Research On Multi-hop Time Synchronization For Underwater Acoustic Networks

With growing concern about the investigation, development and militaryapplication of marine resources, underwater acoustic networks (UANs) began toattract widespread attention from academic, industrial, and military fields. Timesynchronization which is an important supporting technology in the sensor networkplays an important role in underwater scheduling, positioning and hibernation ofsensor nodes. And all of them are built on the basis of time synchronization, so weneed time synchronization.Contrast to wireless sensor network on mainland, UANs has a large propagationdelay, so all the protocols in wireless environment with nearly zero propagation delaybetween sensor nodes is no longer the case for underwater acoustic communication.Thus all of the existing time synchronization protocols for UANs face to node-to-nodetime synchronization; none of them solve multi-hop time synchronization for UANs,therefore new approaches for multi-hop time synchronization in UANs are required.In this paper, we propose a multi-hop time synchronization algorithm forunderwater acoustic networks based on overhearing mechanism which is consist oftwo stages: level discovery stage and synchronization stage.During the level discovery stage, nodes choose the one which has the smallestlevel during the waiting time as node’s father node and assign themselves a level, onegreater than the father’s level. After establishing their own level, they broadcast a newlevel discovery packet containing their own level. This process is continued andeventually every node in the network is assigned a level. For the synchronizationstage, we extend Tri-message algorithm to multi-hop condition and introduce anoverhearing mechanism by which node can not only receive synchronization packetsfrom parent node but also from neighboring nodes to reduce the synchronization error.In this paper, we also give the mathematical analysis of MSUAN.During the modeling of MSUAN, we found that not all timestamps can be used to compensate the time error, so we propose the improvement scheme of our algorithm(IMSUAN) by filtering the timestamps.At the end, we establish a simulation platform by OPNET to evaluate theperformance of level discovery performance, effect of varying number of hops,predicting the time after synchronization and stability of our algorithm.The contributions of this thesis include:(1)We extended node-to-node time synchronization algorithm to a multi-hopcondition which consists of two stages: level discovery stage and timesynchronization stage. In the first stage, we propose a new time constructionalgorithm which can effectively reduce errors generated in the time synchronizationprocess.(2)During time synchronization stage, we propose an overhearing based timesynchronization algorithm which can make good use of the nature of broadcast,mathematical analysis and simulation results show that improvements can be achievedby the overheard timestamps.