Research On Time Synchronization For Mobile Underwater Acoustic Sensor Networks

The main components of the underwater acoustic communication networks are deployed below the water surface,and it is difficult for the underwater sensor nodes to remain static due to the influence of seawater flow.Meanwhile,in order to form the underwater three-dimensional networks usually requires the participation of autonomous mobile nodes such as AUV(Autonomous Underwater Vehicle)or UUV(Underwater Unmanned Vehicle).During the data transmission process,the relative movement among the underwater sensor nodes leads to the dynamic propagation delay,which affects the performance of underwater acoustic communication network.At the same time,due to the error in the estimation of clock skew by time synchronization protocol,there is always clock error among the network nodes,and the clock error accumulated over time will affect the control effect of concurrent media access control protocol on the data transmission time.some studies have found that the clock error of long-term running network equipment shows a nonlinear change pattern,indicating that the clock skew is not constant,and requires the network to periodically resynchronize,which will consume the limited channel resources and node energy.Therefore,in this paper,based on the characteristics of clock error,we propose the high-precision time synchronization protocol and high-efficiency time synchronization protocol based on the difference of the attention to performance of mobile networks,at the same time,the paper studies the influence of the mobile underwater acoustic sensor network performance,and then the improvement scheme of the concurrent media access control protocol is proposed.Firstly,this paper proposes a Doppler-enhanced time synchronization protocol.The existing time synchronization protocols for node movement scenarios are using the underwater acoustic velocity and Doppler frequency shift measurement to estimate the relative moving speed among nodes,and then using the calculated dynamic propagation delay among nodes to improve the time synchronization accuracy,in order to suppress the introduced underwater acoustic velocity error,it is necessary to compensate the underwater acoustic velocity while the execution time synchronization.The proposed Doppler-enhanced time synchronization protocol directly uses the Doppler frequency shift to avoid the adverse situation of using the underwater acoustic velocity directly,and also compensates for the effect of the clock skew on the Doppler frequency shift measurement.Simulation proves that the Doppler-enhanced time synchronization protocol has the higher time synchronization accuracy in the large clock skew scenario where the nodes move relative to each other.Secondly,this paper proposes an adaptive power efficient time synchronization protocol.Through the long time observation of the clock,it has been found that the clock skew has slow time variation,which requires the underwater sensor nodes having completed time synchronization to do periodic resynchronization.The existing time synchronization protocols based on linear clock model to estimate clock skew require multiple rounds of synchronous message interaction,the energy consumption of multiple rounds of message transmissions mainly affects the life cycle of underwater sensor nodes,this paper refers to the general clock model from the terrestrial time synchronization protocol,and applies that model to underwater sensor nodes to achieve time synchronization.The proposed adaptive power efficient time synchronization protocol based on the Kalman filter greatly saves node energy consumption.The simulations show that the new protocol can suppress the influence of node mobility on the time synchronization accuracy and achieve the high energy efficiency resynchronization.Then,by studying the existence of three concurrent-class MAC protocols relying on time synchronization,namely DOTS protocol,FDCA protocol and DS protocol,we further analyze the influence of the two new time synchronization protocols on the performance of short-term mobile networks and long-term mobile networks.First,according to the node time synchronization error existing in the network,reorganize the timing sequence protection for transmission time allocation algorithm introduced by the three concurrent media access control protocol,based on short-term mobile networks scenario and long-term mobile networks scenario,and the simulation results show that the two new time synchronization protocol,can achieve better performance of concurrent medium access control protocol depending on the network time synchronization.Finally,the analysis shows that the DS protocol has the condition of slot,and the management of slot is combined with the working principle of DS protocol.The simulation results show that the better network performance is gained on the scenario of high traffic network.