Localization with mobile anchor points in wireless sensor networks

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Localization with mobile anchor points in wireless sensor networks

  1. 1. Localization With Mobile AnchorLocalization With Mobile Anchor Points in Wireless Sensor NetworksPoints in Wireless Sensor Networks Authors: Kuo-Feng Ssu, Chia-Ho Ou, and Hewijin Christine Jiau Presented by: Md. Kayser Nizam, Md. Habibur Rahman, Md. Monzur Morshed Course: Sensor Networks and Wireless Computing Instructor: Md. Saidur Rahman
  2. 2. Main Idea of this paperMain Idea of this paper  In this paper, authors described a range-free localization scheme using mobile anchor points equipped with GPS moves in sensor field and broadcasts its current position periodically.  For range-free localization, no extra hardware or data communication is needed.  Experiment results showed that authors scheme performed better than other range- free mechanisms.
  3. 3. LocalizationLocalization  What is “localization”? • Determining where a given node is physically located in a wireless sensor network (WSN).  Why do we need to localize a node? • Identify the location at which sensor reading originate. • A sensor reading consists of <time, location, measurement> • In novel communication protocols that route to geographic areas instead of ID.  Localization is a problem in WSNs • Nodes randomly deployed • Location unknown
  4. 4. Localization (cont.)Localization (cont.)  Localization is essential • Necessary for data correlation (e.g. target tracking) • Many MAC, routing, and other protocols use nodes' locations • Helps in understanding the utility of a WSN from its coverage area • Increase network lifetime  Scalability of localization protocol is important • Large networks especially need localization • Many using anchor nodes are non-scalable
  5. 5. Localization (cont.)Localization (cont.)  Problem Formulation • Defining a coordinate system • Calculating the distance between sensor nodes  Defining a Coordinate System • Global • Aligned with some externally meaningful system (e.g., GPS) • Relative • An arbitrary rigid transformation (rotation, reflection, translation) away from the global coordinate system
  6. 6. Localization (cont.)Localization (cont.)  In general, almost all the sensor network localization algorithms share three main phases  DISTANCE ESTIMATION  POSITION COMPUTATION  LOCALIZATION ALGHORITHM
  7. 7. Distance EstimationDistance Estimation  ANGLE OF ARRIVAL (AOA) method allows each sensor to evaluate the relative angles between received radio signals  TIME OF ARRIVAL (TOA) method tries to estimate distances between two nodes using time based measures  TIME DIFFERENT OF ARRIVAL (TDOA) is a method for determining the distance between a mobile station and nearby synchronized base station  THE RECEIVED SIGNAL STRENGTH INDICATOR (RSSI) techniques are used to translate signal strength into distance.
  8. 8. Position ComputationPosition Computation  The common methods for position computation techniques are:  LATERATION techniques based on the precise measurements to three non collinear anchors. Lateration with more than three anchors called multi-lateration.  ANGULATION or triangulation is based on information about angles instead of distance.
  9. 9. Classifications of LocalizationClassifications of Localization MethodsMethods Wireless Sensor Network localization algorithms into several categories such as:  Centralized vs Distributed  Anchor-free vs Anchor-based  Range-free vs Range-based  Mobile vs Stationary
  10. 10. Centralized vs DistributedCentralized vs Distributed  Centralized • All computation is done in a central server  Distributed • Computation is distributed among the nodes
  11. 11. Anchor-Free vs Anchor-BasedAnchor-Free vs Anchor-Based  Anchor Nodes: • Nodes that know their coordinates a priori • By use of GPS or manual placement • For 2D three and 3D four anchor nodes are needed  Anchor-free • Relative coordinates  Anchor-based • Use anchor nodes to calculate global coordinates
  12. 12. Range-Free vs Range-BasedRange-Free vs Range-Based  Range-Free • For achieving coarse grained accuracy • 3 methods of distance estimation • Centroid • DV-hop • Geometry conjecture  Range-Based • For fine grained accuracy • TOA • TDOA • RSSI • AOA
  13. 13. Generic Approach Using AnchorGeneric Approach Using Anchor NodesNodes  Determine the distances between regular nodes and anchor nodes. (Communication)  Derive the position of each node from its anchor distances. (Computation)  Iteratively refine node positions using range information and positions of neighboring nodes. (Communication & Computation)
  14. 14. Phase 1: CentroidPhase 1: Centroid  Idea: Do not use any ranging at all, simply deploy enough beacons  Anchors periodically broadcast their location  Localization:  Listen for beacons  Average locations of all anchors in range  Result is location estimate  Good anchor placement is crucial! Anchors Ref: Nirupama Bulusu, John Heidemann and Deborah Estrin. Density Adaptive Beacon Placement, Proceedings of the 21st IEEE ICDCS, 2001
