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CDM-Qx Satellite Modem Revision 5
Forward Error Correction Options MN/CDMQx.IOM
6–6
6.5 Turbo Product Codec (Hardware Option)
6.5.1 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
.
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.
6.5.2 The Evolution of TPC in Comtech Products
When Comtech EF Data first introduced the Turbo Coding option in 1999, 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.)
In 2002, the new second generation TPC option was released. This has added data rate
capability up to 20 Mbps, in addition to Rate 7/8 and Rate 17/18 capability. The Rate 7/8
TPC is extremely powerful, offering performance very close to the original Rate 3/4
TPC, but using 15% less bandwidth. Note also that the Rate 17/18 TPC adds just 5% FEC
overhead, but yields almost identical coding gain to Rate 1/2 Viterbi at a BER of 1 x 10
-7
.
Below is a listing of all the available TPC modes and rates in the CDM-Qx.