无线传感器网络中的栅栏覆盖研究
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摘要
无线传感器网络(Wireless Sensor Network,WSN)是由大量计算、通信及存储能力有限的传感器节点组成的特殊Ad hoc网络,可感知、采集和实时传递监测对象的各种参数,在军事、工业及民用等领域应用广泛,是目前一个非常活跃的研究领域。覆盖问题是无线传感器网络研究中的关键问题之一,它反映了传感器网络所提供的“感知”服务质量,可以使无线传感器网络的空间资源得到优化分配。论文针对无线传感器网络的栅栏覆盖问题,从监控方(传感器网络)和反监控方(移动目标)两个角度展开了深入研究。前者是解决无线传感器网络如何探测到目标的问题,而后者则关注如何在已有覆盖的基础上提高网络覆盖性能。本文主要工作包括以下几个方面:
     (1)首先,本文从被监测的移动目标即反监控方的角度出发,研究了受时间约束的目标如何安全穿越传感器网络的问题。当智能目标穿越敌方无线传感器网络的穿行时间受限时,现有基于广度优先搜索的穿越算法不能保证路径满足约束条件。为此,论文建立了一种穿越模型,并提出一种启发式的近似数值优化算法(kSP-LAHTP)。算法利用Voronoi图将连续路径问题域离散化,计算Voronoi边的两个衡量指标:暴露度和穿行时间,结合线性聚合的启发式路由机制,使目标实现满足时间约束值的最佳穿越。分析和试验结果表明算法解决了目标穿越时间受限情况下的穿越问题,且随系数k的增加,算法搜索路径更接近实际最佳。
     (2)研究了轨迹长度受约束的目标穿越问题。在实际应用中,可能由于一些不可抗拒因素的原因(如:能耗),目标必须兼顾穿行轨迹的长度,过长的轨迹无疑要消耗更多的能源。尽管当目标以匀速运动前进时,路程的消耗可转换成对时间的消耗,但若目标以变速前进时,路程和目标穿行时间难以进行有效转换。为此,本文采用网格技术,并结合标签设定路由机制,提出一种轨迹长度受约束的目标穿越轨迹算法。该算法可使智能目标在起点至终点的轨迹长度不大于约束阈值的前提下实现近似最优穿越。分析和试验结果表明,算法能有效搜索到满足约束条件的近似最佳轨迹,同时可以通过增加网格阶数来提高轨迹精确度。
     (3)研究了基于有向感知模型的弱栅栏覆盖问题。目前,关于弱栅栏覆盖控制方面的研究一般是针对满足全向性感知模型的传感器网络展开。然而,传感器网络构造栅栏时,仅需用到传感器节点的部分感知区域即扇形区域,显然采用全向感知模型将导致节点能源的浪费。为此,本文采用有向感知模型来展开弱栅栏覆盖问题的研究,分析了传感器网络构造弱栅栏覆盖的概率以及间隙长度(即覆盖缺口)小于目标大小的概率,并提出一种弱栅栏覆盖启发式构造算法。该算法通过计算节点覆盖区域在区域边界的投影,将弱栅栏覆盖问题转化为一维线覆盖问题,且算法可依据目标的大小来调控间隙距离,从而使网络探测到目标的同时最小化节点数。实验结果表明,采用有向感知模型的方案为构造栅栏所需激活节点数少于基于全向感知模型的方案,且构建弱栅栏覆盖所需节点数随着目标大小的增加而减少。
     (4)研究了基于数据融合的强栅栏覆盖问题。对于沿任意路径穿越带状区域的移动目标,分布于区域的无线传感器网络通常构造强栅栏覆盖以检测穿越目标,并利用节点睡眠调度策略有效缓解节点能量受限问题。然而,现有研究一般基于圆盘感知模型且未考虑节点间的协作,容易导致传感器节点的浪费。针对此问题,本文采用概率性感知模型,并利用数据融合技术构造虚拟节点来增加节点覆盖区域。在此基础上,提出一种栅栏覆盖控制算法。算法借助分治法构造栅栏,以减少节点间通信开销;并调度传感器使冗余节点睡眠,达到减少网络能耗和延长网络寿命的目的。实验结果表明,算法在栅栏数、网络寿命等性能上均优于基于节点监测数据未融合的栅栏覆盖控制算法。
     (5)最后,研究了基于概率性模型的k-连通栅栏覆盖问题。覆盖和连通是传感器网络的两个重要属性,在网络设计中扮演着重要角色。然而,目前大部分相关工作仅考虑其中一类问题,而未将覆盖和连通结合研究分析。而且,研究成果大部分是基于圆盘模型,该模型并不能有效利用节点的感知和通信能力。为此,本章基于概率性模型分析了栅栏覆盖与s-t连通之间的关系,并提出一种分布式k-连通栅栏覆盖维持协议。该协议可同时实现栅栏覆盖和s-t连通,且比其它基于圆盘模型的协议更适合真实环境。实验结果表明,协议可通过对探测率阈值,误警率阈值以及包传递率阈值的设置来提供质量可控的连通栅栏覆盖和调节激活节点数。
     综上所述,本文以改善覆盖能力和提高覆盖质量为目标,分别从反监控方和监控方两个角度研究了栅栏覆盖问题,对于推进无线传感器网络栅栏覆盖控制的研究和实用化具有广泛的理论意义和应用价值。
Wireless sensor network (WSN) is a special self-organized ad-hoc networkconsisting of a number of tiny sensor devices with low processing power, lowcommunication capacity, and limited buffer capacity. Each device has sensing,processing and wireless communication capabilities, which enable it to gather usefulinformation about the environment, to generate report messages and deliver messagesto the base station. Wireless sensor networks widely used in military, industrial, civil,etc. is currently an active research area. Sensing coverage is one of the fundamentalproblems in wireless sensor networks, which reflects the quality of service (QoS) ofmonitoring provided by the entire sensor network. Space resources of wireless sensornetwork can be optimized allocation by coverage. This dissertation focuses on thebarrier coverage based on monitoring (wireless sensor network) and anti-monitoring(moving target) in wireless sensor network. The former is to address how to cover themoving target, while the latter is to consider how to provide better QoS of monitoring.The major contributions of the dissertation are as follows:
     (1) Firstly, based on anti-monitoring, the moving target subjected to traversal timeconstraint how to across enemy sensor networks field was studied. The most existingtraversal algorithms based on breadth-first-search are difficulty to satisfy apre-determined time constraint value. Motivated by this reason, a traversal model isconstructed and a heuristic approximately optimization algorithm (kSP-LAHTP) isproposed in this dissertation. The algorithm makes the continuous path problemdomain to a discrete one by Voronoi diagram. By using exposure and traversal time asthe path (trajectory) performance metric, the algorithm combines the linear aggregatedrouting mechanism to find the optimal path. It makes the moving target can traversethe enemy sensor networks field. And the traversal time from the source to thedestination is less a given threshold. Theoretically and experimentally, it is concludedthat the proposed algorithm is able to solve the traversal path problem with timeconstraint. The proposed algorithm (kSP-LAHTP) is able to find a traversal path closerto the optimum with parameter k increasing.
