Mobile Anchor Points for Localization in Wireless Sensor Networks

1. Localization With Mobile Anchor Points 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. Main 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. Localization • 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. 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. 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. Localization (cont.) • In general, almost all the sensor network localization algorithms share three main phases • DISTANCE ESTIMATION • POSITION COMPUTATION • LOCALIZATION ALGHORITHM

7. Distance 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. Position 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. Classifications of Localization Methods 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. Centralized vs Distributed • Centralized • All computation is done in a central server • Distributed • Computation is distributed among the nodes

11. Anchor-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. Range-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. Generic Approach Using Anchor Nodes • 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. Phase 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. 3 4 1 2 3 1 2 1 4 1 2 Phase 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 3 hops B avg hop: 5 C A

16. System 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. Localization 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. Beacon 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. Location Calculation

20. Beacon 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. Chord 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. Obstacle 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. 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. Simulation 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. Simulation Parameters

26. Performance

33. Conclusion 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.

34. Thank you 