Adaptive Space Time - Time Division Multiple Access (AST-TDMA) protocol for an underwater swarm of AUV´s

This paper presents a new MAC protocol: Adaptive Space Time - TDMA (AST-TDMA) for Autonomous Underwater Vehicles (AUV) operating as a swarm in the open ocean. The aim of the protocol is to allow the dissemination of navigational information amongst the vehicles as quickly as possible such that they may operate as a group in a swarm like fashion. The proposed new protocol incorporates a method to handle the unique channel characteristics experienced underwater, in particular low bandwidths and long propagation delays and is designed to operate in a single channel broadcast acoustic environment. The protocol is based on a TDMA approach where vehicles are allocated a slot in a cycle. In AST-TDMA however, the slot size is not fixed but will adaptively change, from slot-toslot and cycle-to-cycle, due to the changes in range and therefore propagation delay resulting from vehicles position in the swarm and the swarm movement. As slot size varies, so will the cycle time for the exchange of each vehicles´ information. AST-TDMA has significant advantages over other time-based protocols as it avoids both the need for guard times and time synchronization, which are both major drawbacks in time based protocols. It also works with the spatial-temporal diversity created in long propagation delay environments to allow `non exclusive channel access´ in a single channel, while maintaining the collision avoidance benefit of contention free protocols. This is demonstrated through simulation. A new metric, NCCP, was developed to test the new MAC protocol for swarm operational effectiveness. It has been shown through simulations that the AST-TDMA protocol outperforms TDMA for swarm operations in its ability to disseminate information in a timely manner and with higher channel utilization. Using AST-TDMA also allows a much higher density of vehicles to operate in a swarm like network.