User's Manual

PMP 450 Planning Guide
Diagramming Network Layouts
pmp-0047 (June 2013)
2-31
Diagramming Network Layouts
Avoiding Self Interference
The following section includes information maximizing tower performance by minimizing self-interference.
Physical Proximity
Two AP clusters co-located on the same tower require a CMM. The CMM properly synchronizes the transmit start
times of all modules to prevent interference and desensing of the modules. At closer distances without GPS
synchronization, the frame structures cause self-interference. Non-synchronized deployments are highly
discouraged.
Furthermore, non-synchronized APs on the same tower require that the effects of their differing receive start times
be mitigated by either
 100 vertical feet (30 meters) or more and as much spectral separation as possible within the same frequency
band range
 the use of the frame calculator to tune the Downlink Data parameter in each, so that the receive start time in
each is the same
The constraints for collocated modules in the same frequency band range are to avoid self-interference that would
occur between them. Specifically, unless the uplink and downlink data percentages match, intervals exist when one
is transmitting while the other is receiving, such that the receiving module cannot receive the signal from the far
end.
The interference is less a problem during low throughput periods and intolerable during high. Typically, during low
throughput periods, sufficient time exists for the far end to retransmit packets lost because of interference from the
collocated module.
Spectrum Analysis
You can use an SM as a spectrum analyzer. See Mapping RF Neighbor Frequencies on Page 2-20. Through a toggle
of the Device Type parameter, you can temporarily transform an AP into an SM to use it as a spectrum analyzer.
SM Automatic Transmit Power Control
The PMP 450 AP automatically sets the transmitter output power in its SMs through a feature named Auto-TPC
(Transmit Power Control). The conceptual reason for this feature is OFDM reception in the AP is sensitive to large
differences in power levels received from its SMs, and by limiting power levels of close-in SMs the overall RF
noise floor is lowered.