SOTDMA is the most complex TDMA access scheme defined for AIS and also provides the backbone for autonomous operation of the network offshore. The complete technical definition of SOTDMA can be found in ITU-R M.1371-4 Annex 2.
The key elements of SOTDMA operation are as follows:
- All stations share a common time reference (derived from GPS time) ensuring they can all accurately determine the start time of each TDMA slot.
- Each data transmission includes indication of the TDMA slot that will be used by the transmitting station for subsequent transmissions. This allows receiving stations to build up a ‘map’ of which slots are in use by which station.
- Each station avoids slots known to be in use by other stations for its own transmissions. This prevents two stations in range of one another using the same slot.
- As mobile stations move from one area to another they encounter new stations with different slot allocations. This may cause the station to modify its own slot allocation leading to a dynamic and self organising system over time and space.
SOTDMA also provides for dynamic and autonomous management of capacity in busy areas. Should a situation occur where all TDMA slots are occupied ‘slot re-use’ rules are applied. This allows the slots occupied by the stations most distant to a particular mobile station to be re-used for its own transmissions. This effectively reduces the size of an AIS ‘cell’ and ensures that position reports from the nearest vessels (which are most relevant to safety of navigation) are not affected.
The essential hardware requirements to support SOTDMA are:
- VHF transmitter capable of operating on any AIS channel in the marine VHF band
- Two VHF receivers capable of operating on any AIS channel in the marine VHF band
- Full time decoding of all received messages in order to populate an internal slot map
- GPS receiver to provide a time reference for TDMA timing
- Sufficient memory (RAM) to store a slot map for at least five minutes of TMDA slot allocations (the allocation status for 22500 TDMA slots)
Designed for Class A devices, SOTDMA transmits data as follows:
- AIS device scans for available slot in the AIS slot map
- Reserves available slot
- Transmits data into the reserved slot and is notifies other AIS equipment of its intention to use this slot for the next transmission