Mocomtech CDM-QX - Manual (Page 171)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
Forward Error Correction Options MN/CDMQX.IOM
7–5
and 16-QAM TPC modes the demod and decoder can remain synchronized 2 – 3 dB below the
Viterbi/Reed-Solomon or TCM cases
.
Comtech 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 The Evolution of TPC in Comtech Products
When Comtech EF Data first introduced the Turbo Coding option in 1999, only Rate 3/4 QPSK
was offered. Further work permitted the addition of Offset QPSK operation. 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)
were then made available.
(These two rates were developed to address transmission from very small antennas, where ITU
flux density limits may be an issue. The combination of code rate and BPSK modulation provides
wide spreading, and hence reduces flux density.)
In 2002, the new second generation TPC option was released. This has added data rate capability
up to 20 Mbps, in addition to Rate 7/8 and Rate 17/18 capability. The Rate 7/8 TPC is extremely
powerful, offering performance very close to the original Rate 3/4 TPC, but using 15% less
bandwidth. Note also that the Rate 17/18 TPC adds just 5% FEC overhead, but yields almost
identical coding gain to Rate 1/2 Viterbi at a BER of 1 x 10
-7 . Below is a listing of all the
available TPC modes and rates in the CDM-Qx.
Table 6-4. Available TPC Modes
Code Rate/Modulation Data Rate Range
Rate 21/44 BPSK 32 kbps to 4.772 Mbps
Rate 5/16 BPSK 32 kbps to 3.125 Mbps
Rate 21/44 QPSK 32 kbps to 10 Mbps
Rate 3/4 QPSK 32 kbps to 15 Mbps
Rate 3/4 8-PSK 288 kbps to 20 Mbps
Rate 3/4 16-QAM 384 kbps to 20 Mbps
Rate 7/8 QPSK 32 kbps to 17.5 Mbps
Rate 7/8 8-PSK 336 kbps to 20 Mbps
Rate 7/8 16-QAM 448 kbps to 20 Mbps
Rate 17/18 QPSK 32 kbps to 18.88 Mbps
Rate 17/18 8-PSK 362.7 kbps to 20 Mbps
7.5.3 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. The table below shows,
for the Modem, the processing delays for the major FEC types, including the three TPC modes.