Comtech EF Data CDM-570A - Manual (Page 599)

CDM-570A/570A Satellite Modem with Optional Packet Processor MN-CDM570A
Appendix P Revision 2
P–9
• Latency for 16k block DVB-S2 ACM at 100 ksps = 349 milliseconds
• Latency for VersaFEC ACM at 100 ksps = 54 milliseconds
• For the example shown, the latency for a 16k block DVB-S2 ACM scheme is
approximately 7 times higher than VersaFEC ACM.
• The latency for a 64k block DVB-S2 ACM scheme is approximately 25 times
higher than VersaFEC ACM.
P.8 Configuring VersaFEC ACM in the CDM-570A/570AL
VersaFEC ACM requires the appropriate FAST code for the maximum operating
symbol rate.
Configuration is very straightforward from the CDM-570A/570AL-IP front panel. For a detailed
overview of modem operations via the front panel, refer to Chapter 6. FRONT PANEL
OPERATION.
To configure the CDM-570A/570AL-IP for VersaFEC ACM operation, do these steps:
Step Task Front Panel VFD Mnemonic
1 (SELECT) CONFIG: TI > IP-ACM
Select the Traffic Interface as IP-ACM. Press ENTER to proceed to the ACM
menu.
Data Interface: RS422 IP
V.35 RS232 G.703 IP-ACM
2 Configure the desired ACM operating parameters. Proceeding from the
(SELECT) CONFIG: TI > IP-ACM > ACM submenu:
A. ModCod: Define the range of ModCods over which the system will
operate. ModCod 00 is BPSK Rate 0.488 (0.49 bps/Hz), while ModCod11
is 16-QAM Rate 0.853 (3.41 bps/Hz).
ACM:ModCod Unlock-Action
Target-SNR-Margin
Min Modcod: 00 BPSK .488
Max Modcod: 11 16QAM .853
If you wish to constrain the system to run at a fixed ModCod, set the Min and Max ModCod values to be
equal.
B. Unlock-Action: Choose the desired action when the remote demod loses
lock. This is important, as the ACM system depends on the feedback of the
SNR metric from the remote demod to determine the optimum ModCod.
The choices are:
Go to min Tx ModCod (recommended)
–or– Maintain Tx ModCod
Distant-end loses lock:
Go to min Tx ModCod ()
C. Target SNR Margin: This is a VERY important parameter. The ACM system
is designed to switch based on thresholds that correspond to a BER of 5 x 10 -8
for each ModCod. However, in order to prevent oscillation around two
ModCods at this exact value, 0.3 dB of hysteresis has been added. The
switch points and the hysteresis are shown in Figure P-3.
Target SNR Margin:
1.0 dB (0.0 – 4.5) ()
The Figure P-3 graph shows the switch points with the Target SNR margin set to 0dB. However,
the switch points can be moved (increased) by configuring the Target SNR margin parameter,
which can vary from 0 to 4.5 dB, in 0.5dB steps. In a fading environment, Comtech EF Data
strongly recommends that you add sufficient margin to maintain an adequate link quality (and
to maintain demod lock) during the interval between the SNR degrading and the ACM
controller responding by lowering the ModCod. See Sect.P.10 Notes and Recommendations.