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Appendix G
CDM-760 Advanced High-Speed Trunking Modem
G.4.8 Notes and Recommendations
•
CDM-760 ACM operation is designed for point-to-point operation only. Future point to multi-
point operation will be introduced in a later release of the product. ACM operation and
purchased option are required on all modems running the link (two in non-redundant mode
and four in 1:1 redundant mode).
•
ACM constitutes a closed-loop control system similar in concept to Automatic Uplink Power
Control (AUPC). It should be remembered that, like all control systems, the speed at which the
system can react is governed by a number of factors, including: the time taken to estimate
Es/No to the required accuracy at the far side demodulator; the transport delay over the
satellite; and the processing of messages and associated action taken at the near side
modulator.
•
The ACM controller algorithm that resides in the CDM-760 does not have to switch through
ModCods sequentially – it can change, if needed, directly from ModCod4 to ModCod28 (or
vice versa).
•
While ACM can do remarkable things, the fundamentals still apply. The demodulator will not
run at a 16APSK ModCod if the Es/No instantaneously drops to 0dB – the demodulator will
lose lock and the system will recover by switching to ModCod4 (if so configured). The
recommended setting for Minimum ModCod is 4. Comtech EF Data also recommends that
you set the Unlock Action to 'Go to minimum ModCod' – this will give the most robust link
availability.
•
Running the ACM link with the Target Es/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, use a
increase the target Es/No Margin 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-760. For
example, suppose the 15 Msps FAST option is installed, and the symbol rate is set to 15
Msps; the theoretical maximum data rate would be 52.2 Mbps at ModCod 23. However, if
CnC is being used with a 40 Mbps FAST limit, the ACM Max ModCod will be limited to
ModCod 18, or 38.6 Mbps. If, for a given symbol rate, it is not possible to set Max ModCod
to the desired value, you should therefore check to determine what other FAST codes may
be limiting Max ModCod.
•
Es/No 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 = Es/No – 10log(Spectral Efficiency).
•
To achieve minimum latency, set the WAN buffer to the smallest practical value. The default
setting is 20ms, and Comtech EF Data recommends keeping it at this level.
•
ACM is 100% compatible with DoubleTalk Carrier-in-Carrier (CnC) and Dynamic Predistortion
(DPD).
•
ACM maximizes throughput not only when Es/No varies due to atmospheric conditions, but it
also mitigates the effects of other impairments such as antenna pointing error, excessive
phase noise, and certain types of interference. However, rapidly fluctuating impairments, such
as scintillation at low antenna look-angles at C-band, will generally not be improved by ACM or
will require abnormally large Target Es/No Margin.
•
ACM modes are not compatible with CCM modes, due to differences in frame preambles.
G–18
Revision 4
MN-CDM760
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