## 4 Performance Evaluation
### Simulation Results  
Figure 5a,b illustrate the variations in aggregate throughput in static and dynamic ship scenarios, respectively. In Figure 5a, the aggregate throughput initially increases as the number of ship stations grows; however, it gradually declines after reaching a certain number of ship stations. Up to 15 ship stations, the aggregate throughput increases similarly regardless of the RI value. This is because the VDES frame can accommodate all VDE-TER data packets transmitted by the ship stations, resulting in an increased total number of packets received by the shore station. However, as the number of ship stations exceeds 15, traffic surpasses the capacity of the VDES frame, causing aggregate throughput to gradually increase and then decrease. In particular, the aggregate throughput is lower and drops more sharply with RI set to 2 than with RI set to 6. This occurs because a smaller RI results in more frequent AIS message transmissions. As a result, VDE-TER data, having lower priority than AIS messages, are more frequently unable to be transmitted. In SOCA, non-interference slots with lower expected slot occupancy probabilities are preferentially selected for VDE-TER data transmissions, whereas the existing VDES randomly selects candidate slots without accounting for interference. Consequently, SOCA results in fewer overlapping slots than the existing VDES, leading to higher aggregate throughput, as shown in the figure. Specifically, the overlapping slot refers to a slot that is redundantly allocated for both AIS message transmissions and VDE-TER data transmissions. Therefore, overlapping slots may result in collisions between AIS message transmissions and VDE-TER data transmissions, or in VDE-TER data transmissions being deferred. Quantitatively, SOCA achieves 19.06% and 8.11% higher aggregate throughput with RI values of 2 and 6, respectively, compared to the existing VDES. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Ship Conditions** & **RI (s)** \\ \hline Ship at anchor or moored and not moving faster than 3 knots & 180 \\ Ship at anchor or moored and moving faster than 3 knots & 10 \\ Ship moving at 0–14 knots & 10 \\ Ship moving at 0–14 knots and changing course & 10/3 \\ Ship moving at 14–23 knots & 6 \\ Ship moving at 14–23 knots and changing course & 2 \\ Ship moving at \textgreater{}23 knots & 2 \\ Ship moving at \textgreater{}23 knots and changing course & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters related to ship conditions. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline Number of ship stations & 5–50 & TDMA frame length & 90 slots \\ Communication range & 50 NM & Slot length & 26.67 ms \\ Data rate of AIS & 9.6 kbps & RI & 2, 6 \\ Data rate of VDE-TER & 38.4 kbps & Rr & 30, 10 \\ AIS message size & 32 bytes & NI & 75, 225 \\ VDE-TER data size & 640 bytes & SI & \(15,45\) \\ Frame length & 2250 slots & \(n_{pkt}\) & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Simulation parameters. In Figure 5, the aggregate throughput in the dynamic ship scenario is generally higher than in the static ship scenario. This is because, in the dynamic ship scenario, the RI values of ship stations change, which leads to reduced collisions between AIS messages. This reduction in collisions decreases unnecessary slot usage caused by AIS message retransmissions, thereby increasing the opportunities for ship stations to transmit VDE-TER data. Consequently, successful VDE-TER data transmissions increase, resulting in 48.19% and 2.88% higher aggregate throughput in the dynamic ship scenario compared to the static ship scenario when the RI is set to 2 and 6, respectively. Furthermore, in the dynamic ship scenario, SOCA achieves 7.30% higher aggregate throughput compared to the existing VDES. Figure 6,b illustrate the variations in average latency in static and dynamic ship scenarios, respectively. In Figure 6, the average latency increases as the number of ship stations grows.