Comtech EF Data CDM-570A - Manual (Page 433)

CDM-570A/570AL Satellite Modem with Optional Packet Processor MN-CDM570A
Appendix B Revision 2
B–5
B.5 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. 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.
B.5.1 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.
Claude Shannon, considered by many to be the father of modern communications theory, first
established the concept of Channel Capacity in his 1948 paper A Mathematical Theory of
Communication. This places an absolute limit on how fast it is possible to transmit error-free
data within a channel of a given bandwidth, and with given noise conditions within that channel.
He concluded that it would only be possible to approach this limit through the use of source
encoding – what is familiar today as Forward Error Correction.
Shannon postulated that if it were possible to store every possible message in the receiver,
finding the stored message that most closely matched the incoming message would yield an
optimum decoding method. However, for all but the shortest bit sequences, the memory
required for this, and the time taken to perform the comparisons, makes this approach
impractical. For all practical purposes, the memory requirement and the decoding latency
become infinite.
For many years, there were few advances in the quest to approach the Shannon Limit. The
Viterbi algorithm heralded a major step forward, followed in the early 1990s by the