K.11 Notes And Recommendations - Comtech EF Data CDM-425 Installation And Operation Manual

K.11 Notes and Recommendations

Appendix B. FORWARD ERROR CORRECTION (FEC) OPTIONS.
•
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-425 Advanced Satellite 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 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.
Appendix K
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.
CDM-425 Advanced Satellite Modem
K–25
Revision 0
MN-CDM-425