改进无线Mesh网系统的时间同步技术研究

本文针对面向ITS应用无线Mesh网络分布式多址接入对时间同步的要求,以及实际应用环境对网络同步的制约因素,通过对已有时间同步机制与技术的分析对比,具体研究在缺乏网络外部高精度同步时钟支持条件下,基于物理层参数解析的分布式WMN全网时间同步方法,给出了改进的无线MESH网网络节点时间同步方案,并就方案性能进行了仿真研究。     

分布式声传感器阵列校准方法综述

分布式声传感器阵列在军事侦察、公共安全监控和人机交互等领域应用广泛.但时钟异步、频率响应失配和节点位置未知问题严重影响后续基于分布式声传感器阵列的语音处理算法的性能,因此近年来学者们热衷于研究校准这三类阵列失配问题的方法.首先,本文介绍了导致分布式声传感器网络各类失配问题的原因和三类问题解决的先后顺序.然后,将现有的解决三类问题的方法进行了综述.其中,时钟同步方法主要包括基于时间戳交换和短时傅里叶变换域线性相位漂移的方法,频率响应校准方法主要包括基于声源导向矢量和自适应滤波器的方法,节点位置/位姿校准方法则主要包括基于节点间几何构型和深度学习的方法.最后,给出了本领域未来研究的方向.     

WSN节点随机运动下的二维测距解析定位

在无线传感器网络节点随机运动的情形下,为适应移动节点所处邻居信标节点数量随时的变化,解决在信标节点稀疏情况下,满足室内环境下高精度测距定位的问题,提出基于次声波和电磁波的非同步TDOA测距解析定位算法。仿真表明,该算法具有较好的鲁棒性,并可实现厘米级定位精度。     

Ad Hoc网络中分布式时隙同步技术研究

该文以介绍快速移动Ad Hoc网络在运动节点间业务交换的应用为基础,在分时隙时分多址（TDMA）接入机制基础上,改进了快速运动的移动节点之间进行分布式时隙同步的技术.该技术通过网络中节点之间定时的交换,获得网络的同步,为高度移动Ad Hoc网路中的节点同步提供了一种非常有效的资源和稳定性.同时,支持可变传输速率的特性,从而使得网络具有更广泛的应用领域.     

无线传感器网络分布式一致时间同步协议的收敛分析及加速设计

该文研究了基于分布式一致的无线传感器网络时间同步协议的收敛和加速问题。通过将其同步迭代过程映射到马尔可夫链的状态转移过程,推导出了分布式一致时间同步协议在循环网中的收敛速度与节点邻居数和网络规模有关。Matlab仿真实验表明该结论对类均匀规则网和类均匀网也是正确的。此外,对于类均匀网,邻居数分布也会影响协议的收敛速度。因此该文提出了基于改变网络邻居数分布的加速算法来提高分布式一致时间同步协议的收敛速度。规模为100个节点的类均匀网络实验结果表明,该文提出的加速算法在没有显著改变节点平均传输半径的情况下可使分布式一致时间同步协议的收敛迭代次数降低约25%。     

基于网络编码的无线网络分布式协作通信机制

 本文提出了一种基于无线网络编码的协作通信机制NCCC.无线网络编码能够在取得合作分集的性能增益的同时,降低网络中断概率.分布式中继节点选择算法是NCCC机制的核心,该算法根据即时信道状态,选择执行网络编码操作的最优中继节点.理论和仿真结果证明, NCCC机制可以提高无线网络双向多信源多信宿传输的性能和鲁棒性.     

利用跳频同步的Ad hoc网络时隙同步

本文针对采用跳频数传TDMA接入的Ad hoc网络，提出了一种利用跳频同步来实现TDMA时隙同步的方案。考虑到时隙同步是为了使得各个节点的时隙基准对齐，并不需要保持节点之间具体时间一致，利用跳频同步采用分隔符对跳频帧的定位，对定位进行时延补偿后，可实现时间基准基本对齐。由于同步偏差的存在，通过设置保护带允许时隙的抖动来防止冲突发生。本文给出了具体的同步实现方案，并且与其他的Ad hoc网络时隙同步方案进行了比较。仿真结果表明，该方案占用了较少的传输带宽，提高了效率，并且可以保证一定的同步精度。     

测量TDOA的完全非同步照射辐射源定位算法

针对现有基于TDOA的定位算法难以适应信号完全非同步到达的辐射源定位问题,提出了一种新的基于TDOA的完全非同步照射辐射源定位算法。由各观测站自动提取辐射源脉冲样本图及开始时间,副站将提取的脉冲样本图及开始时间传给主站,主站利用脉冲样本图匹配及外推技术得到TDOA序列（TDOAs）,解决了非同步照射引起的无法计算TDOA问题。通过计算两条时差双曲线的渐近线交点近似求取初值,通过牛顿迭代法实现对辐射源位置的估计。最后给出的数值仿真结果验证了该方法的有效性。      

