Comtech EF Data CDM-550T - Manual (Page 78)

CDM-550T Satellite Modem Revision 3
Forward Error Correction Options MN/CDM550T.IOM
Table 7-3 Concatenated RS Coding Summary
FOR AGAINST
Exceptionally good BER performance - several
orders of magnitude improvement in link BER
under given link conditions.
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.
Very small additional bandwidth expansion
Significant processing delay (~25 kbits) - not
good for voice.
Easy field upgrade in
CDM-550T
Adds to demod acquisition time.
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 R-S
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 added Rate 3/4 OQPSK operation.
• Firmware Version 1.24 has added Rate 1/2 QPSK operation.
These are excellent choices for applications where maximum bandwidth efficiency is desired, in
combination with high coding gain.
• Firmware Version 1.15 (and later) introduced two further code rates - Rate 21/44
BPSK (very close to Rate 1/2) and Rate 5/16 BPSK (very close to Rate 1/3).
These two rates were developed to address an entirely different case, namely that of transmission
from very small antennas, with limited transmitter power. For a dish antenna, the gain is directly
proportional to its area, and the lower the gain, the less directional the antenna becomes. Thus, in
satellite transmission, even though the dish may be perfectly pointed at the desired satellite, if the
beamwidth is wide enough, adjacent satellites in the orbital arc will also be illuminated. This is a
potential source of interference, and for this reason the ITU (International Telecommunications
7–4