User's Manual

Table Of Contents

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.

NOTE - For quick reference you can use the calculator on www.racom.eu

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. The loss depends on the distance travelled by signal and its frequency.

Longer distance and higher frequency both mean greater attenuation. Free-space loss can be calculated

thus:

FSL = 32.44 + 20log f + 20log D

Where:

FSL

free-space loss (dB)

frequency of the emitted signal (MHz)

length of the link (km)

2.1.2. Link budget calculation

The goal is to design a link producing a received signal 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 strength can be calculated using the

formula for signal propagation in free space as follows:

= P

+ G

- FSL

Where

http://www.racom.eu/eng/products/microwave-link.html#calculation_obsah

Implementation Notes