基于无线传感器网络时钟同步的定位算法研究

讨论了在无线传感器定位节点(文中称为锚节点)位置已知条件下,基于信号到达时间差(TDOA)的时间同步算法。根据所有传感器定位节点彼此之间周期性地广播信号,测量信号到达时间(TOA)来估计时间频率的偏移。当信源发送信号,由于传感器钟差造成的初始TDOA误差可以通过一系列补偿计算来减少误差。在一个加性高斯白噪声模型里,通过卡尔曼滤波提高钟差和频差的估计精度。在此基础上改善标签节点的定位精度。     

脉冲耦合和分布式扩散相结合的时间同步协议

针对WSN同步精度较低、可扩展性差的问题,提出基于脉冲耦合和分布式扩散相结合的协作时间同步协议,将同步分为时钟节拍同步和时间同步两个阶段。时钟节拍同步阶段,通过脉冲信号耦合来改变节点相位实现节拍同步;时间同步阶段,基于双向信息交换模式,把主节点域的平均时间扩散有限的跳数,采用互扩散使节点时间近似同步到网络节点的平均时间上,实现时间同步。结果表明:该协议同步精度更高,收敛速度较快,扩展性较好。     

E2DTS: An energy efficiency distributed time synchronization algorithm for underwater acoustic mobile sensor networks

Time synchronization plays an important role in wireless sensor network applications and energy conservation. In this paper, we focus on the need of time synchronization in underwater acoustic mobile sensor networks (UAMSNs). Several time synchronization algorithms have been carried out in this issue. But most of them are proposed for RF-based wireless sensor networks, which assume that the propagation delay is negligible. In UAMSNs, the assumption about rapid communication is incorrect because the communication is primarily via acoustic channel, so the propagation speed is much slower than RF. Furthermore, the propagation delay in underwater environment is time-varying due to the nodes’ mobility. We present an energy efficiency distributed time synchronization algorithm (called “E²DTS”) for those underwater acoustic node mobility networks. In E²DTS, both clock skew and offset are estimated. We investigate the relationship between time-varying propagation delay and nodes mobility, and then estimate the clock skew. At last skew-corrected nodes send local timestamp to beacon node to estimate its clock offset. Through analysis and simulation, we show that it achieves high level time synchronization precision with minimal energy cost.     

水下传感器网络时间同步技术综述

 时间同步是传感器节点协同工作的基础.水下传感器网络由于采用水声通信方式,具有不同于陆地无线传感器网络的特点,为时间同步算法研究带来了新的挑战.论文首先说明同步问题与同步算法的形式化定义,然后讨论水下传感器网络不同于陆地传感器网络的特点,并指出相关特点对于同步问题的影响;接着综述陆地传感器网络同步算法的研究进展,分析相关算法用于水下环境的不足;进而介绍水下传感器网络同步算法的研究进展,并通过仿真实验完成了相关算法的性能对比;最后总结水下传感器网络时间同步的关键问题,指出进一步的研究方向.     

DRAND: Distributed Randomized TDMA Scheduling for Wireless Ad Hoc Networks

This paper presents a distributed implementation of RAND, a randomized time slot scheduling algorithm, called DRAND. DRAND runs in O(delta ) time and message complexity where delta is the maximum size of a two-hop neighborhood in a wireless network while message complexity remains O(delta ), assuming that message delays can be bounded by an unknown constant. DRAND is the first fully distributed version of RAND. The algorithm is suitable for a wireless network where most nodes do not move, such as wireless mesh networks and wireless sensor networks. We implement the algorithm in TinyOS and demonstrate its performance in a real testbed of Mica2 nodes. The algorithm does not require any time synchronization and is shown to be effective in adapting to local topology changes without incurring global overhead in the scheduling. Because of these features, it can also be used even for other scheduling problems such as frequency or code scheduling (for FDMA or CDMA) or local identifier assignment for wireless networks where time synchronization is not enforced. We further evaluate the effect of the time-varying nature of wireless links on the conflict-free property of DRAND-assigned time slots. This experiment is conducted on a 55-node testbed consisting of the more recent MicaZ sensor nodes.     

基于ZigBee节点的按需时间同步算法

简要阐述近年来无线传感网络时间同步（TPSN）算法的发展概况和影响无线传感器网络时间同步的因素,结合无线传感网络中路由节点与终端节点的特点,提出一种融合了参考广播同步（RBS）与TPSN算法设计,在保持同步精确度前提下,整个网络的功耗大大降低。通过实验采集到的数据分析了协议的可行性,证明该算法较适合于对节点密度高,终端节点较多的无线网络中,如工业有害气体的检测。     

A Wireless MAC Protocol with Collision Detection

The most popular strategies for dealing with packet collisions at the medium access control (MAC) layer in distributed wireless networks use a combination of carrier sensing and collision avoidance. When the collision avoidance strategy fails, such schemes cannot detect collisions and corrupted data frames are still transmitted in their entirety, thereby wasting the channel bandwidth and significantly reducing the network throughput. To address this problem, this paper proposes a new wireless MAC protocol capable of collision detection. The basic idea of the proposed protocol is the use of pulses in an out-of-band control channel for exploring channel condition and medium reservation and achieving both collision avoidance and collision detection. The performance of the proposed MAC protocol has been investigated using extensive analysis and simulations. Our results show that, as compared with existing MAC protocols, the proposed protocol has significant performance gains in terms of node throughput. Additionally, the proposed protocol is fully distributed and requires no time synchronization among nodes.     

