# Zero Energy IoT Devices in Smart Cities Using RF Energy Harvesting
## 3 Materials and Methods
### Testbeds Studies
#### 3.3.6 Optimal Points/Area for Sensor Devices
No, any sensor goes to a dead position when the charger travels to its optimal points for the charger and transmits the energy to the optimal points of sensors. Figure 15 shows the RSSI of a dipole antenna. The dipole antenna works better at our optimal points and receives the energy of the sensor at 60 degrees from the charger all the time. No, any sensor goes to a dead position when the charger travels through optimal points. ### RSSI of Patch Antenna during Circular Tour

Figure 16 is based on the RSSI, which is obtained during the circular tour of the charger in constant motion. In this experiment, the sensor devices with patch antennas are fixed (note that the patch antenna has a 122\({}^{\circ}\) beam pattern), and the charger is mobile (Figure 3). When the charger changes the direction angle, then the sensor device degrades charging, and when the charger angle and sensor device angle are in the line of sight then the sensor

Figure 14: Patch Antenna RSSI. Figure 5:0 Dipole Antenna RSSI. device scavenges more energy. For example, in Figure 16, data1 and data5 are not in the line of sight, while data2, data3, and data4 are in the line of sight. At data3, the sensor device scavenges 100% of the energy from the charger. For this purpose, we can say that Figure 5 is the solution for the optimal placement of the sensor device. ### RSSI of Dipole Antenna during a Circular Tour

Figure 17 is based on the RSSI that is obtained during the circular tour of the charger. Since dipole antennas can receive energy from all directions, so Figure 12 shows the optimal place for a sensor device. Figure 5:1 Patch Antenna RSSI. Figure 5:2 Dipole Antenna RSSI. ## 5 Conclusions

In this paper, we study the optimal points for the placement of chargers and the optimal area for the sensor device problem. In our experiments, we have observed that when a device moves away from the charger, the energy scavenging process degrades and reaches a dead state. To stop a device from entering a dead state, we introduced a mobile charger technique to charge the device and find the optimal place for the charger and sensor devices. During mobile charging, we have also observed that when the directional antenna changes its angle, the energy scavenging process degrades. We have proposed two algorithms for directional and omnidirectional antennae to provide an approximate solution to our problem. We considered coverage requirements and energy requirements. In the proposed algorithms, the charger is movable and the sensor devices are fixed. Moreover, the charger movements are controlled at specific angles in such a way that each sensor device can harvest more energy. Furthermore, we have obtained results from these scenarios and shown that this technique has a promising output. This solution is for a 2D environment and cannot be applied to a 3D environment. We will extend this work to a 3D environment. Furthermore, we will use standard simulation tools to compare with other state-of-the-art solutions and also use this work in routing protocols. Conceptualization, M.G. ; methodology, M.G. ; software, M.G. ; validation, M.G. and M.A. ; formal analysis, W.A. and A.G.; investigation, A.G.; resources, M.G. ; data curation, A.u.R. ; writing--original draft preparation, H.Z. ; writing--review and editing, A.G., S.-J.K. and J.-G.C. ; visualization, A.u.R. ; supervision, M.G. ; project administration, M.G. ; funding acquisition, M.G. and S.-J.K. All authors have read and agreed to the published version of the manuscript. This study was supported by the HEC-NRPU-9189 project (Semi-permanent Wireless Sensor Networks Operated by RF Power Transfer), funded by the Higher Education Commission of Pakistan, Department of Computer Science, Bahria University, Islamabad, Pakistan. This study was also supported by the BK21 FOUR project (AI-driven Convergence Software Education Research Program), funded by the Ministry of Education, School of Computer Science and Engineering, Kyungpook National University, Korea (419990214394).