1

Indoor Location Sensing Using Active RFID

Lionel M. Ni, HKUSTYunhao Liu, HKUST

Yiu Cho Lau, IBMAbhishek P. Patil, MSU

Indoor Location Sensing Using Active RFID

Lionel M. Ni, HKUSTYunhao Liu, HKUST

Yiu Cho Lau, IBMAbhishek P. Patil, MSU

2

MotivationMotivation

Overview of RFIDOverview of RFID

Performance EvaluationPerformance Evaluation

LANDMARC Approach LANDMARC Approach

ConclusionConclusion

3

4

Location-aware Computing

• The location is an important context that changes whenever the object moves

• Location-aware services allow to offer value-added service to the user, depending on their current geographic position and will be a key feature of many future mobile applications

• Sensing the location: explicit and implicit cooperation; outdoor or indoor

5

Location Sensing Techniques

• Triangulation: use geometric properties of triangle to compute object locations– Signal strength: signal attenuation is a

function of distance to the signal source

• Scene analysis: use features of a scene observed from a certain reference point

• Proximity: determine if an object is near a known location

6

Sensing Technologies

• Infrared

• Ultrasonic

• Radio Frequency– RFID– 802.11– Bluetooth

• Others

7

Existing Technologies and SystemsInfrared Example: Active Badge Location

System

• Low power requirements• Low circuitry costs: $2-$5

for the entire coding/decoding circuitry

• Simple circuitry• Higher security• Portable • High noise immunity

• Line-of-sight • Coarse resolution• Short range• Blocked by common

materials• Light, weather sensitive

• Pollution can affect

transmission

8

IEEE 802.11Example: RADAR

• It is using a standard 802.11 network adapter to measure signal strengths at multiple base stations positioned to provide overlapping coverage in a given area

9

• Strength– Easy to set up– Requires few base

stations– Uses the same

infrastructure that provides general wireless networking in the building

• Weakness– Poor overall accuracy:

• scene-analysis: within 3 meters with 50 percent probability

• signal strength: 4.3 meters at the same probability

– Support Wave LAN NIC

Microsoft RADAR

10

Ultrasonic

• Active Bat (AT&T)– ultrasound time-

of-flight measurement

– can locate Bats to within 9cm of their true position for 95 percent of the measurements

11

Cricket Location Support System (M.I.T)

• Ultrasonic time-of-flight and a radio frequency control signal

• Lateration and proximity techniques

• Decentralized scalability

• 4x4 square-foot regions

12

RFID: SpotON

• Objects are located by homogenous sensor nodes without central control

• SpotOn tags use received radio signal strength information as a sensor measurement for estimating inter-tag distance

• No complete system yet

13

LANDMARC Prototype

•Selection criteria–Use commodity products or off-the-shelf components–Low cost–Resolution: no more than 2-3 meters

•Decision: RFID technology

14

What is RFID (Radio Frequency Identification) ?

• RFID is a means of storing and retrieving data through electromagnetic transmission to a RF compatible integrated circuit

• 3 basic components

Card Reader Antenna

Reader/ Programmer

TagAntenna

Ai rI nterface

Transponderor Tag

15

Passive RFID

16

Active RFID

• RF Reader– Range up to 150 feet– Identify 500 tags in 7.5 seconds with the collision

avoidance– Support 8 power levels (function of distance)

• Active Tag system– Emit signal, which consists of a unique 7-character

ID, every 7.5 seconds for identification by the readers– Button-cell battery (2-5 years life)– Operate at the frequency of 303.8 MHz

17

Active RFID AdvantagesLocat i on

Sever

RFTags

RFReadersWi rel ess

Network

• Non-line-of-sight nature

• RF tags can be read despite the extreme environmental factors : snow, fog, ice, paint …

• be read in less than 100 milliseconds

• promising transmission range

• cost-effectiveness

18

Using RFID: First Attempt

• How many readers are needed?– Build an array of

readers: too expensive

• How reliable is the tag detection?– Not very reliable

due to signal attenuation

• Placement of RF readers

• Cannot measure distance directly

RR

R

R

R

R R

R R

19

• the received signal power at distance is given by

• free space loss is given by

20

Difficulties

power l evelcoverage

i n f reespace

i ncompl i catedenvi ronment

21

LANDMARC Approach

• The LANDMARC system mainly consists of two physical components, the RF readers and RF tags

22

The Concept of Reference Tags

1m

1m

Ref erence Tag

Tracki ng Tag

RF Reader

(0, 0)

1 m2 m3 m4 m

1 m

2 m

4 m

3 m

5 m

6 m

7 m

8 m

9 m

23

a b c

d e f

g h i

g k l

RFReader1

RFReader2

FourNearest

tracki ngtag

• Distance estimation• Placement of

reference tags• Selection of k

neighboring reference tags

• Weight of each selected reference tags

Known Reference Tags

24

the placement of the reference tags •

•

Three Key IssuesThree Key Issues

the value of k in this algorithm •

• the formula of the weight•

•

25

Distance Estimation: Signal Strength

• Signal Strength Vector of an unknown tag

• Signal Strength Vector of a reference tag

• Euclidian distance

26

27

28

Effect of the Value k

0%

25%

50%

75%

100%

0 1 2 3

e (meters)

cu

mu

lati

ve

%

k=2,Av e=1.47,Worst=2.68

k=3, Av e=1.13,Worst=1.98

k=4, Av e=1.09,Worst=1.81

k=5, Av e=1.13,Worst=1.99

Cumulative Percentile Of Error Distance When K Value Is 2, 3, 4, 5

29

Influence of The Environmental Factors

Cumulative Percentile Of Error Distance in Daytime & Night

0%

25%

50%

75%

100%

0 0.5 1 1.5 2 2.5 3

e(meters)

cum

ula

tive

%

Daytime,Worst=1.956

Night,Worst=1.783

30

Influence of The Environmental Factors (cont’d)

