CDM-570A/570AL Satellite Modem with Optional Packet Processor
Revision 5
Adaptive Coding and Modulation (ACM) Option P–11 MN-CDM570A
(SELECT) CONFIG: TI IP-ACM Target SNR
Margin
Use the ▲▼ arrow keys to select a margin value from
0.0 to 4.5 dB, in 0.5 dB increments. Press ENTER.
This is a VERY important parameter. The ACM system is
designed to switch based on thresholds that correspond to a
BER of 5 x 10
-8
for each ModCod. However, in order to
prevent oscillation around two ModCods at this exact value,
0.3 dB of hysteresis has been added. The switch points and
the hysteresis are shown in Figure P-2.
1.0 dB (0.0 – 4.5) (▲ ▼)
P.6.3 Switch Points
Figure P-2 shows the switch points with the Target Eb/No margin set to 0 dB. However, the
switch points can be moved (increased) by configuring the Target Eb/No margin parameter,
which can vary from 0 to 4.5 dB, in 0.5 dB steps. In a fading environment, it is strongly
recommended to add sufficient margin to maintain an adequate link quality (and to maintain
demod lock) during the interval between the Eb/No degrading and the ACM controller responding
by lowering the ModCod. See Secion P.9, Notes and Recommendations.
Figure P-2. CDM-570A/570AL-IP ACM ModCod Switch Points
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