Wirnet iBTS
Document title : Installation and
Maintenance Manual
Version : 2.0
Author : SNI
Date : 04/12/2018
Classification
This document is the strict property of Kerlink and shall not be either copied nor sent without express
written authorization of Kerlink
Internal Use
Kerlink m2m technologies reserved rights
Confidential
Kerlink – 1 rue Jacqueline Auriol – 35235 THORIGNÉ-FOUILLARD
Page 148 / 221
Strict confidential
3.3
Fresnel ellipsoid
Radio waves generally travel in a straight line from the emitter to the receiver. This is
obviously true when there are no obstacles between the transmitter and the receiver.
However, there are, most of the time, some obstacles between the transmitter and the
receiver. Then, the radio waves bump into the obstacles and are reflected or diffracted with
dephasing. These diffracted waves when arriving on the receiver can cause phase cancelling
with the straight line signals reducing the received power (fading). The fading effect depends
on the distance between the receiver and the emitter, the nature of the obstacles and the
associated out of phase.
Figure 88 : Fading effects due to obstacles
To minimize the fading effects, obstacles in a “Fresnel ellipsoid” must be avoided.
The Fresnel ellipsoid is a theoretical ellipsoid located between the transmitter and the
receiver.
The radius of the ellipsoid is defined as follows:
𝑟1 = √
𝑑1 ∗ 𝑑2 ∗ 𝑐
𝑓 ∗ (𝑑1 + 𝑑2)
Where:
d1 = distance from Tx antenna
d2 = distance from Rx antenna
f = frequency
c = celerity (3E8 m/s)
r1 = radius at the distance d1
A global rule is that 60% of the Fresnel ellipsoid must be clear of obstacles.
In case of buildings between the end point and the Wirnet iBTS, the antenna height must be
adjusted to make sure the building is not close to 60% of r1.
