# Earth-to-Earth Microwave Rain Attenuation Measurements: A Survey On the Recent Literature
## 4 Rainfall Measurements Through Backhaul Cellular Infrastructure
Specifically, the value of 0.1 dB is the proper one for cellular links. \[\overline{R}\approx a\overline{k}^{-b}, \tag{13}\]  
where \(\alpha=(1/c)^{1/d}\) and \(b=1/d\). The variables \(c\) and \(d\) depend on frequency. Additionally, when \(\overline{R}\) is reported in mm/h, then \(\overline{k}\) is reported in dB/km. In Rotterdam, Netherlands, the measurement of rainfall in urban areas through CML is presented [35]. In their research, data from June and July 2009 from 57 microwave links with a length greater than 0.7 km and an average length of 3.7 km were used. The frequencies ranged from 13 to 39 GHz. Rainfall attenuation is validated with the use of (a) the two Doppler weather radars, working at C-band, and they are operated by the Royal Netherlands Meteorological Institute, (b) a rain gauge network that was adjusting radar-based accumulation. A rainfall retrieval algorithm is also confirmed using data from a 27 GHz research link. [PERSON] et al. used data from 5 microwave backhaul networks in southern Germany [36]. A new method was introduced that detected wet and dry periods through spectral time series analysis. In their research, they did not use data from providers, but instead, special equipment installed in towers received power which was recorded by using a data logger. Data were captured from July to October 2010. The data was then sent via a GSM module to the database server for processing and analysis. Specifically, a Cinterion TC65i that included a GSM/GPRS module and two analog to digital converter (ADC) data loggers were used. The advantage was that as they received the analog voltage with an ADC and they achieved better precision in the order of 0.01 dB. They also measured the value of the analog voltage every 3 s and stored the average value per minute. For comparison with signal attenuation, six rain gauges were used in the area of measurements, three with 0.1 mm resolution and 3 with 0.2 mm resolutions. Validation was also achieved through a C-band weather radar covering a radius of 256 km and generated precipitation data every 5 min. The contribution of this research team to the creation of rainfall maps, even for a whole country, is reflected once again in their work [37]. Data were used from approximately 2400 Links with operating frequencies at the highest rate between 37 and 40 GHz and an average path length of about 3 km. The emitted power was almost constant, while for 15 min, the average value of the received power was recorded based on the maximum and minimum amount with 1 dB of resolution. The data were obtained from June to September 2011. Specifically, acquired data from a gauge-adjusted radar was used for the calibration of the microwave link rainfall retrieval algorithm and for the validation of link-based rainfall maps. Two research teams from Switzerland used data from microwave links in the area of Zurich, Switzerland. The measurements took place for about a year from March 2009 to March 2010, with 14 microwave links and operating frequencies of 23, 38, and 58 GHz. The received power was recorded with a temporal resolution of two and five minutes while the transmitted power was unknown. Fourteen rain gauges were used to validate the data. Detection and quantitative error detection algorithms have been proposed [38]. In turn, the authors developed a new method for estimating the rain rate and its uncertainty by integrating the variational approach by [PERSON] et al. [39] and by applying Kalman filtering. The specifications included radar rain-rate fields with a spatial resolution of 1 \(\times\) 1 km\({}^{2}\) along with 5 min temporal resolution in an urban area of 20 \(\times\) 20 km\({}^{2}\). Moreover, the MeteoSwiss radar network calculated the rain rate (for obtaining the advection field) which was combined to the rates found by 14 RGs and to those estimated by 14 microwave links working at 23, 38 and 58 GHz. Their polarizations were vertical and horizontal, while the power resolution was 0.1 or 1 dB.