1. Localization With Mobile AnchorLocalization With Mobile Anchor
Points in Wireless Sensor NetworksPoints 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 paperMain 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. LocalizationLocalization
 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 (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.)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.)Localization (cont.)
 In general, almost all the sensor network
localization algorithms share three main
phases
 DISTANCE ESTIMATION
 POSITION COMPUTATION
 LOCALIZATION ALGHORITHM

7. Distance EstimationDistance 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 ComputationPosition 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 LocalizationClassifications of Localization
MethodsMethods
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 DistributedCentralized vs Distributed
 Centralized
• All computation is done in a central server
 Distributed
• Computation is distributed among the nodes

11. Anchor-Free vs Anchor-BasedAnchor-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-BasedRange-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 AnchorGeneric Approach Using Anchor
NodesNodes
 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: CentroidPhase 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. Phase 1: DV-hopPhase 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
C
A
B
1
1
1
1
2
2
2
3
3
4
4
3 hops
avg hop: 5

16. System EnvironmentSystem 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 SchemeLocalization 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 SelectionBeacon 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 CalculationLocation Calculation

20. Beacon SchedulingBeacon 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 SelectionChord 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 ToleranceObstacle 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.)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 EnvironmentSimulation 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. Three metrics used to evaluate the
performance of proposed localization
mechanism
• Average location error
• Average execution time
• Beacon overhead
Performance MetricsPerformance Metrics

26. Simulation ParametersSimulation Parameters

27. PerformancePerformance

34. ConclusionConclusion
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.

35. Thank you 