RACOM Ray2, User Manual

Page: 16 / 235

is active we recommend using horizontal polarization in the direction with lower data traffic (typically
up-link).
2.1.5. Multipath fading
Multipath fading is another dominant fading mechanism. A reflected wave causes a phenomenon known
as multipath, meaning that the radio signal can travel multiple paths to reach the receiver. Typically,
multipath occurs when a reflected wave reaches the receiver at the same time in opposite phase as
the direct wave that travels in a straight line from the transmitter.
Multipath propagation gives rise to two kinds of signal degrading effects, i.e., flat fading and frequency
selective fading. Flat fading is a reduction in input signal level where all frequencies in the channel of
interest are equally affected and is dependent on path length, frequency, and path inclination. In addition,
it is strongly dependent on the geoclimatic factor K.
To calculate the probability of outage due to multipath propagation of microwave links the ITU-R
probability model can be used which describes a single frequency (or narrowband) fading distribution
suitable for large fade depths A in the average worst month in any part of the world (based on ITU-R
P.530-14). The calculation for detailed link design is given as follows [1]:
P
0
= Kd
3.4
(1+| ε
P
|)
-1.03
f
0.8
×10
0.00067h
L
-A/10
where:
d link distance (km)
f frequency (GHz)
h
L
altitude of lower antenna (m)
A fade depth (dB)
K is geoclimatic factor and can be obtained from:
K = 10
-4.6-0.0027dN1
The term dN1 is provided on a 1.5° grid in latitude and longitude in ITU-R Recommendation P.453.
The data are available in a tabular format and are available from the Radiocommunication Bureau
(BR). E.g. in Central Europe the values dN1 range from -242 to -362.
From the antenna heights h
e
and h
r
(meters above sea level), calculate the magnitude of the path in-
clination │ε
P
│ (mrad) using the following expression:
where:
d link distance (km)
h
r
, h
e
antenna heights above sea level (m)
RAy2 Microwave Link – © RACOM s.r.o. 16
Implementation Notes