# Zero Energy IoT Devices in Smart Cities Using RF Energy Harvesting
## 3 Materials and Methods
### Experimental Models  
In our testbed model, we are using the Powercast P2110-EVAL-01 development kit, which is used for energy harvesting. The TX 91501-3W-ID power transmitter (transmitter) transmits the power; the P2110 Evaluation Board P2110-EVB (receiver) receives this power; the Wireless Sensor Board WSN-EVAL-01 plugin with the P2110 Evaluation Board P2110-EVB sends the sensed data to the access point; and the hyper terminal is used to show the sensed data on a computer/laptop screen. The TX 91501-3W-ID transmitter is responsible for transmitting the energy signal to the P2110 Evaluation Board P2110-EVB. The evaluation board obtains the energy signal, converts it to DC, and recharges the super capacitor. Then, the harvested energy is used for sending data by the wireless sensor board. In our previous experiment (fixed charger and fixed boards), if the charger went farthest from the evaluation board, then it degraded the sensing and communication processes. So, we need a technique that can cover the farthest and nearest problems. For the solution to this problem, we are going to equip the energy transmitter with a moving toy to make it a mobile charger. The movement will occur in such a way that it can keep in mind our constraints, not going away from the maximum transmission range of the energy transmitter and not crossing a 60-degree area. The sixty-degree area has the beam pattern of the transmitter, which has 60\({}^{\circ}\) widths and 60\({}^{\circ}\) heights. For simplicity, we used 60\({}^{\circ}\) areas. To fulfill these constraints, we are going to present our solution. Our solution is based on two sub-problems: an optimal charging tour to find optimal positions for a charger and discovering the charging area or points that are under the eye of the charger from any point when the charger moves. 1. Optimal charging tour to find optimal positions for charger. The charger moves in circular form with a constant velocity in an anti-clockwise manner and completes its one tour in Tk time. 2. Discovering the charging area or points. As we know, the charger transmits the energy in a fixed directional, i.e., 60\({}^{\circ}\). We need a fixed area that has points that come under the direction of the energy transmission whenever the charger moves; otherwise, the charging device will never receive energy when the charger moves, and it will go to a dead position. For this purpose, we are going to find the fixed area or points for the charging device to obtain energy from every position of the charger during movement. Since we have two antennas, a patch and a dipole, with different receiving patterns, the dipole is omnidirectional, vertically polarized, and its energy pattern is 360\({}^{\circ}\), while the patch is directional, vertically polarized, and its energy pattern is 122\({}^{\circ}\). For simplicity, we are using 360 coverage areas for the dipole and 122 coverage areas for the patch antenna. The Powercast technology company used the Friis equation for energy calculation. It has an online calculator (in the form of \(\cdot\)xls) [28] through which we can obtain the received energy. For example, if the distance is 1 m, then the received energy is 8.11; at 2 m, it is 2.030; and so on. \[P_{r}=G_{t}G_{r}\left(\frac{\lambda}{4\pi(d+\beta)}^{2}\right)P_{t} \tag{9}\]  
where \(P_{t}\) is the transmitted power, \(P_{r}\) is the received power, \(G_{t}\) is the transmitter antenna gain power, \(G_{r}\) is the receiver antenna received power, \(d\) is the T-R separation distance, and \(\lambda\) is the wavelength. Gain power is based on the aperture of the antenna. Keeping in mind the polarization loss in power transfer, signal power should be rectified and converted to electrical energy before it can be used. \[P_{r}=\frac{G_{t}G_{r}\eta}{Lp}\left(\frac{\lambda}{4\pi(d+\beta)}^{2}\right)P_{t} \tag{10}\]  
where \(L_{p}\) represents polarization loss, \(\eta\) is rectifier efficiency, and \(\beta\) is a parameter to adjust the Friis free space equation for short distance transmission.