     (2) Secondly, the moving target subjected to trajectory length constraint how toacross the enemy sensor networks field was studied. In practical applications, due tosome majeure reasons (such as: energy consumption), the target must take into account the length of the trajectory, because long trajectory will undoubtedly consume moreresources. When target move forward with uniform speed, the trajectory length cantranslate into time. When target move forward with uneven speed, the trajectory lengthcan’t translate into time. Thus we can’t search the trajectory with length constraint bythe kSP-LAHTP algorithm. For the reason, utilizing grid and label setting routingmechanism, an approximate optimization approach was proposed for finding traversaltrajectory of target. The algorithm can calculate the approximate optimal trajectory ofintelligent target with a constraint on the trajectory length, where the trajectory lengthfrom start to destination is less than special threshold. Theoretical analysis andextensive experiments results show that the proposed scheme is able to find theapproximate minimal exposure trajectory with constraints and improve accuracy of thetrajectory by increase order of grid.
     (3) The issue on how to achieve weak barrier coverage based on directionalsensing model was studied in the dissertation. At present, most of the existing weakbarrier coverage schemes are based on omni-directional sensing model. However, itcan be known that only a sector of sensing region is required to construct barrier.Therefore, omni-directional sensing model will result in waste of energy formaintaining the sensing capability over the remainder of the sensing region. Motivatedby the reason, based on directional sensing model, we analysis the probability of thesensor network successfully accomplished weak barrier coverage and the probability ofthe gap is less than size of the target. And we propose a weak barrier coverage heuristicalgorithm. The algorithm translate the weak barrier coverage problem into linecoverage problem by projecting sensors’ coverage regions on boundary of region,while the barrier can exist the gaps which smaller than a given size. The performanceof the proposed scheme was evaluated and compared with that of the omni-directionalsensing scheme. The results show that the proposed directional sensing schemeachieved weak barrier coverage is better than the omni-directional sensing scheme, andthe number of activated node will decrease with size of target increasing.
     (4) The issue on the (strong) barrier coverage based on data fusion was studied.Wireless sensor networks usually construct barrier coverage for detecting travel targetwhen sensor random development in a belt region. And sensors scheduling algorithm isused to alleviate the energy limitation problem of sensor in sensor networks. However,most of current researches are based on disk sensing model, and don’t consider thecollaboration between sensor nodes, which will result in waste of sensor nodes.Therefore, in this paper, based on probabilistic sensing model, we construct virtual sensor by information fusion scheme that fuse sensing measurements of two sensors,which can significantly improve coverage region. Based on the theories, a barriercoverage preserving configuration algorithm is proposed. The algorithm providescontinuous barrier coverage for the whole region by divide-and-conquer approach,which can reduce the communication overhead. Moreover, the redundancy nodes arescheduled to sleep for reducing energy consumption and prolonging the networklifetime. Analysis and experiment results show that the algorithm outperforms thebarrier coverage preserving configuration scheme based on information non-fusionsensor, on the number of barrier and network lifetime, etc.
     (5) Finally, the issue on k-connected barrier coverage based on probability modelwas studied. Barrier coverage and network connectivity are two important properties ofwireless sensor networks deployed in belt region. However, most of the previousworks addressed only one kind of the two issues: coverage or connectivity alone.Moreover, many previous works on coverage and connectivity are based on the diskmodel, which can’t fully utilize the sensing and communication capacity of sensors.For these reasons, we analysis the relationship between barrier coverage and s-tconnectivity and proposed a distributed algorithm (k-CBC) for maintaining bothk-barrier coverage and s-t connectivity (refer to as k-connected barrier coverage) underprobabilistic model. The proposed algorithm is more suitable for real environmentsthan other protocols that assume sensing and communication regions (ranges) are diskswith a fixed radius around sensors. The performance of the proposed scheme wasevaluated and compared with other schemes. Compared with other algorithm based ondisk model, our scheme can provide the controllable quality of k-connected barriercoverage and the number of active nodes by detection probability threshold, falsealarm probability threshold, and packet deliver rate threshold.
     In summary, our goal is improve the barrier coverage performance and enhancingthe quality of barrier coverage. This dissertation studies the barrier coverage based onanti-monitoring and monitoring respectively, which has practical value and theoreticalsignificance for advancing the research and practicability of barrier coverage.
引文
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