Localization With Mobile Anchor Points in Wireless Sensor Networks Kuo-Feng Ssu, Chia-Ho Ou, and Hewijin Christine Jiaju IEEE TRANSACTION ON VEHICULAR TECHNOLOGY MAY 2005

Outline Introduction Mobile anchor points localization algorithm System environments and assumption Localization scheme Enhancements Performance evaluations

Introduction The location information is useful for many application, like routing, target tracking… localization schemes classified into Range-based Need node-to-node distances or angles for estimating locations Have higher location accuracy but require additional hardware Range-free

Introduction (cont.) localization schemes classified into Range-based Range-free Do not need the distance or angle information for localization Cannot accomplish as high precision as the range-based, but provide an economic approach This paper develops a range-free localization mechanism with mobile anchor It can work with obstacle but sensor needed to determine signal strength

System Environments and Assumptions Two main assumptions: Each mobile anchor point has a GPS receiver Mobile anchor points are able to move

Localization Scheme The perpendicular bisector of any chord passes through the center of the circle LAB A B C LBC Endpoints of chord AB Center of Circle C C

Beacon Point Selection Each sensor node maintains beacon points list and visitor list T10 T9 When receive beacon sensor check its visitor list If has no id, add T1 to beacon point list If has id, extended the lifetime of the anchor When the life time is expired ( 3*beacon time interval), 1.add last point to beacon points list 2.Remove the id in visitor list T7 T5 T11 T8 T3 T12 T6 T1 T13 T4 T0 T2 T14 Sensor mobile Anchor T15 Visitor list (idk, lifetimek) T16 T17 T18 Beacon points list (idk, locationk, timestampk) Beacon information Anchor ID Location timestamp Mobile anchor’s communication range T19

Location calculation The intersection point of Lij and Ljk is the estimated location of the sensor L1 T7 L2 T1 T14 T17

Enhancements- Beacon Scheduling Mobile anchors broadcasting in this environment may cause collision result incorrect beacon point So the scheduling for broadcasting beacon messages is jittered Beacon interval = beacon interval + jitter time Randomly selected from [ 0, ( 0.01*beacon interval ) ]

Enhancements- Obstacle Tolerance Node S would recorded the wrong beacon point

Enhancements- Obstacle Tolerance2 Use signal strength The beacon point have different signal strength so don’t use it

Enhancements- Chord Selection Incorrect beacon points due to Collision Inappropriate beacon interval Sensor Beacon point Incorrect beacon point

Enhancements- Chord Selection 2 If the length of incorrect chord shorter, it make a bigger location error A threshold (λ) for the length of a chord is used

Analysis- Localization Accuracy Sensor Beacon point

Analysis- Localization Accuracy Incorrect beacon point Incorrect sensor location

Analysis- Localization Accuracy Shorter chord Long chord

Performance Evaluation Normal Environment Sensor field = 100 * 100m2 319 sensor nodes were randomly deployed When all sensor nodes obtained their locations, the simulation was terminated Two schemes were evaluated for performance comparisons Centroid and Constraint

Centroid and Constraint Anchor A Anchor B Sensor S Anchor C

Centroid and Constraint (cont.) Anchor A Sensor S Sensor S Anchor A

Simulation Results- Beacon Scheduling Compares randomized and periodical broadcasting schemes Beacon interval = beacon interval + jitter time random periodical Beacon interval Location Error (m)

Simulation Results- Chord Selection Location error fell down rapidly with the chord selection increase increase

Simulation Results- Radio Range Larger range , reduce more execution time Each sensor had to obtain 200 beacon message for localization Only three appropriate beacon points were needed

Simulation Results- Moving Speed Faster moving speed, reduced more execution time steady reduce

Simulation Results- Number of Anchor Our scheme achieved the best performance Increasing the number of anchor helped to reduce the execution time and beacon overhead

Simulation Results- Obstacle Tolerance

Conclusion Several enhancements can improve performance Chord selection Randomized beacon scheduling Advanced beacon point selection Our mechanism outperformed two previous range-free localization schemes