Accurate Bit-Error-Rate Analysis of Bandlimited Cooperative OSTBC Networks Under Timing Synchronization Errors

The distributed multiple-input–multiple-output (MIMO) system (e.g., intercluster communication via cooperating nodes in a wireless sensor network) is a topic of emerging interest. Many previous studies have assumed perfect synchronization among cooperating nodes and identically distributed communication links. Such assumptions are rarely valid in practical operating scenarios. This paper develops an analytical framework for computing the average bit error rate (ABER) of a distributed multiple-input–single-output (MISO) space-time-coded system with binary phase shift keying (BPSK) modulation affected by timing synchronization errors. The cooperating nodes use data pulse-shaping filters for transmission over generalized frequency-nonselective fading channels. As an illustrative example, the performance evaluation of a 2 $times$ 1 MISO system that uses distributed orthogonal space-time block coding (OSTBC) is presented, although this approach can be readily extended to analyze distributed transmit diversity with a larger number of cooperating nodes. We show that under certain conditions, a distributed MISO system with time synchronization errors can still outperform a perfectly synchronized single-input–single-output (SISO) system.      

Distributed Synchronization for OFDMA‐Based Wireless Mesh Networks

In this paper, we propose a distributed synchronization algorithm for wireless mesh networks based on orthogonal frequency division multiple access. For time and frequency synchronization, a node requests its neighbor nodes for a change of fast Fourier transform starting points, transmission times, and carrier frequencies needed for synchronization. The node also updates its own time and frequency elements through simple formulas based on request messages received from neighbor nodes using a guard interval and a cyclic prefix. This process with the cooperation of neighbor nodes leads to a gradual synchronization of all nodes in the network. Through a performance comparison with a conventional scheme, we obtain simulation results indicating that the proposed scheme outperforms the conventional scheme in random topologies and a grid topology.     

Coherent acoustic array processing and localization on wireless sensor networks

Advances in microelectronics, array processing, and wireless networking have motivated the analysis and design of low-cost integrated sensing, computing, and communicating nodes capable of performing various demanding collaborative space-time processing tasks. In this paper, we consider the problem of coherent acoustic sensor array processing and localization on distributed wireless sensor networks. We first introduce some basic concepts of beamforming and localization for wide-band acoustic sources. A review of various known localization algorithms based on time-delay followed by least-squares estimations as well as the maximum-likelihood method is given. Issues related to practical implementation of coherent array processing, including the need for fine-grain time synchronization, are discussed. Then we describe the implementation of a Linux-based wireless networked acoustic sensor array testbed, utilizing commercially available iPAQs with built-in microphones, codecs, and microprocessors, plus wireless Ethernet cards, to perform acoustic source localization. Various field-measured results using two localization algorithms show the effectiveness of the proposed testbed. An extensive list of references related to this work is also included.     

Doppler auxiliary time synchronization algorithm for underwater acoustic sensor network

Time synchronization problem in underwater acoustic sensor networks (UWSN) was studied.Due to the propagation of acoustic signals in underwater environment and nodes movement bring some problems to time synchronization.A distributed time synchronization algorithm was proposed based on Doppler method,called NU-Sync.NU-Sync solved the problem of uncertainty propagation delay caused by nodes movement through calculating relative velocity.And autonomous underwater vehicle (AUV) was used as beacon node which can save energy consumption in the process of calculation clock skew.Simulation resulted show NU-Sync achieves high level time synchronization precision.     

A Low Duty Cycle MAC Protocol for Directional Wireless Sensor Networks

Directional communication in wireless sensor networks minimizes interference and thereby increases reliability and throughput of the network. Hence, directional wireless sensor networks (DWSNs) are fastly attracting the interests of researchers and industry experts around the globe. However, in DWSNs the conventional medium access control protocols face some new challenges including the synchronization among the nodes, directional hidden terminal and deafness problems, etc. For taking the advantages of spatial reusability and increased coverage from directional communications, a low duty cycle directional Medium Access control protocol for mobility based DWSNs, termed as DCD-MAC, is developed in this paper. To reduce energy consumption due to idle listening, duty cycling is extensively used in WSNs. In DCD-MAC, each pair of parent and child sensor nodes performs synchronization with each other before data communication. The nodes in the network schedule their time of data transmissions in such a way that the number of collisions occurred during transmissions from multiple nodes is minimized. The sensor nodes are kept active only when the nodes need to communicate with each other. The DCD-MAC exploits localized information of mobile nodes in a distributed manner and thus it gives weighted fair access of transmission slots to the nodes. As a final point, we have studied the performance of our proposed protocol through extensive simulations in NS-3 and the results show that the DCD-MAC gives better reliability, throughput, end-to-end delay, network lifetime and overhead comparing to the related directional MAC protocols.