Comtech EF Data CDM-760 - Manual (Page 106)

Front Panel Operation Revision 2
CDM-760 Advanced High-Speed Trunking Modem MN-CDM760
6–20
For Example: An ACM system is running on a Ku-Band GEO satellite with a symbol rate of
5Msps. There is a maximum fade rate of 1.5 dB/sec on this link.
Using this table, note that a 5 Msps link on a GEO satellite requires a minimum processing time
of 863 ms. With a maximum fade rate of 1.5dB/sec this means 0.863 * 1.5 = 1.2945 dB. Target
Es/No Margin therefore needs to be set at 1.3 dB at MINIMUM in order to ensure that, in the
maximum fade scenario, demod lock is not lost and faulty data does not pass.
The ACM algorithm has an extremely fast ACM messaging system – the CDM-760 sends ACM
messages to the far side transmitter every 100 ms. As fast as the ACM messages are
transmitted, there are many other factors in an ACM environment to consider that impact the
overall service. These factors can generally be divided into two categories: message processing,
and round trip time:
• Message processing accounts for all ACM messaging, demodulator processing, ACM
switching engine, and time between ACM messages;
• Round trip time is simply the trip over a satellite and back.
This example shows why a very fast 100ms ACM message requires 863ms of total latency:
1 Last ACM message was just sent Time (T = -1 ms)
2 T=0: Worst fade conditions begin
3 T = 100 ms: ACM message is sent to far side transmitter
4 T = 100 + 280 ms: (GEO) ACM message lands at far side transmitter over 1 satellite hop
5 T= 380 + 50 ms: Far side transmitter processes and begins to send new lower MODCOD
6 T= 430 + 280 ms: New lower MODCOD lands at demod over 2 nd satellite hop
7 T= 710 + 53 ms: (5 Msps) The new MODCOD is being processed by decoder
8 T = 763 + 100 ms: This MODCOD must be valid for 100 ms until next MODCOD arrives
9 T = 863 ms: This is the maximum latency this ACM circuit requires per the Symbol Rate
used and the satellite (GEO) used.
As outlined, even with a very fast 100 ms ACM messaging system, there are still significant
elements that play a part in the overall latency of the ACM system. All elements must be taken
into account in order to keep the demodulator locked and the data error free.