A Hybrid MAC Protocol with Channel-dependent Optimized Scheduling for Clustered Underwater Acoustic Sensor Networks

We propose a novel optimal time slot allocation scheme for clustered underwater acoustic sensor networks that leverages physical (PHY) layer information to minimize the energy consumption due to unnecessary retransmissions thereby improving network lifetime and throughput. To reduce the overhead and the computational complexity, we employ a two-phase approach where: (i) each member node takes a selfish decision on the number of time slots it needs during the next intra-cluster cycle by solving a Markov decision process (MDP), and (ii) the cluster head optimizes the scheduling decision based on the channel quality and an urgency factor. To conserve energy, we use a hybrid medium access scheme, i.e., time division multiple access (TDMA) for the intra-cluster communication phase and carrier sense multiple access with collision avoidance (CSMA/CA) for the cluster head-sink communication phase. The proposed MAC protocol is implemented and tested on a real underwater acoustic testbed using SM-75 acoustic modems by Teledyne Benthos. Simulations illustrate an improvement in network lifetime. Additionally, simulations demonstrate that the proposed scheduling scheme with urgency factor achieves a throughput increase of 28 % and improves the reliability by up to 25 % as compared to the scheduling scheme that neither use MDP nor optimization. Furthermore, testbed experiments show an improvement in throughput by up to 10 % along with an improvement in reliability. 1

A Low-cost Distributed Networked Localization and Time Synchronization Framework for Underwater Acoustic Testbeds

Localization and time synchronization are both essential services for Internet-connected underwater acoustic testbeds. Although the two are mutually coupled, they are often treated separately. We propose a new low-cost distributed networked localization and time synchronization framework for underwater acoustic sensor network testbeds. The proposal is based on decoupling the two problems and solving first the time synchronization then localization using the same set of messages, i.e. with no additional overhead. A coarse, followed by a fine-grained localization algorithms are adopted to accurately estimate the location of an unknown node. The protocol is robust to noisy range measurements. The proposed scheme is implemented in a testbed based on Teledyne Benthos Telesonar SM-975 underwater modems and tested extensively in Lake LaSalle at the University at Buffalo. Experiments and simulations in terms of root mean square error (RMSE) demonstrate that the proposed scheme can achieve a high accuracy for a given energy budget, i.e. for a given number of message exchanges. I