# A Review on Rainfall Measurement Based on Commercial Microwave Links in Wireless Cellular Networks
## 1 Introduction  
Rainfall is one of the major driving forces in the hydrological cycle of the land area. Natural disasters caused by rainfall, such as flash flood, debris flows, landslides, and urban waterlogging, have been damaging people's lives and property over the years [1]. On 20 July 2021, floods and secondary disasters caused by torrential rain had a negative impact on 14.786 million people in Zhengzhou, China, with a direct economic loss of 120.06 billion yuan [2]. Therefore, developing rapid and accurate rainfall information acquisition technologies is extremely urgent and important to achieve precise flood risk assessment and enhance the public safety response capability. Meanwhile, rainfall is one of the most critical climate factors to determine crop growth, and accurate precipitation information can substantially improve crop yield [3]. In addition, rainfall information is also an important reference index for reservoir operations, urban water supply, irrigation planning, and other forms of water resource management [4]. However, due to the high spatial and temporal dynamics of rainfall, it is still challenging to accurately monitor and measure rainfall in real time [5, 6]. Rain gauges (RGs), weather radars, and satellite remote sensing are the three main rainfall measurement methods at present. RG is a point measurement tool with limited density, low spatial resolution, and relatively high construction and maintenance costs. Radar observation of rainfall emits electromagnetic waves into the sky. These electromagnetic waves are reflected by clouds and returned to a ground receiver. Rainfall information is then estimated by comparing the original and returned electromagnetic waves [7]. This method measures the water volume in the clouds, which could be quite different from the rainfall amount on the ground. In addition, this method can be easily affected by ground obstacles at a low elevation angle [8]. Satellite rainfall measurement, on the other hand, has a global scale, but the resolution is relatively coarse in terms of small spatial and temporal scales. Moreover, its accuracy is significantly affected by the density of clouds, and the time lag is high [9]. In the recent years, many countries such as Israel, the Netherlands, France, Germany, the Czech Republic, Switzerland, Italy, and China have adopted commercial microwave links (CMLs) in wireless cellular networks (WCNs) to retrieve the regional rainfall information. The measurement accuracy and ability of capturing the spatial and temporal dynamics of rainfall for the CML method have been verified through a variety of simulations and experiments. The CMLs that have already been deployed in WCNs to achieve fronthaul/backhaul communications among base stations can be considered as a widely distributed environmental monitoring sensor network. Since CMLs were originally designed for the communications purpose and environment sensing is just a by-product, the network of CMLs is also called an opportunistic wireless sensor network (OWSN) [10]. The IMT-2030 (6G) Promotion Group released a research report on the integration of communications and sensing technology in September 2021 [11], which points out that the communications and sensing capabilities will finally achieve the integration and symbiosis in the future 6G wireless networks. According to the statistics of the Industry and Information Technology Ministry, the number of 5G base stations in China has already exceeded one million in 2021. The increasing number of the deployed base stations and microwave links provides great opportunities and supports to develop CML-based rainfall measurement technology [12; 13; 14]. Table 1 provides the comparisons of the existing rainfall measurement methods.