## 2 VDES Overview and Related Work
### VDES Overview  
VDES is a multi-component system for maritime communications consisting of AIS, ASM, and VDES, enabling data exchange between maritime stations, ship-to-ship, ship-to-shore, shore-to-ship, ship-to-satellite, and satellite-to-ship. VDES comprises terrestrial and satellite components, VDE-TER and VDE-SAT, which are interoperable and compatible. It is designed to operate in the VHF maritime mobile band (156.025-162.025 MHz) and supports both simplex and duplex modes. To avoid collisions, ship stations must transmit VDES messages according to the following priority order: AIS > ASM > VDE. Figure 1 illustrates the VDES functions and frequency usage. AIS, the core system of VDES, is primarily utilized to prevent collisions and enhance safe navigation by enabling ships to identify various maritime stations. It mainly operates through two channels (AIS 1, AIS 2). AIS periodically transmits information crucial for ship recognition and navigation, static and dynamic information, and voyage-related data. This information includes the identifier (ID) of the ship, as well as its type, location, speed, and navigation details. In coastal regions, AIS is employed to gather navigation data of ships. Conversely, in offshore areas, long-range AIS ensures safe navigation of ships, even in areas lacking infrastructure, by communicating with satellites. Nevertheless, AIS channels, with a bandwidth of 25 kHz and a transmission rate of 9.6 Mbps, have already become overloaded in some large and congested ports where ships are closely clustered. Accordingly, ASM and VDE were introduced to overcome the resource constraints of AIS and to meet the increasing requirements of various applications. ASM plays a key role in preventing traffic overload of AIS channels and loss of AIS messages important for maritime safety by allowing application messages previously transmitted on the AIS channel to be transmitted on two other channels (ASM 1 and ASM 2). The bandwidth of ASM channels is 25 kHz, the same as AIS, but the data rate is 19.2 kbps, which is twice that of AIS. Environmental, safety, and regulatory information is mainly transmitted through ASM. However, there were limits to accommodating the increasing number of ships and maritime application messages. Therefore, VDE was additionally introduced to enable the exchange of various data to meet the requirements of e-Navigation and MASS services through 12 additional channels and high data rates. Multimedia information such as real-time weather data, audio, images, and video is transmitted through VDE. VDE is divided into VDE-TER and VDE-SAT. VDE-TER supports bandwidths of 25, 50, and 100 kHz and data rates from 38.4 kbps up to 307.2 kbps. On the other hand, VDE-SAT supports bandwidths of 50, 100, and 150 kHz and data rates from 2.1 to 56.4 kbps. The default bandwidths of VDE-TER and VDE-SAT are 100 kHz and 50 kHz, respectively. As a result, VDES enhances the efficiency of ship operations at sea, maritime traffic management, and maritime safety by utilizing components with various characteristics. It enables seamless data exchange and service operation for more ship stations by expanding bandwidth and offering high data rates. Figure 2 illustrates the VDES frame structure. VDES employs a frame structure with a duration of one minute, synchronized to coordinated universal time (UTC). The frame consists of a total of 2250 slots. Each slot is approximately 26.67 ms long and can transmit packets of up to 256 bits. The slots in the frame are numbered from 0 to 2449. In VDE-TER, the VDES frame is divided into 25 sub-frames, which are TDMA frames. The TDMA frame numbers range from 0 to 24. Each TDMA frame consists of 15 hexslots, where each hexslot comprises 6 consecutive slots and lasts for 160 ms. The hexslot number increases by 1 every 6 slots. Time progresses from top to bottom and from left to right within the TDMA frame. Furthermore, each slot within a hexslot has a distinct TDMA channel offset, resulting in the utilization of six TDMA channels in a TDMA frame.