## 3 Clustering Model for UAV Systems
### Network Clustering Process
#### 3.2.2 Time Slot Allocation of Nodes  
1. Time slot allocation in the network partitioning phase During the network partitioning phase, the period T is divided into a GCS time slot and W task time slots, as illustrated in Figure 2. The GCS transmits information during its designated time slot and is responsible for dynamically managing the status of the slot. During the non-GCS time slots, each task node monitors the channel status to determine whether individual task time slots are idle or occupied. Upon detecting idle time slots, the node randomly selects one from the available idle slots to broadcast a Hello message. If a collision occurs, where multiple nodes attempt to broadcast in the same time slot, a backoff mechanism, such as binary exponential backoff, is triggered [37]. The conflicting nodes then perform a randomized delay before reselecting an idle time slot, repeating the process until a successful transmission is achieved.  
Figure 2: Time slot allocation in the network partitioning phase.  
Time slot allocation in the neighbor discovery phase  
During the neighbor discovery phase, in each cycle of duration T, the duration excluding the GCS time slot is divided into K equal-length time frames, each corresponding to one of the K network partitions, as illustrated in Figure 3. Each partition exclusively occupies its designated time frame for communication. Within each time frame, a fixed number L of time slots is further defined, where L is set to be greater than the number of nodes in the corresponding partition to reduce the probability of time slot collisions. During their assigned time frames, nodes first monitor the channel status to determine whether individual time slots are idle or occupied. Upon detecting idle slots, the node randomly selects one of the available idle slots to broadcast a Hello message. If a collision occurs, where multiple nodes attempt to broadcast in the same time slot, a backoff mechanism, such as binary exponential backoff, is triggered. The conflicting nodes then perform a randomized delay before reselecting an idle time slot, repeating the process until a successful transmission is achieved.  
Time slot allocation in the clustering phase  
During the clustering phase, in each cycle of duration T, the duration excluding the GCS time slot is divided into K equal-length time frames, with each frame assigned to one of the K network partitions, as illustrated in Figure 4. Within each frame, a fixed number of N + M time slots is defined, where N is the number of nodes currently present in the corresponding partition, and M is a scenario-dependent redundant value dynamically generated to enhance scalability and reduce potential collisions. During their assigned frame, each cluster head, selected within the partition, is responsible for assigning fixed time slots to itself and its cluster members. To accommodate future node arrivals, a certain number of time slots are reserved as backup slots. These reserved slots are first allocated to newly joining nodes. In the absence of new nodes during a cycle, the backup slots can be opportunistically accessed by existing cluster members through contention, thereby improving channel utilization.  
Figure 4: Time slot allocation in the clustering phase.  
Figure 3: Time slot allocation in the neighbor discovery phase.