Turbo Product Codec (Option); Table 7-3 Concatenated Rs Coding Summary - Comtech EF Data CDM-550T Manual

Forward Error Correction Options
interleaver frame synchronization method can add significantly to the time taken for the
demodulator to declare acquisition.
A characteristic of concatenated RS coding is the very pronounced threshold effect. For
any given modem design, there will be a threshold value of Eb/No below which the
demodulator cannot stay synchronized. This may be due to the carrier-recovery circuits,
or the synchronization threshold of the primary FEC device, or both. In the CDM-550T,
and Rate 1/2 operation, this threshold is around 4 dB Eb/No. Below this value, operation
is not possible, but above this value, the error performance of the concatenated RS system
produces exceptionally low error rates for a very small increase in Eb/No.
CAUTION
Exceptionally good BER performance - several
orders of magnitude improvement in link BER
under given link conditions.
Very small additional bandwidth expansion
Easy field upgrade in
CDM-550T
7.5

Turbo Product Codec (Option)

Turbo coding is an FEC technique developed within the last few years, which delivers
significant performance improvements compared to more traditional techniques. Unlike
the popular method of concatenating a Reed Solomon codec with a primary FEC codec,
Turbo 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 - see below.
Two general classes of Turbo Codes have been developed, Turbo Convolutional Codes
(TCC), and Turbo Product Codes (TPC, a block coding technique). TCC suffers from an
irreducible BER of approximately 1 x 10-7, and consequently, a Reed-Solomon codec
may be added in order to achieve an acceptably low BER. For delay-sensitive
applications this may be unacceptable, and the implementation complexity of the TCC
approach is high. For these reasons 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.
When Comtech EF Data first introduced the Turbo Coding option in the CDM-550T,
only Rate 3/4 QPSK was offered. Firmware Version 1.19 has subsequently added
7–4
Care should be taken not to operate the demodulator near its sync threshold.
Small fluctuations in Eb/No may cause total loss of the link, with the
subsequent need for the demodulator to re-acquire the signal.

Table 7-3 Concatenated RS Coding Summary

FOR
CDM-550T Satellite Modem
AGAINST
Very pronounced threshold effect - does not fail
gracefully in poor Eb/No conditions. Additional
coding overhead actually degrades sync threshold,
and reduces link fade margin.
Significant processing delay (~25 kbits) - not good
for voice.
Adds to demod acquisition time.
Rev. 1.3