Points in Wireless Sensor Networks

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Nov 24, 2013 (3 years and 24 days ago)

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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