# State-of-the-Art Techniques in RF Energy Harvesting Circuits
## 1 Introduction  
Energy harvesting (EH) 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 that would normally use batteries or were connected to a power supply network. This can be achieved by designing autonomous energy systems that can function in a variety of applications, such as in the diagnostic and therapeutical field (medical implants) with very low power consumption. 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. In this work, we focus on RF Energy Harvesting and wireless power transfer (WPT techniques as potential sources in autonomous energy systems) [1].  
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 [2]. A circuit that harvests RF energy, from a dedicated source at a short-range is expected to produce power levels in the 50 nW/cm\({}^{2}\) range. An example is an RFID chip that is powered by an RFID reader. The dedicated source enables embedded devices to recharge batteries. On the other hand, a circuit that harvests RF energy from an ambient source, can exploit this energy to charge various storage systems. This type of circuit is expected to produce power levels in the 2 nW/cm\({}^{2}\) range. Typical ambient sources include Wi-Fi, GSM/Cellular, FM/TV/DTV, Bluetooth, etc. This type of source is common for applications for which is very difficult or impossible to replace batteries [3]. RF energy scavenging circuits are known for more than 60 years, but only a few have been able to harvest energy from ambient RF sources [4].  
[PERSON] formulated the first theoretical knowledge about wireless power transfer in 1864. Almost two centuries ago [PERSON] described how to transfer energy between two points without the need for a physical connection to a power source as \"all-surpassing importance to man\". Hence the idea of wireless power transfer (WPT) has been around since the inception of electricity [5]. [PERSON] in the 1960s with the development of wireless communication technologies developed a rectifying antenna named \"rectenna\". As a follow-up to this invention, his team conducted several experiments in 1968 on the transmission of microwave power, which were successful [6]. Figure 1 illustrates a typical RF energy harvesting system that consists of a transmission antenna and a rectenna that is comprised of receiving antenna, an impedance matching network (IMN), a rectifier, and a power management circuit. From all the parts of the rectenna, we discuss in detail about the synthesis of the rectifier. The challenge, during the design of a low input power rectifier, is the combination of a high output voltage and a high power conversion efficiency.  
The remainder of this review work is structured as follows. Firstly, the details and the parameters of the review process, as well as our contribution to this research topic are introduced in Section 2. In Section 3, a detailed analysis of the rectenna parts is performed and their qualitative and quantitative characteristics are described. Section 4 discusses the main results and remarks of this review work. Finally, Section 5 outlines the concluding remarks of this work.