## 2 VDES Overview and Related Work
### Related Work  
Numerous studies have been conducted to mitigate collisions between AIS messages and other transmissions in MIoT. In [25], [PERSON] et al. proposed a priority determination mechanism that determines transmission priority depending on the message type. However, this mechanism causes long delays in low-priority message transmissions, particularly in dense environments with intense competition for slots and channels.  
In [26], [PERSON] et al. proposed a feedback-based TDMA (FBTDMA) protocol that includes a collision feedback mechanism and a modified time slot structure. A collision feedback mechanism uses feedback signals to notify the neighboring ship stations immediately when a collision occurs. However, this causes additional message overhead for collision feedback, resulting in long delays and network overload. In addition, the modified time slot structure increases the complexity of system design and implementation, as it requires sophisticated frame segmentation and inconsistent slot length. In [27], [PERSON] et al. proposed a feedback-based improved hexslot MAC (FIH-MAC) protocol in which the ship stations periodically monitor and update their neighbors' channel state information (CSI) to assess channel availability for message transmissions. However, since this protocol heavily relies on accurate CSI, incorrect estimation of CSI can significantly increase the probability of collisions. In addition, the process of monitoring and updating CSI may incur additional communication overhead and communication delays. Furthermore, the authors assume that the CSI is obtained via cognitive radio, which requires complex radio processing techniques for implementation. In [28], [PERSON] et al. modified the existing carrier-sense TDMA (CSTDMA) protocol. With this modified protocol, ship stations continuously monitor the occupancy of near-future slots when slot allocations are required for message transmissions. However, this approach makes collisions highly likely, even if only one adjacent ship station simultaneously attempts to transmit a message.  
Figure 3: VDE-TER default slotmap.  
Figure 2: VDES frame structure.  
A hybrid MAC protocol, which combines contention-free and contention-based mechanisms, can be applied to the maritime communication networks. In [29], [PERSON] et al. proposed a topology and traffic adaptive medium access control (TTA-MAC) protocol. TTA-MAC uses a frame structure comprising contention-free and contention-based periods. Under high-traffic conditions, the contention-free period is extended by increasing the slot allocation ratio within the frame. However, in high-traffic conditions, ship stations may experience frequent collisions due to the inclusion of the contention-based period within the frame. Additionally, the use of a hybrid MAC with a dynamic frame structure incurs extra computational overhead and requires additional control message exchanges.  
These studies demonstrate various resource allocation strategies to mitigate collisions between AIS messages and other transmissions in maritime communications. Despite these efforts, they still encounter significant challenges in terms of performance degradation, including prolonged delays, excessive message overhead, and frequent collisions in dense environments. Moreover, there are further challenges related to compatibility with existing VDES systems and implementation complexity.