Cambium Networks PMP 450 AP, Configuration Manual

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Chapter 2: Tools Using the OFDM Frame Calculator tool
Page
2-357
Using the OFDM Frame Calculator tool
The first step to avoid interference in wireless systems is to set all APs/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 APs/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 AP/BHM attempting
to receive the signal from a distant SM/BHS while a nearby AP transmits, which could overpower that
signal.
The following parameters on the AP determine the transmit/receive ratio:
• Max Range
•
Frame Period
• Downlink Data percentage
•
(reserved) Contention Slots
If OFDM (PMP 430, PMP 450, PTP 230) and FSK (PMP 1x0) APs/BHMs of the same frequency band are
in proximity, or if APs/BHMs set to different parameters (differing in their Max Range values, for
example), then operator must 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 various configurable parameter values into the calculator for each proximal AP and
then record the resulting AP/BHM Receive Start value. Next vary the Downlink Data percentage in each
calculation and iterate until the calculated AP/BHM Receive Start for all collocated AP/BHMs where the
transmit end does not come before the receive start.
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/BHM.
For more information on PMP/PTP 450 Platform co-location, see
https://support.cambiumnetworks.com/files/colocationtool/ The co-location is also supported for 900
MHz PMP 450i APs (OFDM) and PMP 100 APs (FSK). Please refer Co-location of PMP 450 and PMP 100
systems in the 900 MHz band and migration recommendations document for details .
Caution
APs/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.
The OFDM Frame Calculator page is explained in Table 109.