Advanced Configuration Features
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Horizon Compact Release 1.01.00 Wireless Ethernet Product User Manual – Volume 2
3.11 Automatic Adaptive Modulation
The two principal modulation schemes used on the Horizon Compact system are QPSK and QAM. QPSK
(the lowest modulation scheme) is ideal for long distance, but has the lowest throughput capability.
Higher throughputs are achieved by using more complex modulation schemes e.g. 16-QAM, 32-QAM,
64-QAM, 128-QAM, 256-QAM. The higher numbers indicate a progressively more complex scheme and a
higher bandwidth (throughput) capability e.g. 256-QAM is more complex than 128-QAM and provides a
higher throughput. More complex modulation schemes are susceptible to noise and thus require a
stronger signal for the demodulator to accurately decode the data stream. Consequently, the more
complex the modulation scheme used, the shorter the distance limitation of the radio link.
If a system is using a given modulation scheme and weather conditions cause signal levels to deteriorate
below acceptable levels (risking a link failure), changing the modulation scheme to a less complex
scheme, will allow the link to remain functional, but with a lower throughput, until weather conditions
improve. The modulation scheme can then be returned to the original scheme and the throughput
returned to normal levels.
The Horizon Compact system can be configured to automatically change modulation schemes if
environmental conditions deteriorate to the point where a wireless link may otherwise fail. This feature is
called Automatic Adaptive Modulation (AAM). Note that AAM cannot be invoked if RLS is enabled.
The system will cause the current system mode (which determines the modulation scheme) to be
changed to the available system mode having the lowest modulation scheme (a number of system
modes, which includes the modulation scheme, are available for any given radio band, see Volume 1), if
the Horizon Compact units lose Modsync for 5 seconds or more.
The original modulation scheme (system mode) will be restored once preset parameters indicate that
conditions are suitable for returning to the original modulation scheme (and return to the original
bandwidth).
Note that if ATPC is active when an AAM downshift is invoked, any reduced power setting incurred as a
result of ATPC will be overridden and the power setting adjusted to the highest possible for the applicable
modulation scheme (system mode). When AAM reverts back to normal modulation, the power level will
remain at that last set during the modulation downshift (high), leaving ATPC to adjust the power to an
appropriate value.
The total outage time due to modulation downshift is typically 1 to 2 seconds on average. The time taken
to return to the original modulation scheme (upshift) is typically 100 mS to 300 mS.
AAM is supported by all radio bands available to the Horizon Compact, however, some jurisdictions may
not allow its use.
There are three CLI commands associated with AAM:
• get aam status
• set aam <on/off>
• diagnose aam <up/down>
The first two commands are shown in the procedure below. The third command is for diagnostic purposes
and enables the user to force the modulation scheme to one state or the other.
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