Comtech EF Data CDD-562L - Manual (Page 208)

CDD-562L/564 Demodulator with IP Module Revision 2
Appendix B MN/CDD564L.IOM
B–2
B.2 Turbo Product Codec
B.2.1 Description
Two general classes of Turbo Codes are available: 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 standalone 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, TPC modes the demod and decoder can remain synchronized 2 – 3 dB below
the Viterbi/R-S or TCM cases.
B.2.2 End-to-End Processing Delay
In many cases, FEC methods that provide increased coding gain do so at the expense of increased
processing delay. However, with TPC, this increase in delay is very modest. Table B-1 shows the
processing delays for the three TPCs at 64 kbps based on the latency through a CDM-570/570L
with IP Module and either a CDD-564/564L or CDD-562L.
Table B-1. Turbo Product Coding Processing Delay Comparison
FEC Mode (64 kbps data rate) End-to-end delay, ms
Turbo Product Coding, Rate 3/4 47
Turbo Product Coding, Rate 7/8 249
Turbo Product Coding, Rate 0.95 69
Note that, in all cases, the delay is inversely proportional to data rate, so for 128 kbps the delay
values would be half of those shown above.