RACOM RAy11, Руководство пользователя

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2. Implementation Notes
2.1. Link calculation
Before a microwave link can be installed, an analysis and calculation of the microwave link must be
made first. The analysis should take place before the site survey itself to get a clear idea about the di-
mensions of the antennas. The analysis consists of the following steps:
• Free space loss calculation
• Link budget calculation
• Rain attenuation
• Multipath fading
• Fade margin
• Fresnel zones calculation
This chapter explains the individual steps and an example of link design is given at the end.
2.1.1. Free space loss calculation
As the electromagnetic waves travel through open space they are attenuated. This attenuation is de-
scribed as Free-space Loss. This loss depends on the distance travelled by signal and its frequency.
Longer distance means greater attenuation and higher frequency means greater attenuation. Free-
space loss can be calculated thus:
FSL = 32.44 + 20log f + 20log D
Where:
FSL free-space loss (dB)
f frequency of the emitted signal (MHz)
D length of the link (km)
2.1.2. Link budget calculation
To goal is to design a link so that the received signal is stronger than the receiver's sensitivity at the
required BER (typically 10
-6
). Since every radio signal in earth atmosphere is subject to fading, some
difference between received signal level under normal circumstances and receiver sensitivity is needed
to serve as a fade margin. The minimum value of fade margin can be calculated from the requirement
for link availability (e.g. 99.999% of the time). The required margin depends on the length of the link
as well as other factors such as rain attenuation, diffraction and multipath propagation.
If we ignore the additional loss along the path, the received signal strenght can be calculated using the
formula for signal propagation in free space as follows:
P
R
= P
T
+ G
T
+ G
R
- FSL
Where
P
R
received power level (dBm)
P
T
transmitted power (dBm)
RAy Microwave Link – © RACOM s.r.o. 10
Implementation Notes