Comtech EF Data CDM-750 - Manual (Page 244)

CDM-750 Advanced High-Speed Trunking Modem Revision 2
Appendix C MN-CDM750
C–8
C.4 CDM-750 ACM Scheme
The description of ACM operation in this section serves as the prelude to configuring the
CDM‐750 for ACM operation. The key items discussed are:
• Target Es/No Margin
• Distortion – allowing for degradation by modulation type
• Pilots – ON/OFF
• Standard and Short Frames
• Monotonic operation
• ModCod selection
C.4.1 Target Es/No Margin
The Target Es/No Margin adds a fixed amount of margin to all of the modem’s specified Es/No
values. Links normally include margin to meet performance criteria. When ACM is enabled, a
method is needed to add margin to the guaranteed Es/No specification so the modem will
switch to the next ModCod in a way that maintains a level of performance, generally near the
QEF point (PER ≈ 10‐7); the Target Es/No Margin provides this. This Es/No margin is added to the
published guaranteed Es/No specification per ModCod for ACM operation – the value of Target
Es/No ranges from 0 to 4.5 dB.
Example: An Es/No Margin of 1.0 dB would mean that the minimum Es/No required to run at
ModCod (5) QPSK 3/5 would be the guaranteed Es/No (2.7 dB) plus the Target Margin (1.0 dB) =
3.7 dB.
The Target Es/No Margin can and should also be used as a way to dial in a maximum fade slope.
ACM messages sent from the far side demod to the near side modulator contain the Es/No
estimate that the demod was seeing at that particular time; the ACM message is sent four (4)
times per second. In the worst case scenario, due to the distance traveled over satellite and the
symbol rate of the link, there can be 300‐400 ms of path delay AND a 250 ms (1/4 second)
messaging gap from the last time the Es/No is measured by the far side demod to the time the
near side modulator can act on this measurement. If the maximum slope of a fade (e.g., rain
fade) is 1.0 dB/s it would desirable to minimally set the Target Es/No to a figure that would be
greater than the maximum possible fade. To help choose the Es/No Margin, Figure C‐4 depicts a
graph of ACM messaging delay (including satellite delay).
Example: A 10 Msps link is run and has a maximum rain fade of 1 dB per second. At 10 Msps
there will be a single hop latency of 300ms and an additional maximum ACM message gap of
250 ms. So, if a 1.0 dB rain fade began just after an ACM message was sent it would be
300+250=550ms before an updated ACM message with the reduced Es/No is received and