Comtech EF Data CDD-562AL, Installation And Operation Manual

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CDD-562AL/564A/L Multi-Channel Demodulator Revision 0
Forward Error Correction MN-CDD562AL564AL
A–2
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 concatenation of a Viterbi
decoder with Reed-Solomon hard-decision block codes. It remained clear, however, that the Shannon
Limit was still an elusive target.
Berrou’s work on Turbo Codes showed, through the use of an ingeniously simple approach (multiple, or
iterative decoding passes) that it is possible to achieve performance close to the Shannon Limit. Berrou’s
early work dealt exclusively with iteratively-decoded convolutional codes (Turbo Convolutional Coding,
or TCC), but in time the iterative approach was applied to a particular class of block codes called Product
Codes – hence Turbo Product Coding (TPC). TPC exhibits inherently low decoding latency compared
with TCC, and so is considered much more desirable for 2-way, interactive satellite communications
applications.
In August 1999, Comtech became the first company in the world to offer satellite modems that
incorporate TPC. Since its inception, Comtech has continued to develop and refine its implementation of
TPC in its products, and now offers a comprehensive range of code rates (from Rate 5/16 to Rate 0.95)
and modulations (from BPSK to 16-QAM).