Comtech EF Data CDM-570 - Manual (Page 172)

CDM-570/570L Satellite Modem with Optional IP Module Revision 12
Forward Error Correction Options MN/CDM570L.IOM
8–6
8.5.3 8-QAM Modulation
What is 8-QAM, and why is it important? Unlike 8-PSK, which comprises 8 equally spaced
constellation points around a unit-circle, 8-QAM is comprised of exactly half of a 16-QAM
signal. Fortuitously, the 8-QAM constellation possesses some unique properties that can be
exploited to permit acquisition and tracking of signals at noise levels 2 - 3 dB worse than is
possible with 8-PSK. This is, then, a perfect match for the expected E
b
/N
o
values that TPC
demands. Naturally, it has exactly the same spectral efficiency as 8-PSK.
While the 8-QAM constellation itself is not new, Comtech has performed much original work
related to the choice of optimum mapping and soft decision decoding, and, of course, on the
techniques for acquiring and tracking 8-QAM signals. This work is the subject of a pending
patent application filed by Comtech EF Data.
The basic performance of uncoded 8-QAM is broadly similar to uncoded 8-PSK, but has a
slightly higher peak-to-average power ratio than 8-PSK (about 0.8 dB). In most linear
transponders, this should not be considered a problem.
A major benefit of Comtech’s implementation of 8-QAM is that it is inherently more immune to
the effects of phase noise than 8-PSK. In L-band applications that use low-cost BUCs and LNBs
this is considered particularly advantageous for lower bit rates, where phase noise can be very
problematic.
8.5.4 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 CDM-570/570L, the processing delays for the major FEC types, including the
three TPC modes:
Table 8-5. Turbo Product Coding Processing Delay Comparison
FEC Mode (64 kbps data rate) End-to-end delay, ms
Viterbi, Rate 1/2 12
Viterbi Rate 1/2 + Reed Solomon 266
Turbo Product Coding, Rate 3/4 47
Turbo Product Coding, Rate 21/44, BPSK 64
Turbo Product Coding, Rate 5/16, BPSK 48
Turbo Product Coding, Rate 7/8 245 *
Turbo Product Coding, Rate 0.95 69
* A larger block is used for the Rate 7/8 code, which increases decoding delay.
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. It can be seen that the concatenated Reed-Solomon cases
increase the delay significantly, due mainly to interleaving/de-interleaving.