Cambium PTP 450 series, Руководство пользователя

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Analyzing the RF Environment PTP 450 User Guide
ptp-450 (July 2014)
2-63
Planning for co-location and using the OFDM Frame Calculator Tool
The first step to avoid interference in wireless systems is to set all BHMs 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 BHMs 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 BHM attempting to receive the signal from a distant BHS while a nearby
BHM transmits, which could overpower that signal.
The following parameters on the BHM determine the transmit/receive ratio:

Max Range

Downlink Data percentage

(reserved) Control Slots
If OFDM (PTP 430, PTP 450, PTP 230) and FSK (PTP 1x0) BHMs of the same frequency band are in proximity,
or if you want BHMs 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 BHM, and then record the
resulting BHM Receive Start value. Next vary the Downlink Data percentage in each calculation and iterate until
the calculated BHM Receive Start for all collocated BHMs are within 300 bit times; if possible, within 150 bit
times. In Cambium Point-to-Multipoint systems, 10 bit times = 1
µ
s.
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 (BHM, BHS, BHM, BHS) to perform FSK
frame calculations for setting the parameters on an FSK BHM and any OFDM module (BHM, BHS, BHM, BHS) to
perform OFDM frame calculations for setting the parameters on an OFDM BHM.
IMPORTANT!
BHMs 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.