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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