DOI: xx.xxxx/sat.xxxx ED IT ORI A L
IJSC&N Special Issue “Opportunities and Challenges of Maritime VHF Data Exchange Systems”: Guest editorial message
Two subsequent World Radiocommunication Conferences, held in 2015 and 2019, have concluded the frequency allocation in VHF bands for the two-way maritime VHF Data Exchange System (VDES) via terrestrial and satellite radio frequency links respectively. The modernization and digital evolution of maritime communications was initiated in 1990’s by adopting Automatic Identification Systems (AIS) for ship to shore, ship to ship and shore to ship communications for a variety of applications, targeting safety at sea. The frequency allocation for VDES has been a significant step forward towards achieving the same goal by facilitating solutions for enhanced navigation, broadcasting essential information and many other emerging applications.
The allocation of frequencies at WRC-19 for VDES via satellite (VDE-SAT), reduced regulatory risks in the deployment of VDES for two-way communications, to and from vessels via satellite. At the same time, the allocated frequency plan has raised new technical challenges for the development of space segment technologies aiming at feasible and viable system solutions for end-to-end system deployment. This has made VDES an area of active research seeking academic as well as industrial solutions for end-to-end VDES terrestrial and satellite components sharing common system resources.
The development and deployment of AIS services in the 1990s were primarily intended to function as a tool for ship identification, collision avoidance and vessel traffic monitoring. However, the AIS success in terms of large-scale proliferation and feasibility quickly spawned a host of additional application, which in turn led to the birth of the Application Specific Messages (ASM) concept. The standardization of ASM allowed for even more innovative use of the AIS technology and increased traffic on the AIS channels. Thus, in the 2000s the traffic load on the AIS channels in dense shipping areas increased significantly. To avoid channel overload and possible loss of safety critical AIS messages, it was deemed logical to relocate non-safety related services away from the AIS channels. The simple solution for that was to move the ASM messages off the AIS channels. In the process of defining the spectrum requirements for ASM, additional maritime communication applications surfaced. As they could not all be accommodated by ASMs, the concept of VHF Data Exchange (VDE) was defined and the VHF Data Exchange System (VDES) was born.
In essence, the VDES provides a variety of means for the exchange of data between maritime stations, ship-to-ship, ship-to-shore, shore-to ship, ship-to-satellite and satellite-to-ship. The VDES is a multi-component system comprising of the automatic identification system (AIS), application specific messages (ASM) and VHF data exchange (VDE) in the VHF maritime mobile band (156.025-162.025 MHz). Although primarily a terrestrial system, VDES also allow for satellite use. In particular, the VDE has defined both a terrestrial component (VDE-TER) and a satellite component (VDE-SAT).
Frequencies for the terrestrial components of VDES, ASM and VDE-TER, was identified and included in the Radio Regulations (RR) at the WRC-15. Consideration of VDE-SAT was deferred to WRC-19 which allocated and identified to VDE-SAT the same frequency bands as for VDE-TER.
Resource usage and management differ between the various components of the VDES. The frequency and time-slot allocation within AIS are primarily based on a self-organizing protocol, where each AIS transponder announces when they will transmit based on a slot map created by listening to other AIS transponders transmission announcements. This allows for an efficient use of the frequency resources without a central resource management. ASM depend on a similar self-organizing protocol as AIS, but with a higher ratio of random access and fixed access allocation transmissions. This is due to the expectation that ASM transmissions from ships will be more one-off transmissions and that ASM transmissions from base stations of navigation and safety information to be repeated regularly will benefit from reserving resources. Thus, ASM is self-organizing like AIS, but with more use of central resource management.
For VDE, including both VDE-SAT and VDE-TER, the link access is controlled by central resource management. However, there are provisions for how ships can utilize the frequency resources when no VDE-TER shore station or VDE-SAT satellite are available to control the central resource management. In VDE resource management and assignment is handled by “bulletin boards” and “Announcement Signaling Channels”. VDE-TER shore stations use the Terrestrial Bulletin Board (TBB) to configure the frequency usage within their control area and VDE-SAT satellites use the Satellite Bulletin Board (SBB). The TBB and the SBB serve the same purpose, but they contain slightly different information to account for the operational distinctions between VDE-SAT and VDE-TER. A VDE-SAT satellite provide a very large coverage area, so at any given time multiple VDE-TER shore stations may be within that coverage area. Given that VDE-TER provides for a higher data throughput, the system and spectrum efficiency of VDES are maximized when vessels take advantage of VDE-TER shore station coverage when that is available. To accommodate this, a VDE-SAT satellite should respect the resource assignments of VDE-TER shore stations.
Requirements and schemes for resource sharing between VDE-TER and VDE-SAT are closely linked to the frequency plan used for VDE. The frequency plan decided at WRC-19 stipulates VDE-SAT up- and down-link in both upper and lower legs of RR Appendix 18 with 50 kHz dedicated in each leg. The remaining 100 kHz in each leg of RR Appendix 18 are dedicated to VDE-TER. These 100 kHz can also be used by VDE-SAT if not used by VDE-TER, i.e. for vessels outside VDE-TER shore station coverage. With separate frequency resources dedicated to both VDE-SAT and VDE-TER, no elaborate sharing scheme between VDE-SAT and VDE-TER is needed and the two VDE components can independently maximize the system and spectrum efficiency. In addition the satellite component can utilize frequency resources not used by the terrestrial component.