User Guide

PMP 450 Planning Guide
Figure 28 One unsynchronized AP in cluster resulting in self-interference
The result is self-interference. In this scenario, the self-interference can be avoided only by synchronizing the
TDD transmit cycles of all APs that operate in the same frequency band.
An AP that is isolated by at least 5 miles (8 km) from any other equipment can generate and pass sync pulse
without GPS timing and not risk that interference will result from the generated sync. In any other type of
link, sync should be derived from GPS timing.
Advantage of GPS synchronization
Although the embedded timing generation capability of the AP keeps a precise clock (configuration parameter
Sync Input set to Generate Sync Signal), no trigger exists to start the clock at the same moment in each AP of a
cluster. So, the individual AP can synchronize communications between itself and registered SMs, but cannot
synchronize itself with other modules, except by GPS timing (shown in Figure 29).
Figure 29 GPS timing throughout the network
Time
Time
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pmp-0047 (March 2014)