Research On Time Synchronization Techniques In Underwater Acoustic Sensor Networks

With the rapid development of information technology and marine science, as anemerging marine information collecting technology, Underwater Acoustic SensorNetworks (UASN) have become a research focus, extensively applied in marineenvironmental monitoring, marine pollution monitoring, marine resources survey, aswell as detecting, tracking and positioning of underwater target objects. The principalcomponents of UASN include surface stations and underwater sensor nodes, deployedin large sea areas with low density. And the underwater sensor nodes rely on acousticchannel to communicate with each other. Generally, by the impact of complex marineenvironmental factors, the underwater acoustic link has a variety of characteristics,such as low propagation speed, high latency, varying delay, high bit-error rate, lowbandwidth, serious multipath effects and high energy cost, etc.Time synchronization is an important supporting technology in the sensornetworks, aiming to provide a unified reference clock for the coordination in sensingand driving across multiple nodes. And it is the foundation of such technologies asnetwork collaborative work/sleep, data fusion, time division multiplexing scheduling,node positioning, etc. Unfortunately, due to the low propagation speed and long linkdistance of underwater acoustic channel, as well as the mobility of nodes, the highlatency and space-time uncertainty occurs in UASN. These features cause thetraditional time synchronization protocols for land-based wireless sensor networks arenot suitable for underwater condition. Therefore, the varying delay in acousticcommunication and relative movement between nodes must be taken intoconsideration in time synchronization for UASN, which brings new challenges for theresearch of time synchronization algorithms.For UASN, present domestic and foreign researches of time synchronizationalgorithms mainly utilize point-to-point synchronization based on single-beacon andsingle-hop mechanism. In point-to-point synchronization mechanism, it assumes thateach node has at least one single-hop route to the beacon node in the entire or acluster of the network; and through multiple message exchanges with the beacon node,it will make estimation of clock drift on each node and complete time synchronizationfor the whole network. However, since the marine monitoring tasks involve wide range in reality, the underwater sensor nodes are often deployed with0.5km~2kmlong interval; and the inter-node communications generally use opportunistic routingbased on multi-hop transmission. In fact, it is unpractical for existing point-to-pointsynchronization algorithms to fit the actual requirements of UASN deployment.Motivated by the problem of synchronization accuracy decreasing with thenumber of hops in multi-hop sensor networks, this thesis has studied and designedmulti-source synchronous message fusion algorithm based on multi-beacon andmulti-hop transmission, taking full advantage of multi-source synchronous messageswith time effectiveness to improve the precision of synchronization for every node inthe networks. In this thesis, a Multi-Source Time Synchronization (MSTS) algorithmfor UASN is proposed. MSTS exploits a simulation investigative methodology toassess the effect of path hops on the time effectiveness of synchronous messages,integrating all the synchronous messages with different time effectiveness in multiplemessage exchanges. And MSTS applies weighted linear regression with message timeconfidence level to estimate skew and offset of the local clock on each node.Simultaneously, in order to evaluate the performance of time synchronizationalgorithms for UASN accurately, an Underwater Sensor Network Simulator(UWSN-Sim) has been studied and developed, which can establish the simulationenvironment of underwater acoustic communication and the mobility pattern of nodes;and it is also able to simulate multi-hop transmission of synchronous messages incondition of multiple beacon nodes deployed. By changing the settings of differentsimulation scenarios and communication parameters in UWSN-Sim, a large numberof simulations have been conducted under diverse network topologies and deploymentscales in virtual UASN. The simulation results confirm that MSTS has highsynchronization accuracy in multi-hop topologies; and in large-scale deployment ofmulti-hop networks, compared with hop-by-hop iterative MU-Sync, MSTS achieves ahigher level of synchronization precision with lower average message overhead andshorter overall run time.