Mocomtech CDM-570 - Manual (Page 195)

CDM-570/570L Satellite Modem with Optional IP Module Revision 4
Forward Error Correction Options MN/CDM570L.IOM
7.5 Turbo Product Codec (Hardware Option)
7.5.1 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 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 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.
7.5.2 TPC modes available in the CDM-570/570L
Table 7-4. 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
7–5