Comtech EF Data CDM-625 - Manual (Page 406)

CDM-625 Advanced Satellite Modem Revision 15
Adaptive Coding and Modulation (ACM) MN-CDM625
17–10
Step Task Front Panel VFD Mnemonic
5
(cont)
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.
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) ()
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 Figure 17-3:
Target Eb/No Margin = 1.0 dB (0.0 – 4.5)
()
Figure 17-3. CDM-625 – ACM ModCod Switch Points