## 2 RF-EH System  
The RF-EH system has received much popularity over recent years, as the method offers an alternative yet the sustainable approach to supply power to low-powered electronic systems [18; 19]. Among the common low-powered electronic systems include the internet of things (IoT)-powered devices, wireless sensor networks (WSN), and smart metering systems. Moreover, RF-EH is a suitable alternative and promising approach to deliver energy to next-generation wireless networks [20]. The energy generated by powerful RF signals can be used to charge an RF-EH circuit's energy storage unit. For instance, supercapacitors or rechargeable batteries are time-efficient compared to weak signals [21]. The use of energy harvesters to provide power for WSNs is a promising solution. Sensor devices operate on the energy that already exists in the environment, rather than relying on centralized power sources to charge their batteries. To power integrated circuits, a holding capacitor or supercapacitor is used to keep the direct current voltage (DC voltage) that is used to power them [22]. The different output of the RF power presents a considerable setback in the configuration of RF-EHs since the harvester performance, including the efficiency, is proportional to the power input and the RF frequency source as a result of the impedance matching and the turn-on voltage of the RF-EH, respectively [22]. While the design methodology to achieve optimal RF-EH performance has been developed [23], the harvester must be capable of monitoring the frequency and the power level of the operating RF energy to consistently achieve optimum efficiency. In comparison to wireless power transmission (WPT), the nonreliance of power from the operator of the harvester is the key advantage of RF-EH, which constitutes a \"free\" energy source. Nevertheless, RF-EH is regarded as the most complex energy source due to the varying frequency over the available harvestable power, location, time, energy source distance, and environmental factors. Furthermore, RF-EH has a low power density, particularly in comparison to other renewable energy sources [24]. Despite these limitations, RF-EH faces only minor density problems, particularly the sub-microwatt state (for example, a cellular GSM base station that generated 0.1 \(\mu\)W/cm\({}^{2}\)[25]), significantly different from other ambient energy sources from the sun, through motion, or electrochemical reaction. The small RF density can only be used in ultrapowered devices under constant operation (non-duty-cycled) or low-powered applications, such as duty-cycled operation and low-powered wireless sensors in delay-limited devices, given that an adequate amount of RF energy must be harvested prior to the operation of the system [25]. A typical RF-EH circuit comprises a rectifier, a voltage multiplier, an antenna, and a device for energy storage. The most vital part of the RF-EH circuit is the rectifier, which significantly influences the system's efficiency. The antenna serves as a transducer to convert the strength of an electric field into a voltage difference, or vice versa. The rectifier, on the other hand, converts RF power to DC power. The voltage multiplier produces a higher output DC voltage level when the sensor or energy storage device is activated. Once the energy has been harvested, batteries or supercapacitors are used to store it. It is critical to measure and investigate the power density of EM fields in the ambient environment before designing an RF-EH circuit. The electromagnetic spectrum has been measured in many countries [26; 27] since the preferred frequency band, or bands for radiofrequency energy-harvesting circuits, are the most powerful. The structure of the RF-EH system is illustrated in Figure 1[28]. Wireless communication systems' RF signals would be the best suitable renewable energy source. Energy sources are based on vibration, light, and heat, and are not continuously present everywhere.