Comtech EF Data CDM-760, Инструкция по установке и эксплуатации

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Appendix G Revision 4
CDM-760 Advanced High-Speed Trunking Modem MN-CDM760
G–11
G.4.3 Pilots
DVB-S2 originally introduced Pilots as a means to combat phase noise. An additional concern
arose because the very powerful Low Density Parity-Check Forward Error Correction (LDPC FEC)
reduced the conventional operating point of a demodulator much closer to the noise floor than
was done historically, increasing the risk of cycle slips.
There are several ModCods where DVB-S2 advised using Pilots to avoid cycle slips:
• 8PSK 3/5 and 2/3 • 16APSK 3/4 • 32APSK 3/5
The use of Pilots was also recommended by DVB-S2 for ACM operation to guarantee continuous
receiver synchronization. Here, the DVB advises that there are ModCod cases that are simply
more reliable with Pilots
on
, even in the case of reasonable phase noise.
For the purpose of data integrity and maximum ModCod selection, the CDM-760 modem
always enables Pilots ON for all ModCods whenever ACM mode is selected .
G.4.4 Long and Short Frames
It is possible to process a mix of Normal and short frames, although is not believed to be very
valuable for high speed point to point trunking applications. In addition, 9/10 ModCod is not
allowed for short frames, and it is not selectable in short frame mode. The CDM-760 can be
configured for either Normal or short frames, and does not support mixing frame types. When
operating in ACM mode, ALWAYS use Normal frames at both ends of the link.
G.4.5 Monotonic vs. Non-monotonic Operation
Non-monotonic ModCods are “pruned” from ACM operation. The advantage afforded such
ModCods, in terms of spectral efficiency, is diminished by the availability of an equivalent or
higher spectrally efficient ModCod at an Es/No that is equal to or less than the current ModCod.
G.4.6 ModCod Selection
Table G-5 , Table G-6 , and Table G-7 show the guaranteed Es/No at QEF for each ModCod in the
DVB-S2, DVB-S2-EB1, and DVB-S2X Network Specifications, along with spectral efficiency and
ModCod “pruning”. The ModCod elements that are shaded in these tables are pruned from
ACM operation because they are not monotonic.
Figure G-5
and
Figure G-6
show considerable
overlap of ModCod combinations for both
the DVB-S2 and DVB-S2-EB1 Network Specifications.
The pruning and overlap depicted in these tables and figures infer omission of some ModCods;
therefore, without dropping some of these ModCods, there is a non-monotonic change of
Bits/Hz with changing Es/No. As this would result in an unstable system, re-sorting the DVB-S2
or DVB-S2-EB1 tables resolves this.
For example: When moving from QPSK 5/6 to the next best ModCod choice, when the Es/No is
increasing, the modem would be better suited to switch to 8PSK 3/5, which requires only 5.8dB
Es/No and provides 1.74 bits/Hz. You have intentionally pruned QPSK 8/9 and QPSK 9/10, which