# Routing Protocols for UAV-Aided Wireless Sensor Networks
## 4 Routing Protocols for UAWSNs
### Network Structure-Based Routing
#### 4.1.2 Hierarchical Routing  
In hierarchical routing, sensor nodes are assigned to perform tasks, such as aggregation and forwarding. In hierarchical routing, high-energy nodes perform data aggregation, and low-energy ones perform data sensing, as shown in Figure 2.  
In a linear WSN, nodes are placed in a single straight line, forming a thin linear sensor network (LSN). Furthermore, if all the nodes remain between two parallel lines that stretch for a long distance relative to the transmission range, they constitute a semi-linear or thick LSN. The type of routing involved with LSNs is defined as linear sensor routing. Cluster-based routing is highly suitable for WSNs because it reduces the number of data transmissions and saves energy. Sensor nodes are arranged into clusters, where a particular node is designated as the CH that is in charge for communication with the base station or sink node. UAV-based liner sensor network (ULSN): Routing the data in a multi-hop fashion in LSNs that can extend even up to hundreds or thousands of kilometers can cause high-energy dissipation. A UAV-based liner sensor network (ULSN) [39] deals with such a problem using UAVs. Four kinds of nodes are defined in the system. They are sensor nodes (SNs), relay nodes (RNs), UAVs, and sink nodes. SNs are basic sensor nodes assigned to gather data from the environment. RNs collect information from the nearest sink nodes, and UAVs move along the LSN to collect data from the sink nodes. Sink nodes are placed at both ends of the LSN. SNs use a normal and non-complex algorithm to transmit data to the nearest RN. RNs act as CHs and transfer data to a moving UAV. Finally, the UAV carries the data to the sink node. This procedure of the ULSN decreases the energy consumption significantly because of the reduction of transmission ranges from the SN to the RN and the use of one-hop transmission from the RNs to the UAV. The ULSN can also reduce the interference among RNs caused by the hidden-terminal problem. One drawback of Reference [39] is that the total travel time that a UAV may take while traversing a big LSN was not considered. Cluster-Based Routing  
In cluster-based networks, nodes are divided into several virtual groups. In the clustering schemes, nodes are allocated a different purpose such as CH, cluster gateway (CGW), and cluster member (CM). Figure 2: Data dissemination and gathering in a UAWSN. The CH functions as a local coordinator for its cluster, performing inter-cluster communication and data forwarding. CGW is a non-CH node with inter-cluster links. CGW is used to access neighboring clusters and exchange information between clusters. Ordinary nodes are called as CMs. It was shown that cluster schemes perform better in a large area with a large number of nodes. There are several objectives for clustering in UAWSNs. Scalability and reliability, fault-tolerance, maximal network lifetime, and energy efficiency are the most common objectives among them. In a multi-UAV network, one ground station is used to control one or more UAVs. The ground station controls the UAV directly without any air-to-air (A2A) communication links. However, this scheme does not perform well on a large scale. A centralized-based scheme does not scale well for large UAV networks. In cluster-based schemes, deployed UAVs may be minimized, and area coverage may be maximized. A cluster-based routing protocol, which provides energy efficiency, and clustering schemes help to increase the network lifetime. In cluster formation, all nodes are allowed to make independent decisions to generate efficient clusters. Clustering properties are divided on the basis of the cluster characteristics such as size of the cluster, the number of clusters, intra-cluster communication, and inter-cluster communication. The number of clusters may be either constant or variable. The size of the cluster may be equal or unequal. In cluster-based routing, the communications within the cluster may be either direct or multi-hop.