Localization Methods in Wireless Sensor Networks

Localization in WSN 1
Localization in WSN
Presented by: Yara Ali
Supervised by: Dr. Ahmed
Akl

Agenda
 Introduction to WSN
 Localization
 Usage
 GPS .. Why not ?
 Localization methods taxonomy
 Classifications of Localization Methods
 Summary
 Future work
 References

Introduction to WSN
 A large number of self-sufficient nodes
 Nodes have sensing capabilities
 Can perform simple computations
 Can communicate with each other

Introduction to WSN (Cont.)
 Beacon (Anchor) node:
It’s a node that’s aware of it’s location,
either through GPS or manual pre-
programming during deployment.

 In a Wireless sensor nodes thousands of
sensors need to know their position
Many applications need position info:
 in-home
 forest-fire detection
 atmospheric (temperature,
pressure, … )
 military (target detection, …)
 police

 Advantages:
1. It avoids a lot of wiring
2. It can accommodate new devices at
any time
3. It's flexible to go through physical
partitions
4. It can be accessed through a
centralized monitor

 Disadvantages
1. It's easy for hackers to hack it as we
cant control propagation of waves
2. Comparatively low speed of
communication
3. Gets distracted by various elements
like Blue-tooth

Localization
 Localization is a process to compute the
locations of wireless devices in a network
 WSN Composed of a large number of
inexpensive nodes that are densely
deployed in a region of interests to measure
certain phenomenon.
 The primary objective is to determine the
location of the target

Localization (CONT.)

Usage
 Coverage
 Deployment
 Routing
 Location service
 Target tracking
 rescue

GPS .. Why not ?
 We need to determine the physical
coordinates of a group of sensor nodes in
a wireless sensor network (WSN)
 Due to application context and massive
scale, use of GPS is unrealistic, therefore,
sensors need to self-organize a coordinate
system

GPS .. Why not ? (Cont.)
1. Expensive
2. GPS satellite signals are weak (when compared to, say,
cellular phone signals), so it doesn't work as well indoors,
underwater, under trees, etc.
3. The highest accuracy requires line-of-sight from the receiver
to the satellite, this is why GPS doesn't work very well in an
urban environment
4. The US DoD (dept of defense) can, at any given time, deny
users use of the system (i.e. they degrade/shut down the
satellites)

Localization methods taxonomy

1- Target/Source Localization
 Most of the source localization methods
are focused on the measured signal
strength.
 To obtain the measurements, the node
needs complex calculating process.

(Cont.)
1. The received signal strength of single
target/source localization in WSN during
time interval t:

(Cont.)
2. The received signal strength of multiple
target/source localization in WSN during
time interval t:

(Cont.)
 The Above methods require transmission of a
large amount of data from sensors which may
not be feasible under communication
constraints.
3-4. The binary sensors sense signals
( infrared, acoustic, light, etc. ) from their
vicinity, and they only become active by
transmitting a signal if the strength of the
sensed signal is above a certain threshold.

(Cont.)
 The binary sensor only makes a binary
decision (detection or non-detection)
regarding the measurement.
 Consequently, only its ID needs to be sent to
the fusion center when it detects the target.
Otherwise, it remains silent.
 So, the binary sensor is a low-power and
bandwidth-efficient solution for WSN.

Taxonomy

2- Node Self-localization
 Range-based Localization: uses the
measured distance/angle to estimate the
indoor location using geometric principles.
 Range-free Localization: uses the
connectivity or pattern matching method to
estimate the location. Distances are not
measured directly but hop counts are used.
Once hop counts are determined, distances
between nodes are estimated using an
average distance per hop and then geometric
principles are used to compute location.

2-1 Range based localization

(Cont.)
1. Time of arrival: (TOA)
 It’s a method that
tries to estimate
distance between 2
nodes using time
based measures.
 Accurate but needs synchronization

(Cont.)
2. Time Difference Of Arrival: (TDOA)
 It’s a method for
determining the distance
between a mobile station
and a nearby synchronized
base station. (Like AT&T)
 No synchronization
needed but costly.

(Cont.)
3. Received Signal Strength Indicator:
(RSSI)
 Techniques to translate signal strength
into distance
 Low cost but very
sensitive to noise

(Cont.)
4. Angle Of Arrival: (AOA)
 It’s a method that allows
each sensor to evaluate
the relative angles
between received radio
signals.
 Costly and needs
extensive signal processing.

