Figure 5-4: Fresnel Zone Obstruction - Alvarion BreezeACCESS LB System Manual

The FSL value is dependent on the range between the LB NI-01 Indoor
units, the type of terrain over which the link is deployed, and whether
or not the link is operating line of sight (LOS), optical line of sight
(OLOS) or near-LOS (NLOS). The LOS FSL calculation is well
understood and easy to calculate, and relies on the fact there is
absolutely no obstacle near the direct path. The precise method for
determining the amount of clearance required for LOS involves making
use of a factor known as the Fresnel zone. A Fresnel zone is defined as
a path difference of λ/2 away from the direct path, as shown in Figure
29. A "cleared LOS" link assumes there are no obstacles within 60% of
the first Fresnel zone of the direct path. The diagram below illustrates
OLOS conditions, where a treetop is within the first Fresnel zone, and a
clear direct path exists between the antennas.
First
Fresnel
Zone

Figure 5-4: Fresnel Zone Obstruction

The formula for calculating the radius of the first Fresnel zone, as
shown below, is given as (in ft):
D * 1 D 2
=
R 72 1 .
+
f ( * D 1 D ) 2
Where,
D1 and D2 are the distances from the LB units to the point of interest
(in miles); and,
f is the frequency (in GHz)
(Note: multiply results by 0.3048 to obtain a solution in meters).
Obstruction within
first Fresnel Zone
Wireless Facts
5-9