rainfade

Rainfade, or rain fade, is the degradation or loss of radio signal strength caused by precipitation, especially raindrops, in the atmosphere. It most commonly affects microwave and millimeter-wave transmissions, such as satellite downlinks and terrestrial point-to-point links. Rain fade occurs when raindrops absorb and scatter electromagnetic energy, converting it into heat and causing diffraction losses, leading to reductions in received power and potential interruptions.

The severity depends on rain rate, drop size distribution, polarization, path length through rain, and the angle of elevation. Attenuation increases with frequency; at higher microwave frequencies, rain is more effective at attenuating the signal, making Ka-band links more susceptible than Ku-band and much more than C-band. Rain fades can last from a few seconds to several minutes, and extreme storms can cause longer outages.

Bands commonly affected include Ku-band (12–18 GHz) and Ka-band (26.5–40 GHz) used for satellite communications, as well as terrestrial microwave links. Prediction and planning use rainfall rate data and propagation models to estimate expected attenuation and required fade margins.

Mitigation strategies include designing links with fade margins, using adaptive coding and modulation, increasing transmit power or antenna gain, and employing diversity techniques such as spatial diversity, frequency diversity, or satellite diversity. Redundancy and path diversity can maintain availability in regions with heavy rainfall.

History and context: Rain fade has been a consideration since early satellite networks, and modern systems design around it through standardized prediction methods and robust link budgets.