Localization With Mobile Anchor
Points in Wireless Sensor Networks
Authors:
Kuo

Feng
Ssu
,
Chia

Ho
Ou
, and
Hewijin
Christine
Jiau
Presented by:
Kayser
Nizam
, Md. Habibur Rahman, Md. Monzur Morshed
Course:
Sensor Networks and Wireless Computing
Instructor:
Md.
Saidur
Rahman
Main 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
.
Localization
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
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
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
Localization (cont.)
In general, almost all the sensor network
localization algorithms share three main
phases
DISTANCE ESTIMATION
POSITION COMPUTATION
LOCALIZATION ALGHORITHM
Distance 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
.
Position 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
.
Classifications of Localization
Methods
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
Centralized vs Distributed
Centralized
•
All computation is done in a central server
Distributed
•
Computation is distributed among the nodes
Anchor

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
Range

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
Generic Approach Using Anchor
Nodes
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)
Phase 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
Phase 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
System 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.
Localization 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
Beacon 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
Location Calculation
Beacon 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
.
•
R
andomized
scheduling prevents
the beacon collision
at sensor
nodes so each node can
efficiently obtain beacon
messages from different mobile
anchor points.
Chord 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
Obstacle 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
Obstacle Tolerance (cont.)
•
Enhanced
beacon point selection
based on the
characteristic of
concentric circles is developed
for tolerating the presence of
obstacles
•
E
xploiting
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
Simulation 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
Simulation Parameters
Performance
Conclusion
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
.
Thank you
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