Comtech EF Data CDM-625A - Manual (Page 700)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix M Revision 3
M–20
M.11 Notes and Recommendations
• VersaFEC and VersaFEC-2 ACM are for point-to-point applications. It is required that both
directions on the link run in IP-ACM or V2-ACM mode, although the symbol rates do not need
to be equal. For VersaFEC, if you wish to constrain one direction to run in CCM, simply set the
Min and Max ModCod to be equal. Note, however, that VersaFEC and VersaFEC-2 cannot be
mixed on the same bi-directional link.
• ACM constitutes a closed-loop control system (very similar in concept to AUPC) 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 1dB/second (more if Target Eb/No margin is
increased).
• The ACM controller algorithm that resides in the CDM-625A 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 ModCod11 to ModCod0 (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 0dB – the demod will lose lock
and the system will recover by switching to ModCod0 (if so configured, in VersaFEC).
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’.
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.
• 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 ACM mode, acquisition is always at ModCod0, so acquisition time is
considerably quicker than VersaFEC.
• 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 0.5dB – more if the fading events are
particularly severe and/or frequent.