Localization Localization Techniques in Wireless Techniques in Wireless

Sensor NetworksSensor NetworksPrepared by: Abdelmounaim DahbiPrepared by: Abdelmounaim Dahbi

In partial fulfillment of the requirements for the courseIn partial fulfillment of the requirements for the course

Wireless Ad Hoc NetworkingWireless Ad Hoc Networking

Instructor: Professor Ivan StojmenovicInstructor: Professor Ivan Stojmenovic

University of OttawaUniversity of Ottawa

(x,y)

(u,v)

d1

(z,t)

d2d3

Outline

Introduction Applications Beacon Nodes (Anchor Nodes) Distance/Angle Measurement Techniques Centralized Algorithms Distributed Algorithms Range-based Localization Techniques Range-free Localization Techniques Iterative Refinement Concluding Remarks Ongoing Research Issues References

What is Sensor Localization? The determination of the absolute or relative position of sensor nodes (geographical locations of sensors)

Introduction

N1(x1,y1)

d1

d2d3

N2(x2,y2)

N3(x3,y3)

x

y

x1

y1β

α

N

N

N

Introduction

sensing data (Phenomena) without knowing the sensor location is most

of the times meaningless

+

Why Sensor Localization?

Large number of randomly

scattered sensor nodes

Introduction

Why Sensor Localization?

Gatew

ay

This is true in any location sensitive application, location aware service…

Introduction

Isn’t GPS just the answer?Yes, but:

Not available indoor Limited in certain environments such as Bush Not accessible from under water Constraints on the cost of sensors Constraints on the size of sensors Constraints on the energy consumption Not very accurate …

Applications

Network Functions: Geographical Routing, Collaborative Signal Processing

Bush Fire Surveillance/Detection Intrusion Detection Habitat Monitoring/Wildlife Tracking (ZebraNet) Water Quality Monitoring Pollution Monitoring Traffic Monitoring Target Tracking (Military: tracking enemy vehicles, and

Civilian: tracking wild animals in wildlife preserves) …

Beacon Nodes (Anchor Nodes)

Ordinary sensor nodes that know their global coordinates a priori

Either hard-coded coordinates Or GPS equipped sensor nodes

Different uses of beacon nodes (Reference, Flooding of their positions and other data…)

Importance of Beacon placement For 2D three and 3D four beacon nodes are

needed But, costly

Distance/Angle Measurement TechniquesAngle of Arrival (AoA)

The angle between the propagation direction of an incident wave and some reference direction

Does not require synchronization

But, costly and requires extensive signal processing…

Distance/Angle Measurement TechniquesReceived Signal Strength Indicator (RSSI)

In theory, the energy of a radio signal diminishes with the square of the distance from the signal’s source.

Low cost: all sensors have radiosBut in practice, RSSI ranging

measurements contain noise (in the order of meters)

Difference in propagation in different environments…

Distance/Angle Measurement TechniquesTime of Arrival (ToA)

c: The propagation speed of the radio signal (speed of light)

Accurate But, requires precise

synchronization

Distance/Angle Measurement TechniquesTime Difference of Arrival (TDoA)

c: The propagation speed of the radio signal

ss: The propagation speed of the ultrasound/acoustic signal

Accurate, No synchronization required

But, costly (Hardware)

Distance/Angle Measurement TechniquesRadio Hop Count (DV-Hop)

A

B

1

1

1

1

2

2

2

2

2

3

3

3

4

4

A-B: 12

hAB:3 hops

dhop: avg hop distance : 4 Connectivity data

hij: Shortest path i,j (number of hops)

dij: Distance i,j

dij <= R x hij

Better estimate: dhop

dij = hij x dhop

Distance/Angle Measurement TechniquesRadio Hop Count (DV-Hop)

nlocal: The expected number of neighbors per node

hij: Length of the shortest path between sensor i and sensor j in terms of number of number of hops

dij: The Distance between sensor i and sensor j

dhop : The Average hop distance

dij = hij x dhop

Distance/Angle Measurement TechniquesRadio Hop Count (DV-Hop)

Distance measurements are always integral multiples of dhop

Environmental obstacles can prevent edges from appearing in the connectivity graph that otherwise would be present

Depends on the density… nlocal

Centralized Algorithms

Migration of internode ranging and connectivity data to a sufficiently powerful central base station

Complex processing of the collected data Migration of the resulting locations back to

the respective nodes. Examples:

SDP: The SemiDefinite Programming MDS–MAP: MultiDimensional Scaling

Distributed Algorithms

Each sensor collects its data Computation is done by the sensors Several iterations might be required Not as accurate as centralized but does no

migrations between a central station and the sensors

Examples: Triangulation Trilateration/Multilateration Bounding Box (Min-Max) Centroid

Range-based Localization Techniques Triangulation

AoA to compute the angles

The number of BSs needed for the location process is less

Compute the linear least-square solution

Assuming (x1,y1)=(0,0) and the x axis defined by the two beacon nodes we have:

y

x

Range-based Localization Techniques Trilateration/Multilateration

Distance measured: RSSI, ToA, TDoA

Requires at least 3 BNs in 2D, and 4 BNs in 3D..

