# Zero Energy IoT Devices in Smart Cities Using RF Energy Harvesting
###### Abstract
As the charger moves from nearest to farthest, it consequently changes the result of the transmission of sensed data from fast to slow. 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. These problems lead us to find an optimal charging tour that guaranty optimal positions for the charger. Figure 1: Energy harvesting model in Smart Cities. As it is clear from our previous practical experiment, the nodes that are near the charger harvest energy and the sensor sends sensed data quickly, while the nodes that are farthest from the charger take a long time to send the sensed data. So, distance plays an important role in charging devices. The main purpose of this report is to develop a charger and sensor placement strategy to provide continuous energy to the sensor board and prevent it from being in an idle position. The previous methods can also prolong the lifetime of the zero-powered energy devices; however, our concentration will be on one charger and multiple sensor boards because we only have one Powercast RF energy transmitter and two sensor boards. The following problems will be taken into consideration and overcome by our strategy:  
1. The Powercast energy transmitter has a central frequency of 815 MHz with 3W EIRP emitting power signals. The power transmitter antenna has 60\({}^{\circ}\) horizontal and 60\({}^{\circ}\) vertical beam patterns. This means that we have constrained the placement of sensors in this direction; otherwise, it will not charge the sensors to transmit the data, and consequently, it will lead the sensor to the idle position. 2. The evaluation board receives this power through patch and dipole antennas and converts it into DC with capacitor-regulated voltage output up to 5.25 V, and output current up to 50 mA to charge the capacitor of size 50 mF. The patch and dipole antennas have 6.1 dBi and 1 dBi of gain power, respectively. The beam pattern of the patch antenna is 122 horizontal and 68 vertical, while the beam pattern of the dipole antenna is 360. So, we have a constraint on the patch antenna that it will only place in 122 horizontal and 68 vertical positions; otherwise, it will not receive the transmitter power, and consequently, it will lead the sensor to the idle position. 3. The distance plays an important role between charger and sensor, and the received signal strength is inversely proportional to the square of the transmission distance, so we also have a constraint on the distance that the distance between charger and sensor should be less. For the prevention of a device being in the idle position, the researchers used several techniques for providing continuous energy to the device, such as single/multiple mobile charger placement, single/multiple fixed charger placement strategies, set covers, and probabilistic charger placement. However, it has its problems, such as cast, charging time, etc. In this work, our focus will be on a single mobile charger with fixed sensors. Our great attention will be on:  
1. The effect of the moving charger on receiving data from the sensors. 2. The speed of the mobile charger. 3. The direction (theta) of the mobile charger.