# Development of Radio-Frequency Sensor Wake-Up with Unmanned Aerial Vehicles as an Aerial Gateway
## 3 Sensor Activation and Related Work
A recent study on the in-band wake-up method [47] claimed that, by using both game theory and reinforcement learning techniques, it achieved very effective sleep/wake-up scheduling. However, it kept the wireless communication channel busy and may have required more energy consumption. From an energy-efficiency perspective, the out-band approach is more suitable for the proposed concept that emphasizes opportunistic aerial-ground sensing. There are many studies that employ the out-band wake-up mechanism [48]. In this paper, they are categorized into two groups according to their communication medium: (1) non-RF based and (2) RF-based. In a non-RF based mechanism, researchers proposed wake-up methods using infrared (IR), optical, and acoustic signals. The authors in [49] developed an IR LED-based wake-up mechanism, in which the receiver was a photo-detector that received IR signal and then generated an interrupt. The authors stated that their IR design only consumed 12 uW while listening. It is noted that the obvious drawback of this prototype is the sensitivity of the circuits to external light and vulnerability to ambient noise. In [50], the authors presented a home-energy management system using infrared, signal-based control over a Zigbee network. In this system, an infrared receiver was attached to the Zigbee gateway. The Zigbee gateway was responsible for communicating with other home appliances, whereas the infrared remote control was the out-band wake-up channel used to wake up the Zigbee network. Unfortunately, this paper did not mention the power consumption of the IR receiver. To our understanding, this type of IR receiver in the paper is commercially available and similar to the one used in our experiment, as shown in this paper, which has a better resistance to noise at the cost of a much higher power consumption, and it may require up to 45 mW according to our experiment. Optical communication is another non-RF option for the secondary wake-up channel. Two efforts in [51; 52] used free-space optical (FSO) communication as the transceiver. The receiver, at idle listening, consumes 317 uW and 695 pW. However, the transceiver and receiver both need to be placed in line-of-sight (LOS), and the data rate is slow. It is impractical for use in a UAV since it mostly does not stay in a position that accurately faces the transmitter. Thus, this option is not suitable for our application. The AG-WSN scenario may also limit the use of acoustics as wake-up methods [53; 54] due to the noise produced by the UAV blades. Ultrasonic, as stated in [55; 56], may avoid the noise made by the UAV. It uses a piezoelectric transducer that converts the mechanical energy into electrical energy for generating wake-up interrupts. However, most ultrasonic communication and ranging efforts, to date, are applied in indoor (short-range) or LOS scenarios [57; 58]. In comparison to the non-RF based wake-up mechanisms reviewed above, RF-based communication has the advantages of not requiring LOS, it has better noise and interference tolerance, a higher data rate, and it is more cost-effective. Research on the RF-based wake-up mechanism can be divided into two designs: passive wake-up and active wake-up, both of which have been well studied in the laboratory environment. In a passive design, the RF receiver harvests energy from the transmitter to power itself, thus, requiring no power supply [59; 60; 61]. In [62], the authors simulated a passive RF wake-up receiver, in which they indicated that comparing with the existing duty-cycle method, their RF wake-up could significantly enhance energy efficiency by up to 70%. There are also simulations on both passive and active RF wake-up circuits, such as [63; 64]; the authors of these efforts later implemented the passive RF circuit into a sensor network with a multi-hop capability [65]. However, among these passive RF-based methods, information on the communication range between the transceivers was not found.