User Guide

Configuration options for TDD synchronization Planning considerations
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pmp-0047 (December 2012)
Alternative to GPS synchronization
A link can operate without GPS sync, but cannot operate without sync. The alternative to GPS sync is to configure
the AP in the link to generate a sync pulse to pass to the SM. Depending on the RF environment in which the link
operates, this latter alternative may or may not be plausible.
For example, in Figure 22, AP4
• is not synchronized with any of the other APs.
• is transmitting nearby the other APs while they are expecting to receive SM transmissions from a maximum
distance.
Figure 22 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
Source 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 23).
Time