# A Robust Routing Protocol in Cognitive Unmanned Aerial Vehicular Networks
## 3 Network Model  
A Flying Ad hoc Network (FANET) is essentially a dynamic mobile network that possesses unique capabilities allowing direct communication between Unmanned Aerial Vehicles (UAVs), satellites, and ground stations. This setup is crucial for ensuring seamless data transmission to control centers and facilitating collaboration among UAVs, enabling large-scale ad hoc networking across multiple UAVs simultaneously. FANETs are designed to accommodate the inherently high mobility of UAVs, which distinguishes them from other types of networks such as Mobile Ad hoc Networks (MANETs) and Vehicular Ad hoc Networks (VANETs). This mobility factor plays a central role in various applications ranging from defence inspection to disaster response and aerial photography. A Cognitive Radio Network (CRN) is essentially a smart communication system that can adapt its operations based on the prevailing network conditions. It is like having a network that can think for itself and adjust its processes accordingly. In CRNs, there are two main types of users: Primary Users (PU) and Secondary Users (SU). Primary users hold the licenses for using specific portions of the spectrum, they have exclusive rights to these frequency bands. On the other hand, Secondary Users are unlicensed users who can utilize unused spectrum bands owned by primary users. This concept, pioneered by [PERSON], aims to make more efficient use of the radio spectrum by allowing secondary users to access unused spectrum resources [23]. Now, Cognitive Radio Ad-hoc Networks (CRAHNs) are a specific type of CRN that focuses on cluster-based networks. In these networks, nodes are grouped into logical clusters, where all the nodes within a cluster are located close to each other geographically and are interconnected. This clustering approach helps in organizing the network more efficiently and managing communications within the cluster effectively [23]. Figure 1: Various connectivities of UAVs. In the proposed Cognitive Radio UAV networks (CR-UAV), intermediate nodes play a crucial role as they are deemed reliable and are strategically selected as central nodes based on their path weight. These central nodes form logical groups of nodes, with each group having a designated Central Node (CN) responsible for overseeing efficient communication within the group. Central Nodes (CNs) are pivotal in managing communication between different groups, facilitating seamless inter-group communication. When Primary Users (PU) become active and interfere with the network, the CR-UAV network dynamically reorganizes itself to find alternative routes and maintain connectivity. In cases where nodes become isolated from their groups, they actively seek re-connection by reaching out to new Central Nodes or groups. To facilitate this process, Central Nodes employ a neighbor discovery mechanism. This involves sending out HELLO messages to potential neighbor CNs and awaiting a REPLY. If no response is received within a set time frame, the message is re-transmitted to ensure robust neighbor discovery and network connectivity. The proposed routing protocol introduced grouping them into logical groups. And every group has its own Central Node considering the node with the highest weight value as the Central Node (CN) and the second-highest as the secondary CN. Central Node (CN) is determined when the group has some networking nodes and needs to be controlled by some other node. CN can make communication between inter-connected nodes also with the intra-connected nodes and other groups' CN. CN also uses the Nearest Node (NN) to communicate with the other CN's NN to minimize the transmission energy and for fast communication. NN is used as a communication router between two different network groups, which is considered a common network or common channel. Whenever the PU becomes activated the whole network needs to reform the group because when a PU comes to the network SU needs to find another route for communication as they are using PU's paid network spectrum.