One-way travel time ranging in dense underwater acoustic mobile networks

The development of small low-cost autonomous underwater and surface vehicles has increased the need for underwater wireless communication and ranging to support swarm operation for collaborative data collection efforts. In the last ten years several simulation studies on Autonomous Underwater Vehicle (AUV) swarm fleet formation have been performed, and some preliminary sea demonstrations of proof-of-concept prototypes were carried out. However, their actual realization is hindered on one side by the high cost of acoustic modems, whose price can easily exceed that of small AUVs and, on the other side, by the difficulties of keeping track of the vehicles' positions due to the long latency required by traditional round trip time ranging measures. One-way travel-time (OWTT) halves the latency, at the cost of a high precision oscillator, such as an atomic clock or an oven controlled crystal oscillator, installed in the modem processing unit. In this Thesis we focus on this aspect analyzing different types of high precision oscillators, such as atomic clocks and oven controlled crystal in order to identify which oscillator best fits the trade off between precision, price and integrability in our proof-of-concept system. Then we present two medium access control (MAC) schemes for dense acoustic networks: a first one based on a time-division scheme with a high-precision oscillator to perform OWTT ranging and a second based on the token bus paradigm; both algorithms goal is to minimize the number of ranging packets transmitted in the network while allowing each node to localize all the other nodes. The two schemes are developed and simulated with the DESERT Underwater Framework: the simulation results present how a network can benefit of OWTT to lower the update time for the positioning of all nodes.