Comtech EF Data CDD-880, Installation And Operation Manual

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CDD-880 Multi Receiver
Router
MN-CDD880
Appendix H Revision 2
H–12
H.6 Notes and Recommendations
•
ACM constitutes a closed-loop control system and 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 SNR to the required accuracy, and the transport delay
over the satellite. Realistically, it can cope with fading and other link impairments that do not
exceed 1 dB/second (more if Target Eb/No margin is increased).
•
The ACM controller algorithm that resides in the CDM-840 modem exhibits a different
response depending on whether SNR is increasing or decreasing. If the SNR is improving, 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 MODCOD 11 to MODCOD 00 (VersaFEC example). 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-ARY MODCOD if the SNR instantaneously drops to 0 dB – the demod will lose lock
and the system will recover by switching to MODCOD 00 (if so configured, in VersaFEC).
To achieve the most robust link performance, Comtech EF Data strongly
recommends that you set the Minimum MODCOD to 00 (the MODCOD of last
resort), and set the Unlock Action to ‘Go to minimum MODCOD’.
•
Running the ACM link with the Target Es/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 Es/No Margin of at least 0.5 dB – more if the fading events are
particularly severe and/or frequent.
• 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 - 10
log
(Spectral Efficiency).
•
All IP features that are available in the CDM-840 (VLAN, QoS, etc) are available while in ACM
mode.
• ACM maximizes throughput not only when Eb/No varies due to atmospheric conditions, but
will also mitigate the effects of other impairments, such as antenna pointing error, excessive
phase noise and certain types of interference. However, rapidly fluctuating impairments (i.e.,
~ less than 1 second) such as scintillation at low antenna look-angles at C-band will generally
not be improved by ACM.