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CDM-625A Advanced Satellite Modem
Appendix M
the ACM controller switches to higher ModCods sequentially until the highest possible
ModCod has been reached . It may, therefore, take several seconds for the link to reach
maximum throughput. Conversely, if SNR is degrading rapidly (in a fade) then it can
change, if needed, directly from ModCod11 to ModCod0. This gives the fastest possible
response and helps keep the link intact under fast fading events.
•
While ACM can do remarkable things, the fundamentals still apply. Don't expect the
demod to run at a 16-QAM ModCod if the SNR instantaneously drops to 0dB – the demod
will lose lock and the system will recover by switching to ModCod0 (if so configured). We
highly recommend setting the Minimum ModCod to 0 (the ModCod of last resort) and
set the Unlock Action to 'Go to minimum ModCod'. This will give the most robust link
performance.
•
When running in ACM mode the demodulator is performing blind acquisition – meaning
that it has no a priori knowledge of the modulation type or code rate. For this reason the
demodulator acquisition time will be slower than in CCM mode. However, the acquisition
time is typically under 1 second for all symbol rates and noise conditions.
•
Running the ACM link with the Target Eb/No Margin set to 0dB will give the best
utilization of link power, but in conditions of fast fading may cause demod unlock
events, or highly degraded BER just prior to the switch to a lower ModCod. In order to
mitigate this, we recommend a Target Eb/No Margin of at least 1dB – more if the fading
events are particularly severe and/or frequent.
•
The value of Max ModCod may be limited by other FAST codes installed in the CDM-
625A. 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. Therefore, if it is not possible to set the Maximum
ModCod to the desired value for a given symbol rate, you should check to determine
what other FAST codes may be limiting it.
•
The BER versus Eb/No performance of the ModCods is identical to the VersaFEC CCM
modes described in the FEC Options Chapter.
•
SNR is the preferred metric for driving the adaptation – this is the value displayed on the
monitor screens. If you want to convert this to Eb/No then remember that the
relationship is simply Eb/No = SNR - 10log(Spectral Efficiency).
•
To achieve minimum latency, set the WAN buffer to the smallest practical value. The
default setting is 20ms, and we recommend keeping it at this level.
•
All IP features that are available in the CDM-625A (VLAN, QoS, etc) are available when in
IP-ACM mode. The Sub-Mux feature, however, is not available.
•
VersaFEC ACM is 100% compatible with Carrier-in-Carrier.
M–14
Revision 0
MN-CDM625A
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