# Noninvasive Suspicious Liquid Detection Using Wireless Signals
## 1 Introduction  
The illegal carrying and transportation of flammable and explosive liquids seriously affects public safety. Flammable and explosive liquids such as gasoline and alcohol are also commonly used in various terrorist activities. Therefore, the safety inspection of flammable and explosive liquids is of great significance for ensuring public safety. It has broad application prospects in the fields of public security, civil aviation, and customs. Liquids cannot be identified by the naked eye, and it is difficult to carry out dangerous liquid inspection in crowded places, which is a great challenge for security personnel. Moreover, there are still some places where the \"taste liquid\" method is used to determine whether the liquid is safe [1]. Forbid passengers to carry large amounts of liquid has become the main method to prevent terrorist attacks. For example, international civil aviation regulations prohibit carrying more than 100 mL of liquid, and trains and high-speed trains prohibit carrying flammable liquids and more than 120 mL of compressed spray. The traditional detection of suspicious items is either manual inspection (such as setting checkpoints at each entrance) or special equipment (such as surveillance cameras, X-ray machines, and ultra-wideband scanners), which is costly, expensive to deploy, and difficult to implement on a large scale [2]. It is necessary to introduce a new suspicious liquid detection scheme which is more economical and covers a wider range. There are several mature liquid detection technologies. A traditional approache, the Raman spectrum analysis, uses the molecular structure to identify liquids according to their scatteringspectroscopy. Raman spectroscopy has many unique advantages, such as wide detection range, sharp spectral peaks, and high resolution [3]. That is, when monochromatic light radiates on an object, the molecules of the substance will scatter, and the spectrum reflected by different substances will be different. Moreover, the method, based on X-ray image technology, is used to identify liquid substances, and can obtain the atomic number of the liquid through its X-ray [4]. However, this method has a certain error rate and the equipment is very expensive. Another method is to detect liquids based on the different absorption and attenuation characteristics of different substances via electromagnetic waves [5; 6]. Microwave detection can achieve noncontact detection with a certain distance between the detected objects, and has high detection sensitivity. However, these approaches rely on expensive and specialized equipment, which does not facilitate wide deployment in practice. Recently, Radio Frequency (RF) based sensing has drawn considerable attention. A couple of studies have explored the feasibility of using RF signals for remote monitoring and controlling during infusion. For instance, a wireless intelligent monitoring system, based on wireless communication and network technology, is put forward, which can monitor drip speed in real time and automatically alarm in abnormal conditions [7]. The ZigBee wireless sensor is used to detect the velocity of liquid droplets [8]. While these approaches mainly focus on exploiting the differences of wireless signal measurements to sense physical morphological changes of liquids, using fine-grained wireless channel information (WCI) to identify types of liquid remains an option. 5G communication technology is a data and information transmission technology developed by technicians based on 4G technology. Its advantages include sound transmission performance, fast transmission speed, high utilization rate of resources, and wide range of coverage. As such, it is favored in modern data and information transmission [9]. Based on ubiquitous wireless signals, the wireless sensing system will provide a variety of high precision, high reliability, high security, and convenient application services, among which the human behavior identification technology is at the core for public use [10].