Horizon Compact Plus Release 1.0.1
Wireless Ethernet Product User Manual
– Volume 2
16.0 Adaptive Modulation
The two principal modulation schemes used on the Horizon Compact Plus system are QPSK and
xQAM. 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 (lower order) scheme, will allow the link to remain functional, although the throughput will be
lower, until weather conditions improve. The modulation scheme can then be returned to the original
scheme and the throughput returned to normal levels. Horizon Compact Plus has a Hitless Automatic
Adaptive Modulation scheme.
16.1 Hitless Automatic Adaptive Modulation
The Horizon Compact Plus system can be configured to automatically switch to lower modulation
schemes if environmental conditions deteriorate to the point where a wireless link may otherwise fail.
Compact Plus
will do this without causing a “hit” or losing data. This feature is called Hitless Automatic
Adaptive Modulation (HAAM).
All radio bands available with the Horizon Compact Plus support HAAM.
Note that HAAM cannot be invoked if RLS, ATPC, or redundancy (not supported in this release) is
enabled.
When HAAM is turned on, regardless of the user configured system mode (includes modulation
scheme), the system switches to the highest modulation scheme available (system mode) for the radio
band being used.
If weather conditions cause the Signal to Noise Ratio (SNR) to drop below the “Down SNR” threshold
for the modulation scheme in use (see
Table 16-1
), the current modulation scheme will switch to the
next lowest modulation scheme available. If the SNR continues to fall and drops below the “Down SNR”
threshold for that modulation scheme, the system will switch again to the next lowest modulation
scheme. If the SNR still continues to fall, the system will continue to downshift through all the available
modulation schemes until it reaches the lowest order scheme for the radio band in use.
Example: Initial modulation scheme is 256QAM. If the SNR falls bel
ow the “Down SNR” threshold for
256QAM, which is 30 dB, the system will switch to 128QAM. The “Down SNR” threshold for 128QAM is
27.17 dB. If the SNR drops below this new threshold, then a modulation downshift will occur again to
64QAM. The system will continue downshifting as the SNR thresholds are encountered until QPSK is
reached.
Table 16-1 Modulation Up/Downshift SNR Thresholds
These values are common to all radio bands
and channel bandwidths.
Regular, 10*-12 performance
Mode
Up SNR, dB
Down SNR, dB
QPSK
22.84
N/A
16QAM
23.68
18.11
32QAM
26.00
21.42
64QAM
29.02
24.18
128QAM
31.53
27.17
256QAM
N/A
30.00
