# Earth-to-Earth Microwave Rain Attenuation Measurements: A Survey On the Recent Literature
## 5 Customized Experimental Setups  
This section provides an overview of recent experimental setups that have been used to measure rainfall. The theoretical background remains the same and goes back to calculating the link budget for dry and rain events. This section also includes works that use the received signal strength level from mobile terminals to estimate rainfall. Specifically, in Durban, South Africa, the one-year experimental data from a 6.73 km path with operating frequency at 19.5 GHz was compared to theoretical results from typical rain attenuation models. Integration time for both the rainfall and the received signal was 1 min [68]. A disaster alarm system was designed based on measurements from wireless link attenuation due to rainfall in the Philippines where typhoons are common phenomena. In order to estimate the spatial distribution of the rain rate, acoustic sensors had been placed along the path of the links with a resolution of 100 m. The microwave links were operating at 26 GHz and were placed at distances of 4 km each other. Three years of measurements were processed in order to establish the attenuation model. The received signal level limit and the duration of rainfall were two critical parameters that triggered the alarm system [69]. Rainfall has an effect on wireless signal quality. It has been observed that during and after rain period wireless links operating at 2.4 GHz were affected, in such a way that some of them had decreased performance but some of them had increased performance [70]. The phenomena were attributed by the authors not to rain itself but in water gathered on antennas changing their radiation pattern, because at such relatively low frequency, the attenuation due to rainfall is expected to be negligible according to existing theoretical models. Following an experiment where water was periodically sprayed on antennas plastic enclosures, it has been found that initially, the received signal strength increases, and then it decreases as the water amount is large enough. An experimental setup was established using microwave links together with rain gauges in order to estimate rain-related attenuation to received power [71]. Transmitters and receivers were placed at distances of about 3-4 km. The double operating frequency was at 13 GHz and 22-23 GHzapproximately. The rain gauges were tipping buckets. The baseline, that is, the received power in the absence of rain, was not assumed to be constant during the rain event as it could be unrealistic. The baseline is not defined only by the rain itself but also by atmospheric absorption depending on specific conditions. The model performed better, considering the variable baseline. The calculation of the baseline was based on a linear low-pass filter approach. Specific attenuation by the wet antenna effect was taken into account as well. The attenuation model was expressed as, \(R=ak^{b}\), where \(R\) corresponds to the rain rate (mm/h) while \(k\) is the specific attenuation (dB/km) and the parameters \(a\), \(b\) are found to be approximately 4.4 and 1.1, respectively. In the customized experimental setup used by [PERSON] et al. to measure precipitation and humidity, they presented, among other things, detailed information about the microwave system they implemented [72]. The system included a combined transmitter/receiver with operating frequencies of 22,235 and 34.8 GHz. The length of the path was 1.32 km. The data were sampled at a rate of 20 Hz via an automatic operating mode with a power resolution of 0.1 dB. For the validation of the microwave data, a Thies Laser Disdrometer had been installed next to the Tx/Rx system which had a time resolution of one minute. Very close to the measurement area, there was a weather station of the type Vaisala WXT520 which gave rain rate data, humidity, pressure, and temperature information with a time resolution of ten min. The experimental setup was located near Fendt, Peissenberg, Germany, and a total of 6 different attenuation cases were analyzed from 2011 data.