# Routing Protocols for UAV-Aided Wireless Sensor Networks
## 1 Introduction  
Recently, unmanned aerial vehicle (UAV) networks emerged and became popular. Due to the rapid deployment and flexibility of UAV devices, UAV networks are being considered as future wireless communication systems. The key reason is low cost and availability of UAV devices. This is because the rapid deployment of technologies such as Internet of things devices, sensors, embedded microcomputers, low-cost Wi-Fi radio interfaces, global positioning systems (GPS), and batteries enables UAVs to be widely used in numerous application areas in military and civilian domains [1, 2]. In recent years, routing protocols for UAV networks were reported in the literature. In a previous work [1], the authors investigated several cluster-based routing protocols in UAV networks. In Reference [2], the authors provided a comprehensive survey of routing protocols in different categories, including topology-based, position-based, and forwarding-based routing in UAV networks. In UAV networks, multiple UAVs communicate with each other and with ground stations. UAV-aided wireless sensor networks (UAWSNs) are quite different from UAV networks in terms of communication entities, communication distance, UAV mobility, dynamic link quality, and frequent topology change. UAWSNs are wireless sensor networks with a UAV sink, where a UAV is used to collect data from ground sensor nodes. Hence, a number of routing protocols for UAWSNs were reported in the literature. Therefore, the routing protocols for UAWSNs, which we present in this paper, are different from those for UAV networks in previous surveys [1, 2]. Originally, UAV devices were used in military applications; however, recently, they were used to enhance the operation of wireless sensor networks (WSNs) in various applications, such as environment monitoring [3; 4], vehicular network monitoring [5], wildfire management [6], remote sensing [7], farming [8], search and rescue [9; 10], emergency communications [11], and infrastructure monitoring [12]. UAWSNs are effectively applied to some geographical scenarios, such as remote, harsh, and isolated areas [13], as well as to data collection from sensors [14], data relay [15; 16], and UAV-aided wireless communications [17; 18]. For collecting the sensed data in WSNs, building a new communication zone or maintaining a communication infrastructure is too expensive and difficult to maintain. The quick and effective transmission of sensor data to the base station (i.e., the sink node) is a challenging task. In traditional WSNs, sensor nodes deliver the data hop by hop until they reach the sink node. In the process of data transmission, the sensor node consumes more energy not only for transmitting the sensed data but also for relaying the data hop by hop. Therefore, the data delivery process is quite burdensome for large-scale sensor networks. Multi-hop transmission consumes more energy, as well as faces high latency. Sensor nodes deployed over a large geographical area impose constraints on data transfer to the sink node [19]. Recently, due to significant technology development, WSNs were enhanced with UAV devices. UAWSNs are a hybrid system architecture for data collection from sensors. The tendency to use UAVs in WSNs is increasing more and more today according to a wide range of applications. To enhance the UAV-WSN collaboration for data acquisition, efficient routing protocols are necessarily required. That is, UAV-based data acquisition is an effective solution for retrieving sensor data, even from inaccessible locations. A UAV can move over the sensor network and retrieve data from the sensor nodes. This reduces energy consumption and avoids long transmission distances and redundant transmissions. The UAV can adaptively adjust the flight path for better channel conditions to collect data from the sensor nodes and transmit the data to the base station.