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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
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Outline Introduction Mobile anchor points localization algorithm
System environments and assumption Localization scheme
Enhancements Performance evaluations
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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
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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
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System Environments and Assumptions System environment
Two main assumptions:1. Each mobile anchor point has a GPS receiver2. Mobile anchor points are able to move
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Localization Scheme The perpendicular bisector of any chord
passes through the center of the circle
A B
C
Endpoints of chord AB
Center of Circle CC
LAB
LBC
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When receive beacon sensor check its visitor listIf has no id, add T1 to beacon point listIf 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 list2.Remove the id in visitor list
Beacon Point Selection
mobileAnchor
Sensor
T0T1
T2
T3
T4
T5
T6
T7
T8
T9T10
T11
T12T13
T14T15
T16
T17
T18
T19
Each sensor node maintains beacon points list and visitor list
Beacon information1. Anchor ID2. Location3. timestamp
Visitor list• (idk, lifetimek)
Beacon points list(idk, locationk, timestampk)
Mobile anchor’s communication range
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Location calculation The intersection point of Lij and Ljk is the estim
ated location of the sensorL1
L2
T1
T7
T14
T17
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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 ) ]
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Enhancements- Obstacle Tolerance
Node S would recorded the wrong
beacon point
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Enhancements- Obstacle Tolerance2 Use signal strength
The beacon point have different signal strength so don’t use it
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Enhancements- Chord Selection Incorrect beacon points due to
1. Collision2. Inappropriate beacon interval
Sensor
Beacon point
Incorrect beacon point
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Enhancements- Chord Selection 2 If the length of incorrect chord shorter, it mak
e a bigger location error
A threshold (λ) for the length of a chord is used
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Analysis- Localization Accuracy
SensorBeacon point
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Analysis- Localization Accuracy
Incorrect beacon pointIncorrect sensor location
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Analysis- Localization Accuracy
Shorter chord Long chord
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Performance Evaluation Normal Environment
Sensor field = 100 * 100m2
319 sensor nodes were randomly deployed When all sensor nodes obtained their locations, th
e simulation was terminated Two schemes were evaluated for performance
comparisons Centroid and Constraint
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Centroid and Constraint Centroid
Anchor A
Sensor S
Anchor B
Anchor C
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Centroid and Constraint (cont.) Constraint
Anchor A
Sensor S
Anchor A
Sensor S
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Simulation Results- Beacon Scheduling Compares randomized and periodical
broadcasting schemes
random
periodical
Beacon interval
Lo
cati
on
Err
or
(m)
Beacon interval = beacon interval + jitter time
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Simulation Results- Chord Selection Location error fell down rapidly with the
chord selection
increase increase
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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
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Simulation Results- Moving Speed Faster moving speed, reduced more execution time
steady
reduce
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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
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Simulation Results- Obstacle Tolerance
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Conclusion Several enhancements can improve
performance Chord selection Randomized beacon scheduling Advanced beacon point selection
Our mechanism outperformed two previous range-free localization schemes
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