# State-of-the-Art Techniques in RF Energy Harvesting Circuits
## 4 Discussion  
Energy harvesting is the process in which energy is harvested from the environment and converted into electrical power. The aim of energy harvesting is the energy autonomy of systems. Energy from the natural resources can be solar, thermal, vibration, friction, or Radio Frequencies (RF) signals and behave differently over time, depending on the operating environment of the system. We focused on RF Energy Harvesting and how an RF EH system, a rectenna, can be designed. There are two different approaches of energy transfer: the dedicated transfer that means high power values or the harvesting of ambient energy that means low power values. A rectenna consists of a receiving antenna, an impedance matching network, and a rectifier. The design process includes the selection of the proper energy harvesting circuit topology, the selection of a suitable diode, and the design of an appropriate impedance matching network. At the end of the design process, there is the optimization and simulation of the whole system. We categorized many designs according to the type of different modules (antennas, impedance matching networks, rectifier circuit) and we mentioned the pros and cons of each topology. Furthermore, we came to some basic conclusions about the design of each unit of the rectenna, which we mention below. Some antennas are made from several materials and in several designs, such as dipole antennas, Yagi-Uda antennas, microstrip antennas, etc. From the bibliographic research that was performed, no design is used or preferred more than the rest. Maybe in the near future and due to their easy fabrication as well as their small size, microstrip antennas will be preferably selected. In terms of the design of the impedance matching circuit the most used matching networks, are the L, the reversed L, the \(\pi\), and the T network. The L matching is frequently used and has only two components, which makes the design and control process match more simple. In some designs it's better to choose a T or \(\pi\) impedance matching network because these topologies achieve wider bandwidth than the L network. Additionally, the T matching network can be used to improve the output voltage levels. In addition to that, the vast majority of the authors chose to use distributed elements in their designs that are preferred at microwave frequencies (3-300 GHz). The lumped elements are generally used at lower frequencies (below 3 GHz). The selection of diodes is one of the most critical steps in the design of the rectifier. Usually, Schottky diodes are used because of their low junction capacitance, low threshold voltage, low barrier, and high switching capacity. A rectifier, which is the fundamental circuit of the rectenna, can be designed with diodes or with MOSFETs. There are benefits to both. But one thing that is clear from all these works is that circuit designers at these frequencies (4\(G\), 5\(G\), etc.) prefer diodes because MOS transistors suffer at high RF power due to a reverse current, which drains some of the scavenged energy back to the input. On the other hand, CMOS technology (BTMOS, DTMOS) sometimes is preferred, because CMOS transistors when operating in low voltages are more sensitive than diodes. Also, as shown in Table 6 above, most designs are voltage doublers and not multipliers because if we use more stages, then we will have a higher drop voltage. In most cases, this is not acceptable. Many designers design reconfigurable rectifiers to overcome some of the limitations such as the low-efficiency and limited working range of the RF energy scavenging systems. Another practice to improve the efficiency in RF energy harvesting systems is to add a boost converter to the output of the rectifier. One of the most basic parameters that characterize the quality of a rectenna system is the power conversion efficiency (PCE), which mainly depends on the load, which in turn can be capacitive, inductive, or purely ohmic.