# AUV-Aided Optical--Acoustic Hybrid Data Collection Based on Deep Reinforcement Learning
## 6 Results and Discussion
### Impact of the Data Arrival Rate on Performance  
Figure 6 shows the effect of different data arrival rates on the weighted average AoI. The velocity of the AUV is set to 1 m/s, and the length of each time slot is set to 6 s. It can be observed from the figure that at lower data arrival rates, the single UAC is superior to the single UOC since the acoustic waves can be deployed for long-range data transmission, which significantly saves the travel time of the AUV. As the data arrival rate increases, the weight of data transmission time on AoI improves, which results in the single UOC scheme being superior to the single UAC scheme in terms of AoI. In addition, the greedy algorithm performs poorly in weighted average AoI as it greedily selects the closest visit location ignoring data importance and AoI. Our proposed multi-modal data collection scheme outperforms the other three schemes for different data arrival rates and is near the single UOC performance when the data size is over 140 Kbits. The main reason for this phenomenon is that the multi-modal scheme selects acoustic communication to reduce the sailing time when the data size is small and optical communication for fast data transmission when the data size is large, and thus, it can adapt to different data conditions and achieve a relatively low weighted average AoI. Figure 5: Effect of AUV velocity on the weighted energy consumption of the task. To verify the superiority of the proposed multi-modal data collection algorithm in terms of energy consumption for data collection, we compare the transmission energy consumption of CHs and AUV energy consumption for different data sizes. In this study, we set the AUV velocity to 1 m/s and the data size to 20-200 Kbits, and the experimental results are shown in Table 2 and Figure 7. It is observed that the average data transmission energy consumption of CHs under a single UOC approach is the smallest, and the AUV energy consumption is the highest. The energy consumption of CHs is the highest for the single UAC and greedy approaches, but the AUV energy consumption is kept at a low level. In the multi-modal scheme, the energy consumption of CH increases and then decreases with the increasing data size, and the AUV energy consumption gradually increases, but the overall energy consumption remains at a low level. The reason for such a phenomenon can be explained as follows. The single UOC requires the AUV to travel to the immediate vicinity of the node, which increases the energy consumption of the AUV for navigation. Fortunately, due to the low energy consumption and high bandwidth of the optical modems, the energy consumption of the CHs owing to the data transmission is low. Similarly, the single UAC and greedy algorithm allow the AUV to collect data over longer distances using acoustic waves, which greatly saves AUV energy consumption. However, with the increasing data size, the low bandwidth and high energy consumption of the UAC make the energy consumption of the CHs significant. In the multi-modal scheme, the AUV selects the UAC to collect data when the data size is small, keeping the transmission energy consumption of the CHs low while reducing the mobile energy consumption. When the data size is larger than 140 Kbits, the multi-modal scheme switches to strictly optical communication mode to reduce the excessive energy consumption of the CHs in order to extend the lifetime of the UWSN. Note that our \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Collected Data (Kbits)** & **Single UAC (mJ)** & **Single UOC (mJ)** & **Greedy (mJ)** & **Multi-Modal (mJ)** \\ \hline
20 & 6659.29 & 0.02 & 5650.31 & 5112.19 \\
50 & 15,403.83 & 0.05 & 12,780.47 & 3968.71 \\
80 & 24,081.09 & 0.08 & 20,852.35 & 5583.11 \\
110 & 32,623.83 & 0.11 & 28,789.69 & 7668.36 \\
140 & 41,099.29 & 0.14 & 379,37.81 & 0.14 \\
170 & 49,103.91 & 0.16 & 46,346.02 & 0.16 \\
200 & 52,399.92 & 0.19 & 55,359.61 & 0.19 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Average data transmission energy consumption of CHs versus the data arriving rate.