Comtech EF Data SLM-5650A - Manual (Page 402)

SLM-5650A Satellite Modem Revision 8
Appendix D MN-SLM5650A
D–8
D.8 Low Density Parity Check (LDPC) coding
D.8.1 Introduction
In the past few years there has been an unprecedented resurgence in interest in 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 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 (i.e., TCC), but in time the iterative approach was applied to TPC. TPC exhibits inherently
low decoding latency compared with TCC, and so is considered much more desirable for two-
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 16QAM). However, in the past few
years, as part of the general interest in Turbo coding, a third class of Turbo coding has emerged:
Low Density Parity Check (LDPC) Codes.
LDPC is more like TPC than TCC in that it is an iteratively-decoded block code. Gallager first
suggested this in 1962 but, at the time, the implementation complexity was considered to be too
great; for decades, it remained of purely academic interest. Further interest in LDPC was
stimulated in 2003, when the Digital Video Broadcasting (DVB) committee adopted LDPC codes
(proposed by Hughes Network Systems) as the basis for the new DVB-S2 standard. Now,
however, with silicon gates being cheap, plentiful and fast, an LDPC decoder can easily be
accommodated in a large Field Programmable Gate Array (FPGA) device.
The LDPC method on its own produces an undesirable ‘flaring’ in the Bit Error Rate (BER) vs.
Eb/No characteristic, and for this reason it is desirable to concatenate a short BCH code with