Comtech EF Data CDM-625 - Figure 17-3. CDM-625 - ACM Modcod Switch Points

CDM-625 Advanced Satellite Modem Revision 13
Adaptive Coding and Modulation (ACM) MN-CDM625
17–10
Step
Task
Front Panel VFD Mnemonic
i.

Min/Max-ModCod:
Define t he r ange o f
ModCods ov er which t he s ystem w ill op erate.
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.
ii.

Unlock-Action: Choose the d esired action
when t he remote demod l oses l ock. T his i s
important, as t he ACM s ystem depends o n t he
feedback of t he S NR me tric f rom the r emote
demod t o d etermine t he op timum M odCod. T he
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) ()
5
(cont)
iii.

Target Eb/No margin:
This i s a V ERY
important parameter. The ACM system is designed
to switch based on thresholds that correspond to a
BER of 5 x 10
-8
for each Mo dCod. However, i n
order to prevent oscillation around two ModCods at
this e xact v alue, 0 .3 dB o f hy steresis has been
added. T he s witch points a nd t he h ysteresis a re
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