2-2 Range-free localization
 DV-Hop is the typical representation
 It doesn’t need to measure the absolute
distance between the beacon node and
unknown node. It uses the average hop
distance to approximate the actual
distances and reduces the hardware
requirements.

2-2 Range-free localization
(Cont.)
 Adv:
Easy to implement and applicable to
large network.
 Disadv:
The positioning error is correspondingly
increased.

2-2-1 DV-Hop
 It is divided into 3 stages:
1. Information broadcast
2. Distance calculation
3. Position estimation

1-Information broadcast
 The beacon nodes broadcast their location
information package which includes hop count and is
initialized to zero for their neighbors.
 The receiver records the minimal hop of each beacon
nodes and ignores the larger hop for the same
beacon nodes.
 The receiver increases the hop count by 1 and
transmits it to neighbor nodes.
 All the nodes in a network can record the minimal hop
counts of each beacon nodes.

2-Distance calculation
 According to the position of the beacon node
and hop count, each beacon node uses the
following equation to estimate the actual
distance of every hop

3- Position estimation
 The beacon node will calculate the average
distance and broadcast the information to
network.
 The unknown nodes only record the first
average distance and then transmit it to
neighbor nodes.
 The unknown node calculates its location
through.

2-2-1 DV-Hop (Cont.)
 A-B: 15
Anchors
 flood network with
own position
 flood network with
avg hop distance
Nodes
 count number
of hops to anchors
 multiply with avg hop distance
A
B
1
1
1
1
2
2
2
3
3
4
4
3 hops
avg hop: 5
C

2-2-1 Modified DV-Hop

2-2-2 Pattern Matching
Localization
 Also called map-based or finger print algorithm.
 It involves 2 phases:
1. The received signals at selected locations are
recorded in an offline database called radio map.
2. It works at the online state.
 The pattern matching algorithms are used to infer
the location of unknown node by matching the
current observed signal features to the prerecorded
values on the map

Classifications of Localization
Methods
 The localization techniques can be classified with
respect to various criteria:
1. Centralized vs Distributed
2. Range-free vs Range-based
3. Mobile vs Stationary
4. Coarse-grained vs fine-grained

Centralized vs Distributed
 Centralized
 Data collected in the whole network are
transmitted to the central unit that calculates the
estimated location of each node in a network.
 Distributed
 Computation is distributed among the nodes
 Each node estimates its own position based on
the local data gathered from its neighbors.

Range-Free vs Range-Based
 Range-Free (connectivity)
 Makes no assumption about the availability or
validity of such information, and use only
connectivity information to locate the entire sensor
network.
 Hop-Counting Techniques
 Range-Based (distance)
 Defined by protocols that use absolute point to
point distance estimates (range) or angle
estimates in location calculation.

Mobile vs Stationary
 Mobile Stationary

Coarse-grained vs fine-
grained
 Coarse-grained:
finding approximate coordinates of
nodes in a network so it provide lower
precision estimates of this coordinates.
 Fine-grained:
Determining precisely the coordinates
but require much more communication
and computation efforts.

Summary
 WSN .. What & Why ?
 Distance estimation VS position computation VS
localization algorithm
 Single/Multiple localization in WSN/WBSN
 Calculating the distance between sensor nodes
( TOA – TDOA – RSSI – AOA )

Summary
 Range-based methods require extra hardware therefore have a
higher cost but provide more accurate distance measurements,
whereas range-free methods use only connectivity information
and so are less accurate.
 Range-free localization ( DV-Hop , Modified DV-Hop , pattern
matching localization )
 The localization techniques can be classified with respect to
various criteria. They differ on the assumed localization
precision, hardware capabilities, measurement and calculation
methods, computing organization, the assumed network
configuration, architecture, nodes properties and deployment,
etc.

Future Work
 Few papers investigate multiple-source
localization in WSN and WBSN

References
1. http://www.hindawi.com/journals/ijdsn/2012/96
2. http://www.docslide.com/wireless-sensor-netw
3. http://www.docstoc.com/docs/32678966/Loc
alization-in-Wireless-Sensor-Network---
ELEC-619B-Presentation

References (Cont.)
4.https://www.cs.virginia.edu/~stankovic/psfiles/wsn.pdf
5.http://www.sersc.org/journals/IJCA/vol6_no3/7.pdf
6.http://www.docstoc.com/docs/130374399/Localization
-in-Wireless-Sensor-Networks
7. http://www.degruyter.com/view/j/amcs.2012.22.issue-
2/v10006-012-0021-x/v10006-012-0021-x.xml

Any Questions?

Thank You !

Localization Methods in Wireless Sensor Networks

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