User's Manual

LOS
Fresnel Zone clearances
Figure 10-6: Line of Sight (LOS) plus some clearance.
10.4 Link Budgeting
This section is intended to give an overview of what an RF engineer has to
consider in determining the expected performance of an RF link.
The Link Budgeting starts with the output of a transmitter power amplifier. That
signal goes through a cable to the antenna. The antenna radiates it in a certain
pattern. It then travels through the air, where there may be additional attenuation
from rain. It finally arrives at the Receive antenna, down a cable, and into the
receiver itself. The receiver is usually rated for its “Sensitivity” at a certain bit
error rate. Any signal larger than that is assumed to have been received without
error.
The difference between the received signal and the Receive Sensitivity is the
margin for error, called Fade Margin.
A typical calculation of Fade Margin is:
Access Point Power amplifier output = 20 dBm
Access Point Transmit antenna gain = 14 dBi
Free Space Path Loss (2 miles @ 5 GHz)= 117 dB
Rain fade = 1 dB
CPE Transceiver Receive antenna gain = 22 dBi
CPE Transceiver Gain = 38 dB
ARCXtend manual, August 2003 10-6