Compute the linear least-squares solution

Multilateration if more than three beacons are used to estimate the sensor’s position

Range-free Localization Techniques Bounding Box (Min-Max)

Distance based on Radio Hop Count (DV-Hop)

Simple

But less accurate…

Range-free Localization Techniques Centroid Algorithm

Nodes localize themselves to the centroid of their proximate reference points

Xi1,Yi1

Xi,Yi

Xi2,Yi2

...

..Xik,Yik

x

Iterative Refinement

Node obtains initial position

Node broadcasts its position

Position is refined iteratively using:Distances to neighborsNode’s previous positions

Concluding RemarksWhat is the best localization algorithm?

No best algorithmDepends on:

Error in range measurementConnectivityNetwork topologyNode capabilitiesApplication requirements...

Concluding RemarksPros and Cons Two main types of distributed localization

algorithms: Range-based

Estimating the coordinates based on the collected information of distances or angles among nodes

Merit: Relatively high accuracy Drawback: Costly (Hardware, Power consumption)

Range-free Estimating the coordinates based on the connectivity relations Merit: Cost-effective Drawback: Not as accurate (But: coarse accuracy is sufficient

for most sensor network applications)

Hardware/Energy Cost vs Location Precision

Ongoing Research Issues

Noisy distance measurementCostly distance measurement

(hardware, energy)Few beacons…ScaleMobility

Questions

Q1: Triangulation is based on the law of sines which states

(sin a)/A=(sin b)/B=(sin c)/CProve the law of sines

Answer:Sin a = L/B, sin b = L/AB . sin a = A . sin b (sin a)/A=(sin b)/B…

L

Questions

Q2: The Radio Hop Count (DV-Hop) distance estimation technique is based on the average hop distance dhop and the hop count hij (the length of the shortest path in the graph between si and sj in terms of the number of hops). This technique has a major drawback related to environmental obstacles which can prevent edges from appearing in the connectivity graph that otherwise would be present. Give an example of a graph where such drawback is highlighted..

Answer:In this diagram, hAC = 4. Unfortunately, hBD is alsofour, due to an obstruction in the topology.

Questions

Q3: Knowing that dhop=3 and that an obstruction is affecting the connectivity in a number of edges as shown in the figure.

Give an estimate for the ditances dAB, dBC and dAC

Answer:dAB = hAB x dhop = 3 x 3 = 9dBC = hBC x dhop = 2 x 3 = 6dAC = hAC x dhop = 5 x 3 = 15

A

B

C

Questions

BN3 (0,0)

BN1 (x1,y1)

BN2 (x2,y2)

SN

(xs,ys)

r3

r2r1

Q4: Assuming accurate distance measurements between nodes, apply the trilateration technique to determine the SN coordinates (unknown) using the three BNs coordinates and the r distances (known). Let BN3 be the origin of the coordinate system.

Answer:

Questions

References

I. Stojmenovic, Handbook of Sensor Networks, Chapters 9 and 14, John Wiley & Sons, 2005

T. HE, C. HUANG, B. 11. Blum, J. A. Stankovic, and T. Abdelzaher, "Range-free localization schemes for large scale sensor networks,“ Proc. 11obiCom'03, Sep. 2003, pp. 81-95.

N. Bulusu, 1. Heidemann, and D. Estrin, "GPS-less low cost outdoor localization for very small devices," IEEE Personal Communications Magazine, vol. 7, 11ay. 2000, pp. 28-34.

Boukerche, A.; Oliveira, H.A.B.; Nakamura, E.F.; Loureiro, A.A.F.; , "Localization systems for wireless sensor networks," Wireless Communications, IEEE , vol.14, no.6, pp.6-12, December 2007

Sayed, A.H.; Tarighat, A.; Khajehnouri, N.; , "Network-based wireless location: challenges faced in developing techniques for accurate wireless location information,"Signal Processing Magazine, IEEE , vol.22, no.4, pp. 24- 40, July 2005

Thank you!

Questions!