Localization with mobile anchor points in wireless sensor networks
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
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
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.