Mocomtech CIM-550 - Manual (Page 119)

CiM-550 IP Enabled Satellite Modem Rev. 2
Forward Error Correction CD/CIM550.IOM
93
A fixed computational clock is used to process input symbols, and to search backwards
and forwards in time to determine the correct decoding path. At lower data rates there are
sufficient number of computational cycles per input symbol to permit the decoding
process to perform optimally. However, as the data rate increases, there are fewer cycles
available, leading to a reduction in coding gain. This is clearly illustrated in the
performance curves which follow. For data rates above ~1 Mbps, Viterbi should be
considered the better alternative.
Table 7-2. Sequential Decoding Summary
FOR AGAINST
Higher coding gain (1 -2 dB) at
lower data rates, compared to
Viterbi. Pronounced threshold effect - does not
fail gracefully in poor Eb/No conditions.
Higher processing delay than Viterbi
(~4 k bits) - not good for low-rate coded
voice.
Coding gain varies with data rate - favors
lower data rates.
7.4 T URBO P RODUCT C ODEC (O PTION )
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
has to 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.