# Opportunistic Weather Sensing by Smart City Wireless Communication Networks
## 3 OISAC in Smart Cities  
Smart cities have come a long way in terms of infrastructure in recent years. Current and future smart cities rely heavily on wireless communication channels operating at mmWave frequencies and, as such, can be utilized as opportunistic weather sensors. Anything from low-power communication devices (such as sensor reporting and utilizing services for citizens and the management of the city, e.g., full trash cans, bus arrivals, traffic light statuses, etc. ), to high-bandwidth communication links for autonomous vehicles and street cameras are all interconnected in a vast xhaul line-of-sight grid. Future mobile devices (5G and above) will also implement mmWave modems, giving even more diversity of channels in dense connected urban environments. Until a few years ago, CML networks were mainly used for the purpose of establishing a stable and static backhaul of cellular networks and indeed, 3G and 4G cellular networks rely heavily on such CMLs at frequencies in the K-band (6-40 GHz). In recent years, as the need for a larger bandwidth arose, higher frequencies in the E-band wireless links started to be deployed. Moreover, such E-band CMLs offer a higher throughput, and thus, are a good solution for the city-wide networks used for designated smart-city applications such as street camera feeds, public transportation real-time control, Wi-Fi services to the residents, etc. As the carrier frequency increases, it is obvious that E-band links' capacity is higher than K-band links, but at the cost of higher propagation losses of the signal. Therefore, E-band CMLs are generally shorter--a fact that by itself introduces additional challenges for opportunistic rain monitoring--from additional non-negligible other-than-rain attenuation phenomena such as wet-antenna effects [49; 50] through the general phenomena of overestimation of the rain intensity when using the standard power law (of Equation (1)) [29]. On the other hand, these frequencies are more sensitive to precipitation (See Figure 2). Recent studies showed, however, that SCWN can be exploited for opportunistically generating rain measurements that, in turn, lead to accurate near ground spatiotemporal rain maps [51]. It can also be used for the near-real time estimation of storm front movement [52], a capability that is of the utmost important for real-time city management and security service responses, especially during heavy and extreme weather events in cities. Based on our recent works [14; 53], we present a demonstration of how using signal-level measurements from an actual SCWN improves the spatiotemporal resolution of rain monitoring in urban environment significantly. In Figure 3a, a setup of an actual SCWN in the city of Rehovot in the central district of Israel is depicted. There, a total of 66 links at frequencies of 70 GHz to 80 GHz are deployed at an area of about 10 km \(\times\) 10 km. The shortest link in this network has a length of only 53 m, and the longest one spans over 2.4 km [53]. A histogram of all the SCWNs' path lengths is depicted in Figure 3b [53], and an image of the actual Tx/Rx terminals is presented in Figure 3c (taken from [54]). The full hardware description of the terminals is available in [54]. Overall, most of the links are shorter than 1 km. As the power law (of Equation (1)) was empirically proven to be problematic in terms of its accurate rain intensity estimation when dealing with short links [55], we performed a calibration of the power law, by using the longest available link as a calibration reference [29] (as longer links are not affected by the short-link inaccuracy of the power law). In Figures 4-6, the full estimation workflow's results are presented,during three days in which rain occurred--a heavy rain event (59.8 mm within 24 h), a moderate rain event (28.6 mm within 24 h), and a light rain event (3.9 mm within 24 h).