# Development of Radio-Frequency Sensor Wake-Up with Unmanned Aerial Vehicles as an Aerial Gateway
## 3 Sensor Activation and Related Work  
Although solar power or other intermittent energy supply techniques exist, battery power continues to be considered as the most reliable source for powering sensors and robots. By implementing the commonly adopted duty-cycle method, wireless sensor nodes could be pre-programmed to wake up and communicate with the gateway, then go back to sleep after communicating. This approach for extending battery life has been treated as a default function in many commercial wireless sensors. A number of researchers also attempted to optimize power management to further extend the battery life of WSNs [37; 38; 39; 40]. However, one key problem that prevents us from realizing long-term aerial-ground sensing is the opportunistic nature of deploying the UAV (gateway) and the sensor nodes. In an AG-WSN, the gateway (as a payload of the UAV) is deployed spatially to proceed toward the ground sensors on a non-scheduled basis or randomly upon abrupt events. This further implies that the ground sensors do not have 'knowledge' or are not programmable to realize duty-cycle sensing. If the ground sensors are turned on to include at least the microcontroller and communication units (whereas the sensing units may be on or off according to the duty cycles), the battery of the sensor nodes may be drained quickly. One straightforward approach to such energy inefficiency issues is to wake up ground sensor nodes when the UAV is deployed as needed to approach the sensors without any preprogramming. In this paper, we first propose to use a radio frequency (RF)-based out-of-band wake-up mechanism. Then, comparative studies are conducted to investigate their energy saving performance against two other wake-up mechanisms. Using a traditional star-like sensor network, the analytical and experimental studies show solid evidence that the RF-based wake-up mechanism outperforms the other two solutions on energy consumption. Earlier efforts reveal that data transmission in a WSN is generally very expensive in terms of energy consumption, whereas data collection (or the sensing itself) consumes significantly less [41]. For this reason, various methods are developed to extend the life of battery-powered WSNs by reducing the power consumption of the wireless modules. A significant number of efforts were found that focused on developing lower level network protocols by adopting duty-cycle based solutions [42; 43; 44]. These studies aimed to optimize the network protocols, specifically through reducing the energy consumption during the idle or the listening time of the wireless modules. For example, the authors in [45] proposed an adaptive medium access control (MAC) protocol, which introduced a flexible duty-cycle method and claimed to reduce 96% of energy use compared with traditional protocols. However, the core concern for these duty-cycle solutions is that the wireless modules do not know when the data transmission is coming or required, and the node must listen periodically to limit data latency; thus, the duty-cycle ratio cannot go arbitrarily low [46]. Also, duty-cycle methods may have problems with delay and synchronism and, hence, the protocol is relatively complicated. As such, using waking-up mechanisms to answer this concern has been extensively studied. A number of sensor activation methods have been proposed to date. Essentially, such an activation approach features a waking-up mechanism for activating sensing modules in an as-needed (or on-demand) basis. There are two categories of methods when considering wake-up mechanisms for use in wireless networks: in-band and out-band. If an in-band method is used, a special value is transmitted through the data channel to send out the wake-up signal. By contrast, a separate channel is needed to realize such a waking-up mechanism in an out-band method. Using in-band methods can reduce the complexity and cost of the implementation.