Comtech EF Data CDM-750 - Manual (Page 109)

CDM-750 Advanced High-Speed Trunking Modem Revision 2
Front Panel Operation MN-CDM750
5–31
5.2.2.5.3 (CONFIG) ACM Config: Target-Es/No-Margin
The ACM system is designed to switch based on thresholds that correspond to a
QEF ≈ 10
‐7 PER for each ModCod.
Target Es/No Margin = 1.0 dB (0.0 – 4.5)
(
v)
Use the ST arrow keys to select a margin value between 0.0 and 4.5 dB, in 0.1 dB increments.
Press ENTER when done.
The Target Es/No Margin value, used as a tool to compensate for Rain Fade, is added to the QEF
specification for all ModCods. The QEF specification for a ModCod plus the Target Es/No Margin
plus the Modulation Degradation for a modulation type will set the “Shift Down” Es/No point in
ACM operation.
For example: The far end side of a link has a maximum rain fade slope of 1 dB per second. ACM
messages are sent roughly every 250 ms and it will take 290 ms for the message to reach the
near side modulator. Assuming worst‐case scenario of a strong rain fade that begins just after
sending an ACM message:
• There could be 250 ms of delay before the next message;
• The message will take 290 ms to traverse across the satellite link;
• There will be some minimal processing time of 20 ms to act on the new message and
change to the new ModCod;
• The new ModCod signal will land at the far end site in another 290 ms.
This equates to
250+290+20+290=550 ms . If there is a maximum rain fade of 1dB per second,
there could be as much as .55 dB of fade before the new ModCod is received at the fading site’s
demodulator. To ensure that missing a single ACM message does not drop the link, the user may
want to add another 250 ms, so now the max fade could be 800 ms – or 0.8 dB – of fade. For this
reason, it is preferred that the Target Es/No margin be at least 0.8 dB. Factory default is set to 1dB.
5.2.2.5.4 (CONFIG) ACM Config: Degradation
Degradation: QPSK=0.0 8PSK=0.0
16QAM=1.0 32APSK=2.0 
v
An additional level of signal margin can be added to the QEF switch points in ACM operation by
adding a degradation figure per modulation type. This degradation figure further increases the
Es/No required to switch to a ModCod having a degradation figure above 0.0.