Analyzing the RF Environment
PTP 450 Configuration and User
pmp-0815 (August 2015)
4-15
Anticipating Reflection of Radio Waves
In the signal path, any object that is larger than the wavelength of the signal can reflect
the signal. Such an object can even be the surface of the earth or of a river, bay or lake.
The wavelength of the signal is approximately
2 inches for 5.4 GHz and 5.8 GHz signals.
3.4 inches for 3.5 GHz and 3.6 GHz signals.
A reflected signal can arrive at the antenna of the receiver later than the non-reflected
signal arrives. These two or more signals cause the condition known as multipath.
Multipath may increase or decrease the signal level and so overall attenuation may be
higher or lower than that caused by the link distance. This is problematic at the margin of
the link budget, where the standard operating margin (fade margin) may be compromised.
Noting Possible 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 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:
Downlink Data percentage
(reserved) Contention slots
If OFDM (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 then you must use the Frame
Calculator to identify compatible settings.
