Configuring Versafec Acm In The Cdm-570A/570Al - Comtech EF Data CDM-570A Installation And Operation Manual

CDM-570A/570A Satellite Modem with Optional Packet Processor
Appendix P
•
Latency for 16k block DVB-S2 ACM at 100 ksps = 349 milliseconds
• Latency for VersaFEC ACM at 100 ksps = 54 milliseconds
•
•
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
1 (SELECT) CONFIG: TI > IP-ACM
Select the Traffic Interface as IP-ACM. Press ENTER to proceed to the ACM
menu.
2
Configure the desired ACM operating parameters. Proceeding from the
(SELECT) CONFIG: TI > IP-ACM > ACM submenu:
ModCod: Define the range of ModCods over which the system will
A.
operate. ModCod 00 is BPSK Rate 0.488 (0.49 bps/Hz), while ModCod11
is 16-QAM Rate 0.853 (3.41 bps/Hz).
Unlock-Action: Choose the desired action when the remote demod loses
B.
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:
Target SNR Margin: This is a VERY important parameter. The ACM system
C.
is designed to switch based on thresholds that correspond to a BER of 5 x 10
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.
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.
If you wish to constrain the system to run at a fixed ModCod, set the Min and Max ModCod values to be
equal.
Go to min Tx ModCod (recommended)
–or– Maintain Tx ModCod
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.
-8
P–9
MN-CDM570A
Revision 3
Front Panel VFD Mnemonic
Data Interface: RS422 IP
V.35 RS232 G.703 IP-ACM
ACM:ModCod Unlock-Action
Target-SNR-Margin
Min Modcod: 00 BPSK .488
Max Modcod: 11 16QAM .853
Distant-end loses lock:
Go to min Tx ModCod
()
Target SNR Margin:
1.0 dB (0.0 – 4.5)
()