User Guide

PMP 450 Planning Guide Analyzing the RF Environment
pmp-0047 (December 2012)
2-23
Planning for co-location and using the OFDM Frame Calculator Tool
The first step to avoid interference in wireless systems is to set all APs to receive timing from a synchronization
source (Cluster Management Module, or Universal Global Positioning System). This ensures that the modules are
in sync and start transmitting at the same time each frame.
The second step to avoid interference is to configure parameters on all APs of the same frequency band in
proximity such that they have compatible transmit/receive ratios (all stop transmitting each frame before any start
receiving). This avoids the problem of one AP attempting to receive the signal from a distant SM while a nearby AP
transmits, which could overpower that signal.
The following parameters on the AP determine the transmit/receive ratio:
• Max Range
• Downlink Data percentage
• (reserved) Control Slots
If OFDM (PMP 430, PMP 450, PTP 230) and FSK (PMP 1x0) APs of the same frequency band are in proximity, or
if you want APs set to different parameters (differing in their Max Range values, for example), then you should use
the Frame Calculator to identify compatible settings.
The frame calculator is available on the Frame Calculator tab of the Tools web page. To use the Frame Calculator,
type into the calculator various configurable parameter values for each proximal AP, and then record the resulting
AP Receive Start value. Next vary the Downlink Data percentage in each calculation and iterate until the
calculated AP Receive Start for all collocated APs are within 300 bit times; if possible, within 150 bit times. In
Cambium Point-to-Multipoint systems, 10 bit times = 1 µs.
The calculator does not use values in the module or populate its parameters. It is merely a convenience application
that runs on a module. For this reason, you can use any FSK module (AP, SM, BHM, BHS) to perform FSK frame
calculations for setting the parameters on an FSK AP and any OFDM module (AP, SM, BHM, BHS) to perform
OFDM frame calculations for setting the parameters on an OFDM AP.
IMPORTANT!
APs that have slightly mismatched transmit-to-
receive ratios and low levels of data traffic may
see little effect on throughput. A system that was not tuned for co-location may work fine at
low traffic levels, but encounter problems at higher traffic levels. The conservative practice is
to tune for co-location
before traffic ultimately increases. This prevents problems that occur as
sectors are built.