Know about Rain attenuation and Solutions for it

1. What is Rain attenuation？

During the operation of the satellite, if it encounters a rainy area, the radio waves emitted by the satellite will not only be partially absorbed by the rain, but the rain will also scatter the radio waves to a certain extent. Under the joint intervention of the two effects, the satellite radio waves will be attenuated. In addition, the scattering effect may also cause some interference to the radio, resulting in a depolarization effect. These phenomena can be called rain attenuation.

Rain attenuation is a common, yet often misunderstood weather phenomenon that interrupts wireless communication signals in the presence of rain. It occurs with all types of satellite systems e.g. Geostationary Earth Orbit (GEO), Medium Earth Orbit, Low Earth Orbit and Global Positioning System.

2. Why there is Rain attenuation?

Due to the higher operating frequency for a Ku or Ka band SATCOM system, the signal’s wavelength is generally shorter as compared to the C band. Therefore, it is more susceptible to signal degradation as the wavelength approaches the size of a typical raindrop. The two major causes of rain fade are:
Absorption – water molecules in a rain droplet absorb portions or all of the signal energy of the passing radio wave. With shorter wavelength, there will be more interaction between the radio wave and water molecules, leading to increased energy losses.

Scattering-this is a physical process, caused by either refraction or diffraction, in which the direction of the radio wave deviates from its original path as it passes through a medium containing raindrops. This disperses the energy of the signal from its initial travel direction.The accumulation of these different reactions ultimately leads to a decrease in the level of received signal, thus resulting in rain attenuation.

3. Effect of rain fade

During rainfall, not only radio wave attenuation will occur, but thermal noise will also have some impact on the earth station. If the rainfall noise is at the input end of the satellite receiving antenna, the antenna will be affected by thermal noise, thereby improving the satellite’s signal reception process. Cut-to-noise ratio. In general, when the antenna elevation angle is relatively low, the rain noise will also increase. The reason is that the shorter the path of the radio wave in the rain, the smaller the attenuation. If there is no rain attenuation, the noise temperature will remain unchanged. If there is no guided wave loss, the only factor that can affect the noise temperature is the attenuation. In addition, there is a phenomenon of depolarization in the process of rainfall. The absorption and scattering of radio waves in the rain are actually related to the polarized waves of the incident waves. In the process of rainfall, the air will have a certain resistance to the rainfall. Therefore, the shape of the raindrops in the air is slightly flat, and its upper and lower axes will also have a certain attenuation effect, which is a differential attenuation, but this attenuation does not It will not have a great impact on the transmission system of single polarization.

4. How to decrease the influence of Rain attenuation？

4.1 Link reserve

The redundancy of the link is a commonly used method in the design of traditional communication links. For example, a 3dB margin is usually reserved for the C-band satellite communication link, and a 6dB margin is usually reserved for the Ku-band satellite communication link. In some areas with less rainfall (such as desert areas), the system availability requirements can be met entirely through link margins; in high rainfall areas, it is unrealistic to rely entirely on this method. On the basis of comprehensive consideration of other methods. However, this method will take up too much satellite resources, waste resources when the sky is clear, and may not be enough when it rains heavily.

4.2 Technology of encoding and decrease rating

When the rain attenuation is large, forward error correction coding (FEC forward error correction) can be used to reduce the bit error rate of transmission. The coding gain can be improved by reducing the coding rate. For example, Convolutional codes with an encoder rate of 1/2 can achieve a coding gain of up to 5dB when using Viterbi decoding. Of course, the coding rate must be reduced. There is a limit. On the one hand, when the coding rate is reduced to a certain level, if the coding rate is further reduced, the additional coding gain will be improved very little; on the other hand, reducing the coding rate will lead to a decrease in the system capacity. In addition, the effect of rain fading can also be overcome through adaptive rate reduction (ARP), which increases channel capacity by reducing the data rate of the attenuated channel. The gain brought by reducing the rate is proportional to the rate reduction. When /4, the gain is 5dB, and the error correction coding and rate reduction technology can be used to compensate for different degrees of rain attenuation; but with the increase of depth, the effective available capacity decreases.

4.3 Space/site diversity

In areas with rainy or very low satellite elevation, the rainfall attenuation is very large, and space diversity technology (also called site diversity technology) can be used, which is a very effective method. Its principle is based on the uneven spatial distribution of rainfall, set up earth stations at two locations separated by a certain distance, and perform diversity reception of signals through two earth stations, which is similar to the spatial diversity technology of terrestrial cellular mobile communication, and can also be switched independently. Single-link communication to earth stations with less rain attenuation. The quality of diversity improvement is mainly measured from two indicators: diversity improvement factor and diversity gain. The effect of diversity improvement increases with the increase of the distance between two stations. But beyond a certain distance, the degree of improvement is very small. While space diversity brings benefits, it also comes at a cost. The cost of network investment has risen sharply, and very complex network control technology is required. It should be pointed out that the space diversity technology is not limited to two sites, and multiple sites can be used for simultaneous diversity reception. Of course, the cost is higher.

4.4 Polarization and Antenna Selection
Different raindrop shapes attenuate the signal differently. As the volume of the raindrop increases, the diameter of the raindrop in the horizontal direction also increases gradually. At this time, the attenuation of raindrops to the horizontally polarized wave is greater than that of the vertically polarized wave, which also means that at frequencies above 10 GHz, the vertically polarized wave has better anti-rain attenuation performance than the horizontally polarized wave. In addition, the antenna aperture will also affect the antenna gain to a certain extent. Therefore, it is necessary to adjust the size of the antenna aperture in a targeted manner according to the actual situation, so as to achieve the purpose of improving the antenna income. However, it should be noted that although increasing the antenna aperture will improve the gain and system coverage, the equipment cost will also increase accordingly.

4.5 Choose LNB with low noise and high gain value

Under normal circumstances, the noise figure of LNB used in the Ku-band is generally 0.7dB, but if LNB with a noise figure of 0.6dB is selected, the system gain can be effectively improved. However, in the actual application process, the method of increasing the diameter of the antenna is preferred. If the diameter of the antenna cannot be increased due to constraints, the method of replacing the high-quality LNB, which can also reduce the cost to a certain extent.

4.6 Some other methods for lowering rain attenuation

Based on our experience, to decrease the influence to satellite transmission system, the uplink stations of satellite transmission system mainly adopt solutions such as improve transmission power, increase transmission antenna area etc. For the dual polarization transmission system with cross polarization multiplexing, it needs to improve the system cross polarization isolation as possible. The most effective solution to decrease rain attenuation of the downlink receive station is using antenna with larger area, LNB and receiver with higher sensibility etc. Meanwhile, there are some additional solutions as below:

-Fine adjust the antenna El &AZ angle and LNB Pol. Angle to the optimum position. Make the earth station antenna align to satellite as possible. Although the live satellite adopts circular pol. Mode, the signal quality can be changed through fine adjusting the pol. So that the received signal strength can be strongest.

-If the antenna is installed outdoor, users can add a rain-proof shed for protection. For parabolic prime focus antenna, users can addrain-proof equipment between feed system and LNB. Attention: The added rain-proof equipment cannot affect the signal reception. It is only used to protect the rain from staying on the surface of panel antenna or feed film so that the rain attenuation can be decreased.

Welcome to leave comments to us if you have some other measures or suggestion on rain attenuation via sales@antesky.com Thanks you.