# Earth-to-Earth Microwave Rain Attenuation Measurements: A Survey On the Recent Literature
## 4 Rainfall Measurements Through Backhaul Cellular Infrastructure  
The primary advantages of rainfall monitoring with the use of microwave links from cellular backhaul networks are the existing infrastructure as well as the abundance of such links around the world. At the same time, they cover a wide range of frequencies. The majority of microwave backhaul links operate in the 6-23 GHz and 32, 38 GHz frequency bands. There are new structures that operate at E-band 70/80 GHz, while in the future, the W and D bands will be used for lengths of no more than 1 km [32]. The distances of the links generally range between a few km and a few tens of km for smaller continuities. Figure 5 presents the microwave backhaul spectrum from cm-Wave to mm-Wave and information concerning bandwidth on each frequency band. The main disadvantage is that the microwave links are made to transfer information and not to measure rainfall, so the accuracy of the measurements is limited. It also limits the sampling time of the receiving signal, which can be varied once more and every 24 h. Additionally, the difficulty of accessing such data managed by companies is another disadvantage. The non-uniformity of rainfall,  
Figure 4: Strong fluctuations of the received power after a very high rainfall event during 30/06/2015. Figure 5: Microwave backhaul spectrum from cm-Wave to mm-Wave. especially in long links and the impossibility of correlation with the rainfall measurements from a conventional rain gauge, adds to significant limitations that need to be addressed. In this section, signal attenuation measurements are presented due to rainfall from backhaul microwave links. Information is presented concerning the transmission and reception parameters, the sampling of signal measurements, the geographical, the active path, and the methodology followed for the correlation of rainfall with the signal attenuation. The studies and development of proper procedures for exploiting signal attenuation, by using microwave links (cellular or not), are striving to find a proper place in the measurement procedure relevant to the rainfall calculation and prognosis. Below are reported essential works relevant to this matter of research and development (R and D) such as the one that follows and that is relevant to the review study on opportunistic remote sensing of rainfall [26] whereas it is described with a non-expert view style (as the authors declare) the history behind rainfall monitoring using microwave links MLs and its principles. This technique is very important as it could empower the measurement procedure in cooperation with other already known devices such as rain gauges, disdrometers, weather radars, and remote sensors in general. Typical working frequencies of cellular devices could be between 1 to 2 GHz (L-band). On the contrary, MLs of a K-band utilize frequencies of higher values (GHz bands) with wavelengths proportional to 1 cm. These kinds of signals are prone to attenuation due to atmospheric conditions such as rainfall. Additional attenuation could occur by absorption and the microwaves' scattering by raindrops. Less attenuation may occur in the presence of hail or snow in the condition that hail stones or snowflakes are not melting [33]. Additionally, water vapor and oxygen exhibit absorption frequency bands, at 22.2 and 60 GHz respectively, in the microwave range of cellular links. The previous fact could be misinterpreted of being attenuation due to rainfall. Albeit the fact that the meteorological area of expertise had the knowledge from the past years that the calculation of the microwave attenuation could contribute to rainfall estimation [34], the great awareness on this matter was accomplished by Equation (13) because it interpreted the fact of millions of commercial MLs to be considered as path-average rain gauges [13]. As far as the typical values of variables for measurement purposes are the 15 min of rate between storing the signal values along with power resolution between 0.1 to 1 dB.