# RF Energy Harvesting Wireless Communications:
## 3 RF-EHWC Hardware
### Concerns in the Design of Energy Harvesting Circuits  
The performance of the energy harvesting module is usually evaluated in the following aspects:  
* _Power conversion efficiency_ is defined as the ratio of the DC output power to the incident RF power. It characterizes the ability of energy harvester to convert RF energy to DC power and is mainly determined by the structure of the energy harvesting circuit, the frequency and the strength of incident waves. * _Energy sensitivity_ is the minimum incident power to activate the energy harvester. It is affected by the voltage gain of the voltage multiplier. Energy sensitivity is an important metric to guarantee that an RF-EHWC system can work reliably in an RF environment with a low power density. It is worth noting that there is a tradeoff between energy sensitivity and power conversion efficiency, as illustrated in Figure 10[3]. Specifically, although a voltage multiplier with more stages can provide higher voltage gain, the parasitic capacitance in the circuit also increases linearly, which reduces the power conversion efficiency of the antenna, thereby decreasing the input voltage at the initial stage of the multiplier. Therefore, the number of stages in the voltage multiplier needs to be carefully determined based on the incident power of RF signal to maximize the amount of energy that can be harvested. Another important feature is that the power conversion efficiency of the energy harvester does not monotonically increase with the intensity of the incident power, due to the nonlinearity of diodes and the parasitic influence of the used elements [49]. When the incident energy is very strong, the power conversion efficiency will have a sharp decrease as the voltage swing at the diode exceeds the breakdown voltage. In this case, the diode conducts in the reverse direction. The nonlinear feature of varied energy harvesting circuits is presented in Figure 11. Due to the high spreading loss of Electromagnetic waves in the free space, the RF-EHWC devices may receive RF signals of strongly varying incident power at different locations. In this case, the breakdown effect needs to be considered carefully. Figure 10: Output DC voltage with respect to the number of stages in a voltage multiplier (recreated from Figure 9b in [48]). Take a real RF-EHWC system, Powercast P2110-EVB (from Powercast, PA, USA), as an example. The Powercast P2110-EVB achieves the highest power conversion efficiency at 900 MHz [52]. Assume the power of the radiated signal from a dedicated energy source is 60 W (47.8 dBm) [26], then the incident power is only 4.2 uW (\(-\)23.8 dBm) at 100 m, which lies in the low power region in Figure 11. However, when the Powercast P2110-EVB is close to a mobile phone, which is an energy source providing 250 mW (the maximum power that mobile phones are permitted to transmit at 900 MHz frequency band is 2 W, but the average power transmitted by a phone is commonly lower than one-eighth of this maximum value, i.e., 0.25 W [26]), then the strength of the incident RF power can reach 7.9 mW (9 dBm) at 15 cm distance. According to Figure 11, this input power at this level is high enough to trigger the breakdown effect in the diode. To make an RF-EHWC system work efficiently in a high dynamic RF environment, a feasible solution is to use multiple complementary energy harvesting modules. In [3], a dual-stage energy harvesting circuit is introduced, where two separate energy harvesting modules are optimized for different input power. The first harvester has an HSMS-2822 diode based seven-stages voltage multiplier that has a low forward voltage (150 mV) but a small breakdown voltage (2 V) to achieve high efficiency at the low input power. The second harvester uses the HSMS-2822 diode that has a high breakdown voltage (15 V) but a large forward voltage (340 mV), which works efficiently at a high power density.