## 2 Related Works  
A common mechanism for reducing power consumption in listening mode is duty-cycling. When duty-cycled, devices wake up at selected times, listen for defined time slots, and go back to a sleep mode; hence, the battery lifetime could be extended. Unfortunately, the planned duty-cycling technique is always a trade-off between energy consumption, sensitivity, and a higher mean communication latency [20]. Since the WuRx is usually in listening mode for a long time period, the selection of the circuit elements of WuRx is very critical, especially for high-frequency signals where the energy consumption, the sensitivity, and the reliability of the received packets are significantly challenging, especially with regard to their interactions between each other. The WuRx hardware is generally based on special electronic circuits designed to reduce power consumption without reduction in sensitivity, reliability, and data rate. The hardware design of WuRx can be based on integrated complementary metal-oxide semiconductor (CMOS) circuits [21; 22; 23], where they show a complicated fabrication process that may need a longer development time period within expensive procedures, in contrast to circuits based on COTS components that are usually cheaper in setup, maintenance, expansion, and development. In order to save required energy in listening mode, the WuRx based on COTS uses generally passive components for the continuous listening to the radio channel to activate the MCU [24]. Various WuRx circuit architectures have been developed to reach a compromise between the sensitivity and data rate, as well as power consumption. Analyzing the data communication and wake-up functionalities into one WuRx hardware was presented in [25]. The latency, the current consumption, and the overall operation range performance under different transmit power levels were characterized. The authors developed a WuRx without an LF amplifier. The system was evaluated in an outdoor setup with a performance range up to \(40\,\mathrm{m}\) by a transmission power of \(11\,\mathrm{d}\mathrm{B}\mathrm{m}\). The presented approach of WuRx without any amplification stage and no RF bandpass filter can affect the sensitivity and reliability, where interferences from other RF devices can affect the system dramatically. Ref. [26] presents a WuRx with a comparator situated between envelope detector and low-power microcontroller to sense smaller input signals and generate the interrupt to the MCU. A wide range of sensitivity with the received packets sequences has been tested in the case of two working frequencies, \(433\,\mathrm{M}\mathrm{H}\mathrm{z}\) and \(868\,\mathrm{M}\mathrm{H}\mathrm{z}\). Results present experiments evaluated within the same region of the nodes up to \(24\,\mathrm{m}\), a packet receiving sequence of \(99\%\) and \(96\%\) for the outdoor and indoor scenarios, respectively. The proposed WuRx achieved a sensitivity of \(-55\,\mathrm{d}\mathrm{B}\mathrm{m}\), but the data rate is not mentioned. An approach to enhance the sensitivity focuses on the use of voltage multiplier to collect the weak RF energy from the antenna. Proposed in [27], the envelope detector circuit with two or more voltage stages acts as a charge pump. However, increasing the number of voltage stages will reduce the whole efficiency of the envelope detector. The voltage at the input decreases because of smaller input resistance of envelope detector and higher conductance [28]. Another technique is the use of an RF amplifier, or low noise amplifier (LNA) situated between antenna and detection circuit. The authors in [29] propose an LNA circuit that consists of a commercial component with a low shutdown current, short turn-on time, and high amplification. The developed circuit reaches a sensitivity up to \(-71\,\mathrm{d}\mathrm{B}\mathrm{m}\), but with a current requirement of \(5.8\,\mathrm{m}\mathrm{A}\) when permanently active.