Comtech EF Data CDM-840, Инструкция по установке и эксплуатации, Страница: 64 / 110

CDM-840 Remote Router Revision 0
Ethernet Management MN-CDM840
5–18
• VersaFEC MODCOD 2 – QPSK 0.631 • VersaFEC MODCOD 8 – 16-QAM 0.731
•
VersaFEC MODCOD 3 – QPSK 0.706
•
VersaFEC MODCOD 9 – 16-QAM 0.780
•
VersaFEC MODCOD 4 – QPSK 0.803
•
VersaFEC MODCOD 10 – 16-QAM 0.829
•
VersaFEC MODCOD 5 – 8-QAM 0.642
•
VersaFEC MODCOD 11 – 16-QAM 0.853
Tx Scrambler – Use the drop-down menu to select the Tx Scrambler as Off or Normal .
Roll Off – The Tx Alpha Rolloff (
α
) dictates how fast the spectral edges of the carrier are
attenuated beyond the 3 dB bandwidth. With 20% rolloff the edge falls off more quickly than
with 25% and 35%.
Use the drop-down menu to set the expected filter Tx Alpha Rolloff (
α
) of the carrier as 20% ,
25% , or 35% . The default selection is 20% .
Spectrum Invert – Use the drop-down menu to select the Tx Spectrum Invert as Normal or Tx
Spectrum Inverted.
Power Level [-40 to 0 dBm] – Enter the Tx power level in dBm.
Carrier State – Use the drop-down menu to select the Tx Carrier State as either Off or On .
Once the desired configuration settings have been made on this page, click [Submit] as needed to
save these changes.
IMPORTANT
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 advises using Pilots to avoid cycle
slips:
•
8PSK 3/5 and 2/3
•
16APSK 3/4
•
32APSK 3/5
The use of Pilots is also recommended by DVB-S2 when the symbol rate is
below 5000 ksps, and for CCM 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-800 Gateway Router always enables Pilots ON for all ModCods whenever
CCM mode is selected.