Comtech EF Data CDM-625A User Manual
Page 690
CDM-625A Advanced Satellite Modem
MN-CDM625A
Appendix M
Revision 3
M–10
Step
Task
Front Panel VFD Mnemonic
5
(SELECT) CONFIG: ACM
Configure the desired ACM operating parameters,
by selecting the
ACM submenu from the CONFIG:
menu branch.
Proceeding from the
CONFIG: ACM submenu:
CONFIG: All Mode Tx Rx Clocks ACM
CnC EDMAC Misc Mask Remote IP
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ACM Config: Min/Max-ModCod Unlock-Action
Target-EbNo-Margin
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a) Min/Max-ModCod: Define the range of
ModCods over which the system will operate.
ModCod0 is BPSK Rate 0.488 (0.49 bps/Hz),
while ModCod11 is 16-QAM Rate 0.853 (3.41
bps/Hz).
Min Modcod: 00 (B 0.488 0.488 bps/Hz)
Max Modcod: 00 (B 0.488 0.488 bps/Hz)
If you wish to constrain the system to run at a fixed ModCod, set the Min and Max
ModCod values to be equal.
CAUTION – The value of Max ModCod may be limited by other FAST codes installed.
For example, suppose the 4100 ksps FAST option is installed, and the symbol rate set
to 4100 ksps, the theoretical maximum data rate would be 14 Mbps at ModCod 11.
However, is CnC is being used, with a 10 Mbps FAST limit, the ACM Max ModCod will
be limited to ModCod 7, or 9.6 Mbps.
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
When distant-end demod loses lock:
Go to min Tx ModCod (Maintain,Min)
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c) Target Eb/No 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
Target Eb/No Margin = 1.0 dB (0.0 – 4.5)
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