Avoiding Interference

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 de-sensing 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 may 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 a SM 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.

Avoiding Other Interference

Where signal strength cannot dominate noise levels, the network experiences:

  • Packet errors and retransmissions.
  • Lower throughput (because bandwidth is consumed by retransmissions) and high latency (due to resends).

Regular spectrum analysis is critical to RF planning. The integrated spectrum analyzer can be very useful as a tool for troubleshooting and RF planning, but is not intended to replicate the accuracy and programmability of a high-end spectrum analyzer, which you may sometime need for other purposes.


When you enable the Spectrum Analyzer on a module, it enters a scan mode and drops any RF connection it may have had. Scanning mode ends when either you click Disable on the Spectrum Analyzer page, or it times out after 15 minutes and returns to operational mode.

For this reason:

  1. Do not enable the spectrum analyzer on a module you are connected to via RF. The connection will drop for 15 minutes, and when the connection is re-established no readings are displayed.
  2. Be advised that, if you enable the spectrum analyzer by Ethernet connection, the RF connection to that module drops.

You can use any module to see the frequency and power level of any detectable signal that is within, just above, or just below the frequency band range of the module.


Vary the days and times when you analyze the spectrum in an area. The RF environment can change throughout the day or throughout the week.