Trango Systems TrangoLINK GigaPlus User Manual page 36

ACM Detailed Description
If the channel conditions degrade due to multipath or fading, the MSE may be affected.
This typically occurs during heavy rain and is more pronounced with higher frequencies
and longer transmission paths.
If the degrade threshold shown in the table is exceeded, the ACM engine will coordinate
a shift of the modulation level down to the next level shown. No packets will be
dropped during this transition since the two ends of the link are coordinating with each
other to ensure that the switch occurs at the same time. Since lower modulation levels
are more robust against channel fading and multipath, they can tolerate a higher MSE as
is evident from the table.
If the link conditions continue to degrade, the shifts downward will continue until the
lowest modulation level, QPSK, has been reached. If further degradation occurs above
the maximum MSE (shown previously in Table 5) for QPSK, the link will be dropped and
packet loss will occur. If the link MSE starts to improve, however, the ACM engine will
gradually shift the modulation level up as the improve thresholds are passed.
When ACM is active the modulation level may be asymmetrical, meaning that one
direction may be running at QPSK and the other may be running at the originally set
higher modulation such as 256 QAM. This is because the ACM engine acts
independently at each endpoint of the link. A low level Binary Phase Shift Keying (BPSK)
channel is maintained out of the data path between the two ends of the link to allow
very tight coordination of the switches in modulation. Only data traffic capacity in the
direction towards the affected receiver will be reduced, maintaining performance on
the unaffected direction.
During all ACM operation the transmitter power remains at the set level if ATPC is off. If
enabled, ATPC acts normally when ACM is active and may increase or decrease the
transmit power based on RSSI levels.
The acm command provides configuration of the ACM and must be done on both sides
of the link. The linktest command from the CLI can be used to view the current TX and
RX modulation levels.
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Trango Systems, Inc. GigaPlus User Manual Rev 3.1