Rain Fade on Microwave Links

Rain fade describes the absorption of microwave radio frequency (RF) signals by atmospheric precipitation such as rain, snow or ice, with significant impact at frequencies above 11 GHz. It also involves signal degradation due to electromagnetic interference from a storm front’s leading edge.

Rain fade can occur at either the uplink or downlink location, and it doesn’t need to be raining at the site for the signal to be affected. The transmitted signal may pass through precipitation far away, particularly with low-angle satellite dishes. Additionally, 5 to 20 percent of signal loss may result from rain, snow, or ice accumulating on the antenna’s reflector, radome, or feed horn. Rain fade affects not only satellite links but also terrestrial point-to-point microwave links.

To mitigate the impact of rain fade, various strategies are used, including site diversity, uplink power control, variable rate encoding, using larger (higher-gain) receiving antennas than normally required, and applying hydrophobic coatings to antenna surfaces.

Two models for Rain fade are considered, Crane and ITU. Generally, ITU model is favoured by microwave planners. The following Rain Fade Map is used in the ITU model for Microwave Link planning for Rain Fade:

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