Rain fade

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Rain fade refers primarily to the absorption of a microwave Radio Frequency (RF) signal by atmospheric rain, snow or ice, and losses are especially prevalent at frequencies above 11 GHz. It also refers to the degradation of a signal caused by the electromagnetic interference of the leading edge of a storm front. Rain fade can be caused by precipitation at the uplink or downlink location. However, it does not need to be raining at a location for it to be affected by rain fade, as the signal may pass through precipitation many miles away, especially if the satellite dish has a low look angle. From 5 to 20 percent of rain fade or satellite signal attenuation may also be caused by rain, snow or ice on the uplink or downlink antenna reflector, radome or feed horn.

Possible ways to overcome the effects of rain fade are site diversity, uplink power control, variable rate encoding, receiving antennas larger than the requested size for normal weather conditions, and hydrophobic coatings. Only superhydrophobic, Lotus effect surfaces repel snow and ice.

Contents 1 Uplink power control 2 CCIR interpolation formula 3 ITU-R frequency scaling formula 4 References 5 See also

[edit] Uplink power control

The simplest way to compensate the rain fade effect in satellite communications is to increase the transmission power: this dynamic fade countermeasure is called uplink power control (UPC). In any case, it has a limited use since it cannot provide large margins: transmitting amplifiers behave non-linearly under increased power, and the output power is limited. The limitations are more severe for a transmitter in a satellite, so power control is used only in uplink.

[edit] CCIR interpolation formula

It is possible to extrapolate the cumulative attenuation distribution at a given location by using using the CCIR interpolation formula^[1]:

A_p = A₀₀₁ 0.12 p^{-(0.546 - 0.0043 log₁₀ p)}.

where A_p is the attenuation in dB exceeded for a p percentage of the time and A₀₀₁ is the attenuation exceeded for 0.01% of the time.

[edit] ITU-R frequency scaling formula

According to the ITU-R^[2], rain attenuation statistics can be scaled in frequency in the range 7 to 55 GHz by the formula

where

and f is the frequency in GHz.

[edit] References

1. ^ CCIR [1990] Report 564-4 "Propagation data and prediction methods required for earth-space telecommunication systems".
2. ^ “Propagation Data and Prediction Methods Required for the Design of Earth-Space Telecommunication Systems,” Recommendations of the ITU-R, Rec. P.618-7, 2001.

[edit] See also

• Fresnel zone
• Diversity scheme

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