## 1 Introduction
For each scenario, we performed more than 1600 measurements of E-fields in an urban area in two cases: when the car was in an open space without a direct line of sight to a base station, and when the car was in underground parking. This paper's content is organized as follows: Section 2 presents a brief literature review regarding main concepts such as RF energy harvesting, IoT devices and Wireless Sensor Network (WSN) usage in the modern economy. This part also stresses the potential usage of energy harvesting from EMFs as a future solution for the optimization of the usage of IoT devices and WSNs. Section 3 describes the measurement equipment, methodology, measurement procedure and scenarios. Section 4 highlights the main results of the experiment and proposes a discussion around the potential usage of RF energy harvested from a controlled environment, such as a vehicle's interior. Results from the experiment confirmed the hypothesis that the EMFs emitted by wireless devices in a car can be used as a new energy source to power battery-less wearable or IoT devices, and they can be used to design energy harvesting systems with maximum conversion efficiency. Section 5 presents the main conclusions along with the research limitations and future directions of research, also stressing the value of the present research before the previous literature in the field. Brief Literature Review on RF Energy Harvesting and Its Importance for IoT Devices and Wireless Sensor Networks  
In the following, we present a brief introduction to the RF energy harvesting issue to clarify the premises that were the basis of our research. The experiment that we propose tests the hypothesis that RF energy that accumulates in the car cabin during an outgoing voice call, video call or data transmission through 2G, 3G, 4G and 5G networks is sufficient to be captured and converted into usable electrical energy. The originality of the research related to our article consists of the method of measurement chosen, the specific setting (the interior of the car) and the proposed scenarios (underground parking, normal parking, 4G, 3G emission, etc.). The subsequent applicability to the design of a complete RF energy harvesting system that allows the effective use of the electromagnetic field that exists in the vehicle by capturing radio waves is quite a large issue. Being a common situation,often encountered in the context of the urban population (frequent trips by car throughout the day, considerable time spent in the car, intensive use of mobile phones in the car), we consider it a very plausible example of generating EMFs, which can be used to substantially reduce the consumption of conventional electricity involved even in daily activity during driving with vehicles (sensors, GPS devices, tablets, etc.). Identifying the most efficient renewable energy sources is a constant concern for energy policymakers worldwide. Today, sources such as solar, wind, hydroelectric, thermal, chemical, kinetic and radio frequency energy offer opportunities to capture and convert ambient energy into usable electrical energy, enabling greater energy efficiency, sustainability and the development of autonomous and self-powered systems [25]. Moreover, a novel communication mechanism that enables devices to communicate by backscattering ambient RF signals was also introduced [26; 27]. This innovative and appealing possibility can reduce costs in a very promising manner because backscattering TV signals and other sources of RF signals can have, at the same time, the role of a power source and a means of communication [27]. In completion, RF energy harvesting offers a renewable and environmentally friendly approach for powering low-power wireless devices and sensors, enabling wireless connectivity and reducing the reliance on traditional power sources. There are several factors contributing to the growing interest in RF energy harvesting. The main factor among them is the proliferation of wireless devices that can be seen as EMF sources [10; 15; 28; 29; 30; 31].