Mocomtech CDM-570 - Manual (Page 196)

CDM-570/570L Satellite Modem with Optional IP Module Revision 4
Forward Error Correction Options MN/CDM570L.IOM
7.5.3 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 Eb/No
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.
7.5.4 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 7-5. 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.
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