## 4 Performance Evaluation
### Simulation Results
When the number of non-interference slots is sufficient relative to the number of slots required for transmission by ship stations (i.e., when the number of ship stations is small), the collision ratio of SOCA approaches 0%. In contrast, the existing VDES does not account for the overlap between the SIs of ship stations and the expected slot occupancy probabilities of candidate slots, resulting in a relatively higher collision ratio than SOCA. Quantitatively, when RI is 2 and 6, the collision ratio of SOCA is 27.61% and 39.59% lower than that of the existing VDES, respectively. In Figure 7b, the collision ratio in the dynamic ship scenario increases as the number of ship stations grows, similar to the static ship scenario. However, the collision ratio in the dynamic ship scenario is consistently lower than in the static ship scenario, regardless of the RI and network size. This is because the ship stations move at different speeds, resulting in varying transmission intervals of AIS messages. Consequently, collisions between AIS messages are reduced, leading to fewer AIS message retransmissions and a lower overall collision ratio in the network. Quantitatively, the collision ratio in the dynamic ship scenario is 62.90% and 10.11% lower than in the static ship scenario when the RI is set to 2 and 6, respectively. Furthermore, in the dynamic ship scenario, SOCA achieves a 39.27% lower collision ratio than that of the existing VDES. Figure 8a,b illustrate the variations in the packet delivery ratio in static and dynamic ship scenarios, respectively. The packet delivery ratio is defined as the ratio of the number of successful VDE-TER data transmissions to the total number of VDE-TER data transmissions. In Figure 8a, the packet delivery ratio decreases as the number of ship stations increases, regardless of the RI. This decline is primarily due to increased collisions between AIS messages. These collisions lead to retransmissions of AIS messages, creating numerous overlapping slots in the VDES frame and resulting in further collisions between AIS messages and VDE-TER data transmissions. In particular, the packet delivery ratio decreases more rapidly when the RI is set to 2 compared to when it is set to 6. This is because the frequent transmission of AIS messages by ship stations significantly increases the number of AIS messages in the network. Overall, SOCA achieves a higher packet delivery ratio than the existing VDES. SOCA identifies the durations during which the AIS messages of  
neighboring ship stations will be transmitted based on its NIT and avoids transmitting VDE-TER data in those durations whenever possible. By doing so, SOCA ensures high reliability in data transmission and enables a large number of ship stations to utilize channel resources more efficiently. In contrast, the existing VDES allocates slots for VDE-TER data transmission without accounting for interference caused by transmissions from neighboring ship stations. Consequently, the existing VDES suffers from frequent collisions and retransmissions, leading to a degradation in overall network performance. Quantitatively, when RI is 2 and 6, the packet delivery ratio of SOCA is 32.29% and 12.99% higher than that of the existing VDES, respectively. In Figure 8b, the packet delivery ratio in the dynamic ship scenario is higher than in the static ship scenario, regardless of the RI. In the dynamic ship scenario, fewer AIS messages are transmitted than when the RI values of all ship stations are set to 6. Accordingly, collisions and retransmissions caused by AIS messages are reduced because the ship stations do not transmit AIS messages with the same transmission interval. As a result, the channel resources occupied by transmission of AIS messages are reduced, allowing more VDE-TER data to be transmitted successfully. Therefore, the shore station in the dynamic ship scenario receives more VDE-TER data than in the static ship scenario. Quantitatively,  
Figure 7: Collision ratio: (**a**) static ship scenario and (**b**) dynamic ship scenario.