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Appendix G
CDM-760 Advanced High-Speed Trunking Modem
G.4
CDM-760 ACM Scheme
The description of ACM operation in this section serves as the prelude to configuring the CDM-
760 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
G.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.
For example: An Es/No Margin of 1.0dB 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 300-400ms of path delay AND a 250ms (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/second 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 G-4
depicts a graph of ACM messaging delay (including satellite delay).
For example: A 10 Msps link is run and has a maximum rain fade of 1 dB/second. At 10 Msps
there will be a single hop latency of 300ms and an additional maximum ACM message gap of
250ms. So, if a 1.0dB rain fade began just after an ACM message was sent it would be
250 = 550ms
upon by the near side modulator. The near side modulator would then process the request
(20ms), change its ModCod, and the signal would be sent back to the far end demodulator
landing at the far side in another 300ms. In this example, the maximum time it takes from when
the fade begins to receipt of the new ModCod is
maximum fade of 1dB/second and with 850ms of total round trip time to act on an ACM
message, the minimum ACM margin you would require would be
0.87 dB
."
before an updated ACM message with the reduced Es/No is received and acted
300 + 250 + 20 + 300 = 870ms
1dB/second * 870ms =
G–9
Revision 4
MN-CDM760
300 +
. At a
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