# Zero Energy IoT Devices in Smart Cities Using RF Energy Harvesting
###### Abstract  
The invention of batteries made it possible to store electricity for many purposes. One of the purposes is to keep the operations of WSN running without any interruptions. The main drawback of sensor nodes is their limited energy sources. The researcher introduces the energy harvesting (EH) concept for IoT-based WSNs to overcome energy limitations and charge the IoT devices. Many energy harvesting techniques have been introduced, such as solar, thermal, and flow-based ones, but radio frequency (RF)-based EH techniques received great attention from researchers due to their easy availability (from TV, radio, and wireless frequencies). In this paper, we have conducted a real-world experiment on Powercast energy harvesting devices and examined the behavior of sensed data in different scenarios, such as indoor, distance (feet/meters), and directional antennas. In our experiments, we have observed that when a device is removed from the charger, the energy scavenging process degrades and reaches a dead state. To stop a device from entering a dead state, we introduce a mobile charger technique to charge the device and find the optimal place for chargers and sensor devices. During mobile charging, we have also observed that when the directional antennas change their angle, the energy scavenging process degrades. To tackle these problems, we introduced two algorithms for directional and omni-directional antennas that efficiently solve the problem. Furthermore, we have obtained results for these scenarios and show that this technique has a promising output. energy harvesting; IoT; EHWSN; smart cities; RF 20231248.10.  
frequently, which can increase operational expenses [4; 5]. The researchers tried to find the solution to this problem and introduce the concept of energy harvesting techniques from renewable energy sources such as solar, wind, thermal, radio frequency (RF), etc. The RF energy harvesting technique has obtained the attention of emerging technology platforms, such as Google, WISP, PoWiFi, Powercast, and EnOcean STM 300 [4], due to its clean, green, and freely available nature in continuous form and its introduction of wireless charging devices (WCD). WCD is a suitable choice to prolong the lifetime of IoT-based WSNs to near-perpetual. Figure 1 shows the overview of energy harvesting in smart cities. In previous works, data collection and energy transfer were done through wireless power communication networking (WPCN) and simultaneous wireless information and power transfer (SWIPT). In WPCN, the uplink is used for wireless information transfer (WIT), and the downlink is used for wireless energy transfer (WET) [6]. In SWIPT, data collection and energy transfer were assumed to run simultaneously. However, the commercial wireless charging technology platforms, such as Powercast [7] and WISP [8], do not use the WPCN or SWIPT concepts. These two platforms used capacitors for energy buffering due to being environment-friendly, having longer lifetimes, higher recharge cycles, a broader range of voltage and current, low cast [9], and higher performance in low temperatures over batteries [3]. The Powercast platform used the recharge-then-transmit procedure and, at any time, deprived the sensor node of working and harvesting energy (recharging). At this stage, the sensor node halts its normal work and waits for recharging. These phenomena put the sensor node in the idle position. After getting the required time, the sensor node becomes active and starts its normal work. The idle and active positions of the sensor node make the network topology dynamic, which leads to several problems such as dynamic topology, charging latency, communication latency, and farthest-nearest node problems. The farthest-nearest node problem is one of the most important problems in charging IoT devices. This means that a node that is near the charger can harvest more energy as compared with the farthest node.