1. 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

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

3. 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

4. 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

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

6. 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

7. 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

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

9. 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 ) ]

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

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

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

13. 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

14. Analysis- Localization Accuracy
Sensor
Beacon point

15. Analysis- Localization Accuracy
Incorrect beacon point
Incorrect sensor location

16. Analysis- Localization Accuracy
Shorter chord
Long chord

17. 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

18. Centroid and Constraint
Anchor A
Anchor B
Sensor S
Anchor C

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

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

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

22. 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

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

24. 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

25. Simulation Results- Obstacle Tolerance

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