Comtech EF Data CDM-570 - Manual (Page 171)

CDM-570/570L Satellite Modem with Optional IP Module Revision 12
Forward Error Correction Options MN/CDM570L.IOM
8–5
8.5 Turbo Product Codec (Hardware Option)
8.5.1 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 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 E
b
/N
o
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 output error rate becomes unusable. This is considered to
be an advantageous characteristic in 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/R-S or TCM cases
.
With this release of the CDM-570/570L, Comtech EF Data 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.
8.5.2 TPC modes available in the CDM-570/570L
Table 8-4. Available TPC Modes
Code Rate/Modulation Data Rate Range
Rate 5/16 BPSK 2.4 kbps to 0.937 Mbps
Rate 21/44 BPSK 2.4 kbps to 1.430 Mbps
Rate 21/44 QPSK, OQPSK 4.8 kbps to 2.860 Mbps
Rate 3/4 QPSK, OQPSK 7.2 kbps to 4.500 Mbps
Rate 3/4 8-PSK, 8-QAM 10.8 kbps to 6.750 Mbps
Rate 3/4 16-QAM 14.4 kbps to 9.000 Mbps
Rate 7/8 QPSK, OQPSK 8.4 kbps to 5.250 Mbps
Rate 7/8 8-PSK, 8-QAM 13.6 kbps to 7.875 Mbps
Rate 7/8 16-QAM 16.8 kbps to 9.980 Mbps
Rate 0.95 QPSK, OQPSK 9.1 kbps to 5.666 Mbps
Rate 0.95 8-PSK, 8-QAM 15.3 kbps to 8.500 Mbps
Maximum rates are subject to the appropriate FAST codes being installed