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CDM-600L Satellite Modem
Forward Error Correction Options
7.6
Turbo Product Codec (Hardware Option)
7.6.1
Introduction
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 a Reed Solomon 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
With this release of the CDM-600L, Comtech now provides the best Forward Error
Correction technology currently available, offering a very broad range of TPC code rates,
combined with the entire range of modulation types, from BPSK to 16-QAM.
7.6.2
The Evolution of TPC in Comtech Products
When Comtech EF Data first introduced the Turbo Coding option in the CDM-550T,
only Rate 3/4 QPSK was offered. Further work permitted the addition of Offset QPSK
operation. Two further code rates - Rate 21/44 BPSK (very close to Rate 1/2) and Rate
5/16 BPSK (very close to Rate 1/3) were then made available.
(These two rates were developed to address transmission from very small antennas,
where ITU flux density limits may be an issue. The combination of code rate and BPSK
modulation provides wide spreading, and hence reduces flux density.)
When the CDM-600L was first released in July 2001, it was limited to these original code
rates, at data rates up to 5 Mbps. In November 2001, 16-QAM was added (Viterbi/Reed-
Solomon), while at the same time, Rate 3/4 TPC was added for both 8-PSK and 16-
QAM. This is the so-called Low Rate TPC option.
In April 2002, the new High Rate TPC option was released. This has added data rate
capability up to 20 Mbps, in addition to Rate 7/8 and Rate 0.95 capability.
7–6
Revision 0
MN/CDM600L.IOM
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