Comtech EF Data CDM-570A Installation And Operation Manual page 663

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The ACM controller algorithm that resides in the CDM-570A/570AL-IP does not have to
switch through ModCods sequentially – it can change, if needed, directly from ModCod0 to
ModCod11 (or vice versa). When the demod first locks at ModCod0 the ACM controller will
examine the SNR from the remote end and switch directly to the ModCod that maximizes
throughput.
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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).
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For VersaFEC 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.
For VersaFEC-2 running in AMC mode, acquisition is always at ModCod 0, so acquisition
time is considerably quicker than VersaFEC.
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Running the ACM link with the Target Eb/No Margin set to 0 dB 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.
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For VersaFEC, the value of Max ModCod may be limited by other FAST codes installed
in the CDM-570A/570AL. For example, suppose the 3000 ksps FAST option is installed,
and the symbol rate set to 3000 ksps, the theoretical maximum data rate would be
10.239 Mbps at ModCod 11. However, if CnC is being used with a 5 Mbps FAST limit,
then ACM Max ModCod will be limited to ModCod 4, or 4.83 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 same applies
for VersaFEC-2.
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The BER versus Eb/No performance of the ModCods is identical to the VersaFEC CCM
modes described in the FEC Options Chapter.
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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 - 10
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To achieve minimum latency, set the WAN buffer to the smallest practical value. This will
depend on how well the connected equipment implements rate limiting, and the variation in
IP packet size. The default setting is 20 ms for VersaFEC, and 100 ms for VersaFEC-2.
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ACM and AUPC are supported at the same time.
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All IP features that are available in the CDM-570A/570AL-IP (VLAN, QoS, etc) are available
when in IP-ACM mode. The Sub-Mux feature, however, is not available.
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VersaFEC ACM and VersaFEC-2 are 100% compatible with Carrier-in-Carrier (CnC).
Adaptive Coding and Modulation (ACM) Option
CDM-570A/570AL Satellite Modem with Optional Packet Processor
For VersaFEC to achieve the most robust link performance, Comtech EF
Data strongly recommends that you set the Minimum ModCod to 0 (the
ModCod of last resort) and set the Unlock Action to 'Go to minimum
ModCod'. This allows the most robust link performance.
VersaFEC-2 automatically fixes the minimum ModCod to 0, and the Unlock
action is fixed to force the Tx side to automatically go to ModCod 0.
(Spectral Efficiency).
log
P–17
Revision 5
MN-CDM570A