  15. 15. Phase 1: DV-hopPhase 1: DV-hop • Anchors • flood network with own position • flood network with avg hop distance • Nodes • count number of hops to anchors • multiply with avg hop distance C A B 1 1 1 1 2 2 2 3 3 4 4 3 hops avg hop: 5
  16. 16. System EnvironmentSystem Environment • Sensor network consists of sensor nodes and mobile anchor points • Randomly distributed • Can receive messages from sensor nodes and mobile anchor points • Mobile anchor points can traverse for assisting sensor nodes to determine their locations • Each mobile anchor point has a GPS receiver and sufficient energy for moving and broadcasting beacon • Messages during the localization process.
  17. 17. Localization SchemeLocalization Scheme • Inspired by the perpendicular bisector of a chord conjecture. • Perpendicular bisector of any chord passes through the center of the circle • Localization problem can be transformed based on the conjecture • Sensor node location: center of the circle • Sensor nodes communicate with mobile anchors through the radius of the circle
  18. 18. Beacon Point SelectionBeacon Point Selection • At least three endpoints on the circle should be collected for establishing two chords • Anchor point periodically broadcasts beacon messages when it moves • Beacon message contains the anchor node’s id, location, and timestamp • Node maintains a set of beacon points & a visitor list • Beacon point is considered as an approximate endpoint on the sensor node’s communication circle
  19. 19. Location CalculationLocation Calculation
  20. 20. Beacon SchedulingBeacon Scheduling • Broadcasting in wireless ad hoc networks may cause destructive bandwidth congestion, contention, and collision • Collision at sensor nodes could occur due to beacon messages in the mechanism • Solution: the scheduling for broadcasting beacon messages is jittered. • Randomized scheduling prevents the beacon collision at sensor nodes so each node can efficiently obtain beacon messages from different mobile anchor points.
  21. 21. Chord SelectionChord Selection  Localization will be accurate if the selected beacon points are exact on the communication circle  Incorrect beacon points could be chosen due to collision or inappropriate beacon intervals.  Chords generated using the beacon points thus fails to estimate the position of the sensor  When length of the chord is too short, probability of unsuccessful localization will increase rapidly  A threshold λ for the length of a chord is used to solve the problem  The length of a chord must surpass the threshold for reducing the localization error
  22. 22. Obstacle ToleranceObstacle Tolerance • Obstacles in the sensor field cause radio irregularity in the sensor network • Radio irregularity could degrade the performance of localization protocols so most localization schemes require a non-obstacle sensing area • Original mechanism may choose inappropriate beacon points if obstacles exist
  23. 23. Obstacle Tolerance (cont.)Obstacle Tolerance (cont.) • Enhanced beacon point selection based on the characteristic of concentric circles is developed for tolerating the presence of obstacles • Exploiting chords on one of its concentric circles can also compute the center of the circle • B3, B4, and B5 are on the same concentric circle and can form two suitable chords to determine the center of the circle • Signal strength of a received beacon is in inverse proportion to the distance with the sender
  24. 24. Simulation EnvironmentSimulation Environment Six sets of simulations for evaluation: •Beacon scheduling •Threshold for the length of a chord •Radio range •Moving speed •Number of anchor points •Obstacles
  25. 25. Three metrics used to evaluate the performance of proposed localization mechanism • Average location error • Average execution time • Beacon overhead Performance MetricsPerformance Metrics
  26. 26. Simulation ParametersSimulation Parameters
  27. 27. PerformancePerformance
  28. 28. ConclusionConclusion In this paper, authors found that ……………..  Range-free localization mechanism without using distance or angle information was also able to achieve fine-grained accuracy.  The sensor nodes can calculate their positions without additional interactions based on the localization information from mobile anchors and the principles of elementary geometry.  All computation is performed locally, and beacon overhead only occurs on mobile anchors so the mechanism is distributed, scalable, effective, and power efficient.  Execution time for localization mechanism can be shortened if the moving speed, the radio range, or the number of mobile anchor points in increased.
  29. 29. Thank you 

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