# Earth-to-Earth Microwave Rain Attenuation Measurements: A Survey On the Recent Literature
## 4 Rainfall Measurements Through Backhaul Cellular Infrastructure
The combination of short commercial microwave links in an urban environment and rain gauges is a promising solution to improve space-time resolution of rain monitoring [52]. An exponential law usually models the attenuation due to rainfall. For frequencies between 20-40 GHz, the exponent parameter is close to unity. The authors propose a more straightforward model combining linear approximation for rain attenuation and wet antenna attenuation and that is \(R=\gamma(k-\Delta)\ [k>\Delta],\ R=0\ [k\leq\Delta]\), where \(\gamma\) is an empirical parameter related to raindrop attenuation, \(k\) is a specific attenuation after baseline separation, and \(\Delta\) is offset parameter correcting both for wet antenna attenuation and baseline inaccuracy. This model performs well with aggregation intervals of few minutes to 1 h but not for higher intervals. Additionally, the range of the links should be 1-2 km or less. Several techniques used to estimate rainfall were compared, namely rain gauges, radars, and microwave links providing datasets into a rainfall-runoff simulation model for lowland catchments in the Netherlands. It was found that seasonal variations caused errors in the microwave links attenuation model. The events were underestimated in summer but overestimated in autumn [53]. In Sao Paulo, Brazil, CML measurements were taken for 81 days from 20 October 2014 to 8 January 2015 [54]. The measurements came from 147 Huwai and Ericson CML with a power resolution of 0.1 dB. The analysis of the data was done in path links smaller than 20 km. For the validation of the rainfall attenuation, 152 rain gauges of the National Early Warning and Monitoring Center of Natural Disasters (CEMADEN) in Brazil, were used to measure rain amount per 10 min of integration time. The operating frequencies of the links were ranged from 8 to 23 GHz. Rain rates were derived from attenuation measurements through Rainlink. The rain gauges validated thirty min of rainfall estimations, and the possibility of rainfall was confirmed in subtropical climates such as that of Sao Paulo. A work that could be considered of being complimentary to [38] and to similar ones, is the vertical precipitation estimation which was introduced in [55]. This estimation was due to the Virga phenomenon as the latter can mislead the proper estimation of precipitation at the ground. Consequently, a new method was suggested combining commercial microwave links' (CMLs) measurements to those of weather radars at different heights. In turn, a vertical profile of the rain rate (in arid areas) and of the cloud base level (CLB) was accomplished. The estimation was validated by taking into account the RGs measurements, thus comparing the proposed CLB to that of real CLB (weather station). The mentioned method exhibited a maximum correlation of 0.9615 relevant to measurements from RGs and from two observed storms. The location of the experiment included a CML path of 16 km of length and recording procedures of every 15 min while the utilized radar elevations had a total range of 355 to 2862 m above ground. As far as for clarification purposes, three different kinds of apparatus/measurement conditions were used and i.e., CML and radar beams observing two different elevation ranges constituting the aforementioned total range. An immediate result from this proposed method could be flash flood warnings. A comparison of measured and predicted rain attenuation was presented by [PERSON] et al. [56]. The measurements took place between 2014 and 2016. A total of 17 microwave links were used in the Northeast area of Prague, Czech Republic. The operating frequencies were centered around 32 and 38 GHz. The minimum length of the links was 186 m, while the maximum was 1810 m--the transmitted and received power levels were collected by a customized software that runs on the mobile server operator. The sampling was done every 1 min, and the power resolution was 0.33 dB. Rainfall data was available from the rain gauge network of Prazska vodohospodarska spolecnost.