# Routing Protocols for UAV-Aided Wireless Sensor Networks
## 4 Routing Protocols for UAWSNs
### Network Structure-Based Routing
#### 4.1.2 Hierarchical Routing
In clustering approaches where the cluster size is large and the number of CHs is small, multi-hop communication between CH and CMs may be needed. Since the nodes are equipped with long range, they require a multi-hop approach for communication. CH election is a major part of a clustering scheme. The election of CHs has a substantial effect on the cluster-based routing performance. In cluster-based routing, CHs may be either stationary or moveable. In general, CHs may move within a limited area. For moveable CHs, the cluster topology is a complex task. CHs may perform a vital role such as relay, aggregation, and fusion in clustering approaches. A CH performs as a relay node for controlling the data, data aggregation operation, and synchronization. In cluster-based routing, CH position is important. The position of the CH may be distributed, dynamic, centralized, or hybrid. The distributed approach is widely used because all nodes get the opportunity to serve as a CH in this approach. In UAWSNs, the UAV acts as a mobile sink node. This is a smart approach to collect data from sensors. CH election is the noteworthy process in clustering approach. CH can be elected by various processes. Most of the clustering approaches use proactive clustering rather than reactive clustering. The proactive routing tables are updated through the exchange of periodic messages, whereas the reactive clustering is based on the data-centric method. In reactive routing, maintaining a routing table is not necessary and, when a node requires sending data, it finds a new route. In highly dynamic routing, finding a new route repeatedly results in colossal routing overhead. Based on the resource of sensor nodes, UAWSN can be classified into two categories of homogeneous and heterogeneous networks. In the homogeneous network, all sensor nodes have the same capabilities of energy, communication resources, and computation power, and CHs are selected according to a random manner. On the other hand, in the heterogeneous network, nodes have unequal capabilities. For this reason, CH selection is important in heterogeneous networks. Normally, the node with more capabilities is selected as a CH in heterogeneous networks. UAV-assisted routing protocol (URP): URP [40] is a crop health-monitoring system developed using a UAV-aided clustered WSN. Collecting data from WSNs deployed in farm fields is challenging because of extreme weather conditions, like high temperature, dry weather, sandstorms, and remote locations. To overcome these challenges, the authors of Reference [40] proposed a dynamic data-collection mechanism using UAVs as shown in Figure 3. The dynamic clustering in URP is illustrated in Figure 3. In URP, every CH node participates in the CH selection process based on node probability calculated using a Bayesian classifier. In the routing phase, nodes are classified into three types, namely, CMs, candidate clusters (CCs), and candidate CHs (CCHs). Finally, to nominate a node as a CH, the UAV and CCHs participate in the CH selection process. The system has a large number of sensors deployed to monitor different parameters of the crops, soil, and environment. The role of the UAV in the system is to create a communication infrastructure between the end users and sensing devices. Sensing devices are grouped based on their geographical locations using a clustering method. After that, nodes near the pathway of the UAV are selected as CHs to transmit data to the UAV. The most significant advantage of URP is that it can be deployed anywhere without any prior infrastructure, and it massively increases network lifetime. The protocol can be implemented in a multi-hop manner in the future. Cooperation-based UAV and WSN (C-UAV-WSN): It is easier to manage UAV and WSN as separate independent units. Combining two different paradigms can lead to increased performance, but it makes the network complex to handle. To deal with this issue, the cooperation between UAV and WSN (C-UAV-WSN) [41] can lead to optimal cooperation between the WSN and UAV in two ways.