,
and the 1993 landmark paper, Near Shannon Limit Error Correcting Coding and Decoding – Turbo
Codes. FEC is considered an essential component in all wireless and satellite communications in order to
reduce the power and bandwidth requirements for reliable data transmission.
Correction (FEC) Options
A–1
Revision 0
MN-CDD562AL564AL
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