Points in Wireless Sensor Networks

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
