4.2.3 Rain Attenuation
The rain attenuation occurs as a result of the absorption and scattering of electromagnetic waves by rain particles. The attenuation on any given path depends on Rainfall rate, the frequency of that wave, polarization, temperature, path length and the size of raindrops. Above 10 GHz frequency, rain drops fade in the form of absorption and scattering become a serious contribute to transmission losses. When designing a Line of Sight microwave link or a satellite link operating at frequency above 10 GHz, z, the absorption and scattering by rain along the transmission path should be considered. Rain attenuation prediction models take into account of path reduction factor, the product of path reduction factor and the physical path length of a microwave link is the effective path length which is smaller than the actual physical path length. The modified effective path length features both path length and rainfall rate.
Rain attenuation models have been studied based on experimental measurement data from different regions worldwide. The ITU-R and other models have collected decades of rain data from around the world, but The ITU-R model is mostly widely used. It should be noted that rain attenuation models based on measurement data from different regions worldwide show good agreement with ITU-R recommendations which have taken into consideration of averages of statistics over decades due to the variability of rainfall statistics.18 26
By using this information it is possible to design radio links able to overcome even the worst weather events or to predict the durations of weather related outages of radio links operating at specific frequencies. The ITU rain zone classification map shows the expected rainfall rates in alphabetical order. While areas that experience the least rainfall rate are classified as “Region A,” the highest rainfall rates are in “Region Q”. In ITU-R recommendation, an approximate relationship between attenuation Lrain (dB/km), R distance and rain rate R (in mm/h unit) is given by ITU-R recommendation P.838-3 is given by the following equation (4.3):
Lrain = a R^b (4.3)
Where a and b are functions of frequency given by ITU-R recommendation. Figure 4.2 shows several rain attenuation with different values of rainfall rate given by ITU-R.
As shown, Radio propagation within the millimeter band (30–300 GHz) is characterized by very high rain attenuation which practical limits the link distance. For example, heavy rain with rainfall rate 25 mm/h can lead to over 10 dB/km attenuation at E-band frequencies. For the case of very severe tropical rainfalls (100 mm/h), the attenuation can reach up to 30 dB/km at the same frequency. In addition, we can see that rainfall attenuation increases with frequency in general and rain attenuation is very significant in millimeter waves. However, this generally occurs only in short bursts and is associated with a severe weather event that moves quickly affecting the link performance. Therefore, rainfall rate is always associated with up time probability or link availability of the average year. 18 26
Fortunately, most intense rain tends to fall in limited parts of the world, mainly the equatorial and tropical countries. In other countries, such severe weather generally occurs only in short bursts. It tends to fall in small clusters within a larger and lower-intensity rain, and is usually associated with a severe weather event that moves quickly across the link path. Therefore, rain outage tends to be short and is only problematic on the long distance transmissions. 10