airMAX - Plan an outdoor wireless link

Overview


Readers will learn how the tools and concepts for planning a outdoor wireless link. Distance is one important factor when you consider link planning. However, there are also other important factors, such as Fresnel zone and Fade Margin.

The Fresnel zone

The Fresnel zone defines the area surrounding a line of sight link that must be free of obstructions that might cause out of phase reflections that can significantly degrade signal quality. The below image shows an example of an obstruction that is within the Fresnel zone of the link:

airLink

In many cases, calculating the Fresnel zone is a complicated task, since it’s not a simple ‘Line of Sight’ from one end of the link to the other. Therefore, Ubiquiti developed airLink (www.ubnt.com/airlink), a link budget software which allows installers to estimate the link viability, providing important information such as Fresnel zone clearance, link distance and estimated signal strength and performance.

airView

A site survey should be conducted on-site to examine environmental noise of an area. The easiest way to analyze spectrum interference is using airView, you can find a step-by-step guide here. In case the entire spectrum is very crowded you should consider using a different band. For example, if 2.4GHz band is crowded, it may would be a good idea moving to 5.8GHz or 3.65GHz band.

Fade Margin

Signals strength may fluctuate with environmental changes (change in season, rain patterns, etc.). Therefore, operators usually take into account a 15dB “fade margin”, in order to guarantee a high uptime rate and more robust link.