Chapter 3: System planning
Radio Frequency planning
Page
3-19
Obstructions in the Fresnel Zone
The Fresnel (pronounced fre
∙
NEL) Zone is a three-dimensional volume around the line of sight of an
antenna transmission. Objects that penetrate this area can cause the received strength of the
transmitted signal to fade. Out-of-phase reflections and absorption of the signal result in signal
cancellation.
The foliage of trees and plants in the Fresnel Zone can cause signal loss. Seasonal density, moisture
content of the foliage, and other factors such as wind may change the amount of loss. Plan to perform
frequent and regular link tests if you must transmit through foliage.
Planning for co-location
Co-location between OFDM (450 Series) and FSK (PMP/PTP 1x0)
The first step to avoid interference in wireless systems is to set all AP/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 AP/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/BHM transmits, which could overpower that signal.
The following parameters on the AP/BHM determine the transmit/receive ratio:
•
Downlink Data percentage
•
Frame Period
•
Max Range
•
(reserved) Contention slots
If OFDM (450 Platform Family) and FSK (PMP/PTP 1x0) APs/BHMs of the same frequency band and
channel bandwidth are in proximity, or if you want BHMs set to different parameters then you must use
the Frame Calculator to identify compatible settings for APs/BHMs.
The co-location is also supported for 900 MHz PMP 450i Series APs (OFDM) and PMP 100 Series APs
(FSK).
The Frame Calculator is available on the web management interface Tools > Frame Calculator. To use
the Frame Calculator, type into the calculator various configurable parameter values for each proximal
AP/BHM and then record the resulting AP/BHM Receive Start value. Next vary the Downlink Data
percentage in each calculation and iterate until a calculated AP/BHM Receive Start for all co-located
AP/BHMs where the transmit end does not come before the receive start.
Cambium also provides co-location tool which helps in co-location planning:
Note
Refer to Frame Alignment Legacy Mode parameter of Configuration > Radio > Advance tab
for legacy product settings (See Configuration Guide
)
.