Change The Placements Of Tracking Tags

1m

1m

Reference Tag

Tracki ng Tag

RF Reader

(0, 0)

1 m2 m3 m4 m

1 m

2 m

4 m

3 m

5 m

6 m

7 m

8 m

9 m

31

Influence of The Environmental Factors (cont’d)

Cumulative Percentile Of Error Distance When Changing The Placement Of Tracking Tags

0%

25%

50%

75%

100%

0 0.5 1 1.5 2 2.5 3

e(meters)

cum

ula

tive

%

original setup,Worst=1.81

changeTrkTag,Worst=1.82

32

Effect of The Number of Readers

Cumulative Percentile Of Error Distance With 3 or 4 Readers Data

0%

25%

50%

75%

100%

0 0.5 1 1.5 2 2.5 3

e (meters)

cum

ulat

ive

%

4 readers data, Worst=1.81

3 readers data, Worst=2.59

33

The Effect of Placement of Reference Tags

Without Partition

a b c

d e f

g h i

g k l

RFReader1

RFReader2

FourNearest

tracki ngtag

34

Effect of Placement of Reference Tags (cont’d)

With Partition

a b c

d e f

g h i

g k l

RFReader1

RFReader2

Fournearest

Part i t i on P

realposi t i on

computedposi t i on

35

Effect of Placement of Reference Tags (cont’d)

With Partition

a b c

d e f

g h i

g k l

RFReader1

RFReader2

m n

o

Ori gi nal Reference Tags

New Reference Tags

Tracki ng Tag

36

Placement of Reference Tags

Replacements of the Reference Tags with a Higher Density

1m

1m

Ref erence Tag

Tracki ng Tag

RF Reader

1m

1m

near 1 near 2

37

Effect of Higher Density Reference Tags

Cumulative Percentile Of Error Distance With Higher Reference Tag Density

0%

25%

50%

75%

100%

0 0.4 0.8 1.2 1.6 2

e(meters)

cum

ula

tive

%

Original, Worst=1.81

near1, Worst=1.76

near 2, Worst=1.69

38

Lower Density of Reference Tags

Replacements of the Reference Tags with a Lower Density

1m

1m

Ref erence Tag

Tracki ng Tag

RF Reader

1m

1m

f ar 1 far 2

39

Effect of Lower Density Reference Tags

Cumulative Percentile Of Error Distance With lower Reference Tag Density

0%

25%

50%

75%

100%

0 0.5 1 1.5 2 2.5 3 3.5e (meters)

cum

ula

tive

%

Original, Worst=1.81

far 1,Worst=2.59

far 2,Worst=3.17

40

• Using 4 RF readers in the lab, with one reference tag per square meter, accurately locate the objects within error distance such that the largest error is 2 meters and the average is about 1 meter.

41

Conclusions

• RFID can be a good candidate for building location-sensing systems

• Able to handle dynamic environments• Suffer some problems

– Difference of Tags’ Behavior– RFID does not provide the signal strength of

tags directly – Unable to adjust emitting interval– Standardization

42

Questions?

43

44

DeskRefrigerator

Desk

Desk SofaRefrigerator

2. Tracking

movement

3. Notify where you are (Location sensing)4. Notify your eating schedule

7. Time sensing8. immobility sensing

11. Proximity sensing

16. Notify

“ok to eat”

1.walk

5.Stop eating

6.Back to desk

10. Walk around

9. Notify to move

12. Walk away from sofa

13. Distance& Time

sensing

14. Notify to stop

15. Go back to desk

17. Go to kitchen

18. Refer

Healthy food

19. Eat matched food

45

Triangulation

– Lateration• Direct

• Time-of-flight

• Attenuation

– Angulation

46

(2) Scene Analysis• use features of a scene observed from a

certain reference point

(3) Proximity• determine if an object is near a known

location

47

Project Motivation

• GPS’s inability for accurate indoor location sensing

• Develop a cost-effective indoor location sensing infrastructure

• Enables location-based Web services for mobile-commerce (m-commerce) environment

• Plenty of other application scenarios, depending on your imagination and creativity

48

Passive RFID vs. Active RFID

Reader

Ai rI nterface

RF Si gnal toTag

Tag DataAntenna

Transponder

or Tag

Active tag System

49

A Triangulation Approach

Tag

length 3

l ength 1

Length 2

Reader 1

Reader 2

Reader 3

50

I nternetweb/ database

serversl ocati onservers

A B C D E F G H

S EL E CT E D

O N -L IN E

A B C D E F G H

S EL E CT E D

O N -L IN E

A B C D E F G H

S EL E CT E D

O N -L IN E

A B C D E F G H

S EL E CT E D

O N -L IN E

Wi red Network Connecti onWi

red

Netw

ork

Conn

ecti

on

Wire

d Ne

twor

k Co

nnec

tion

WAP

WAPWAP

WAP

Ad Hoc PDARouter

802. 11b

802. 11b

802. 11b

802. 11b

802. 11b

802. 11b

RF Communi cat i on

RF Communi cat i on

PDA/RF Tag

PDA

PDA/RF Tag

regi strati onservers

Ref erenceTag

802. 11b

RF Communi cat i on

802. 11b

RF Communi cat i on

802. 11b

RFReader

RFReader

RFReader

RFReader

51

Active RFID

• RF Reader– Range up to 150 feet– Identify 500 tags in 7.5 seconds with the collision

avoidance– Support 8 power levels (function of distance)

• Active Tag system– Emit signal, which consists of a unique 7-character

ID, every 7.5 seconds for identification by the readers

– Button-cell battery (2-5 years life)– Operate at the frequency of 303.8 MHz