Comtech EF Data CDM-625 - Manual (Page 409)

CDM-625 Advanced Satellite Modem Revision 15
Adaptive Coding and Modulation (ACM) MN-CDM625
17–13
17.11 Notes and Recommendations
• VersaFEC ACM is for point-to-point applications. It is required that both directions on the link
run in IP-ACM mode, although the symbol rates do not need to be equal. If you wish to
constrain one direction to run in CCM, simply set the Min and Max ModCod to be equal.
• ACM constitutes a closed-loop control system (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-625 modem 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.
• 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). We highly
recommend setting the Minimum ModCod to 0 (the ModCod of last resort) and set the
Unlock Action to ‘Go to minimum ModCod’. This will give the most robust link performance.
• When 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.
• 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 1dB – more 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-625. For
example, suppose the 4100 ksps FAST option is installed, and the symbol rate set to 4100
ksps, the theoretical maximum data rate would be 14 Mbps at ModCod 11. However, if CnC
is being used with a 10 Mbps FAST limit, then ACM Max ModCod will be limited to ModCod
7, or 9.6 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 BER versus Eb/No performance of the ModCods is identical to the VersaFEC CCM modes
described in the FEC Options Chapter.