The acoustic propagation speed under water poses significant challenges to the design of underwater sensor networks and their medium access control protocols. Similar to the air, scheduling transmissions under water have significant impacts on throughput, energy consumption, and reliability. Although the conflict scenarios and required scheduling constraints for deriving a collision-free schedule have been identified in the past, applying them in a scheduling algorithm is by no means easy. In this paper, we derive a set of simplified scheduling constraints and propose two scheduling algorithms with relatively low complexity for both known and unknown orders of transmissions. Our experimental results show that scheduling without slots is on average 22% better than scheduling with slots for large packet sizes, while for small packet sizes scheduling without slots is about 40% better. We also compare our "smallest delay first" heuristic algorithm with the "highest transmission load first" heuristic of ST-MAC [1] and show that our heuristic algorithm performs on average 13% better.

Performing real world experiments with underwater communication is difficult and time-consuming. Input for evaluation of localization and time-synchronization derived from experiments is not readily available.Using real-world experiments we evaluate the performance of our cooperative combined localization and time-synchronization approach called aLS-Coop-Loc and a non-cooperative approach. We perform experiments using the SeaSTAR Proteus node and a Commercial Off-the-Shelf (COTS) node from Kongsberg Maritime at a lake and at Strindfjorden in Norway. These experiments provide realistic insight into ranging performance in real-world environments.Evaluation shows that the cooperative approach outperforms non-cooperative approaches in terms of accuracy of localization and time-synchronization. aLS-Coop-Loc provides about about 2% to 34% better position accuracy and 50% improved timesynchronization.

In this paper we describe a design for an underwater MAC protocol which combines localization, time-synchronisation and communication. This protocol is designed for small-scale clustered networks in which all nodes are able to communicate with each other. We consider an integrated design of localization, time-synchronisation and communication important because scheduled communication, localization and time-synchronisation require estimation of propagation delays between nodes to operate accurately and efficiently. For localization we use multidimensional scaling because it does not require the use of reference nodes.Our MAC design consists of two phases, i.e. an unscheduled coordination phase and a communication phase. During the first phase, propagation delays are estimated, relative positions are calculated using multidimensional scaling, and timesynchronisation is performed. During the communication phase, sensor-data is transmitted using scheduled communication.Using simulation we evaluate the feasibility of such a design. By measuring the time required for the coordination phase at different modulation rates, we derive the required modulation rate for acceptable coordination phase times.

In this article we introduce a MAC protocol designed for underwater localization and time-synchronisation. The MAC protocol assumes a network of static reference nodes and allows blind nodes to be localized by listening-only to the beacon messages. Such a system is known to be very scalable. We show localization and time-synchronization algorithms can be designed for listen-only blind nodes by adapting the equations from GPS. We extend the set of GPS equations with angular information to reduce the number of beacons a blind node has to receive before it can determine its position.We evaluate scheduled and unscheduled communication for sending reference-beacon message using simulation mechanisms. Our experimental results show that when energyconsumption of the beacons is of concern or when the modulation rate is low (≤ 1kbps), scheduled communication is preferred. For systems which are not concerned with the energy-consumption of the beacons, unscheduled communication delivers more messages to the blind-nodes and localization and time-synchronization are performed faster in case of high modulation rates (5kbps and 10kbps).

The acoustic propagation speed under water poses significant challenges to the design of underwater sensor networks and their medium access control protocols. Similar to the air, scheduling transmissions under water has significant impact on throughput, energy consumption, and reliability. In this paper we present an extended set of simplified scheduling constraints which allows easy scheduling of underwater acoustic communication. We also present two algorithms for scheduling communications, i.e. a centralized scheduling approach and a distributed scheduling approach. The centralized approach achieves the highest throughput while the distributed approach aims to minimize the computation and communication overhead. We further show how the centralized scheduling approach can be extended with transmission dependencies to reduce the end-to-end delay of packets. We evaluate the performance of the centralized and distributed scheduling approaches using simulation. The centralized approach outperforms the distributed approach in terms of throughput, however we also show the distributed approach has significant benefits in terms of communication and computational overhead required to setup the schedule. We propose a novel way of estimating the performance of scheduling approaches using the ratio of modulation time and propagation delay. We show the performance is largely dictated by this ratio, although the number of links to be scheduled also has a minor impact on the performance.