Received-Signal-Strength-Based Logical Positioning

Resilient to Signal Fluctuation

Thomas LocherRoger Wattenhofer

Aaron Zollinger

SAWN 2005 2

Overview

• Problem statement– RSSI-based positioning– Related work– Logical positions instead of coordinates

• Experimental observations– signal fluctuation

• System– Calibration phase– Operation phase

• Practical evaluation

• Problems and limitations

• Conclusion

SAWN 2005 3

Problem Statement

• Positioning based on RSSI

b1

b2

b3

b1 b2 b3

SAWN 2005 4

Related Work

• [Bahl, Padmanabhan, Infocom 2000]

• [Bahl, Padmanabhan, Balachandran, MSR Tech.Rep. 2000]

• [Youssef, Agrawala, Shankar, PerCom 2003]

• [Youssef, Agrawala, CNDS 2004]

• [Schwaighofer, Grigoras, Tresp, Hoffmann, Adv.Neur.Inf.Proc.Sys. 2004]

• [Krishnan, Krishnakumar, Ju, Mallows, Ganu, Infocom 2004]

• [Ganu, Krishnakumar, Krishnan, WCNC 2004]

• [Roos, Myllymäki, Tirri, Misikangas, Sievänen, 3rd IEEE WLAN Workshop 2004]

• …

• Problems– Presence of base station position information

– Antenna and radio propagation characteristics

– (Moving) obstacles (walls, buildings, cars…)

Compute coordinates of mobile user

SAWN 2005 5

Related Work

• Important exception:

Nibble [Castro, Chiu, Kremenek, Muntz, UbiComp 2001]

• Information about not required

Compute logical position of mobile user

Room, floor, area…

– base station positions– antenna / radio characteristics– obstacles

Our approach

SAWN 2005 6

Overview

• Problem statement– RSSI-based positioning– Related work– Logical positions instead of coordinates

• Experimental observations– signal fluctuation

• System– Calibration phase– Operation phase

• Practical evaluation

• Problems and limitations

• Conclusion

SAWN 2005 7

Factor 1.5!Factor 7.5!

• The measured signal strengths vary over time

• 15 minutes later:

Experimental Observations

Factor 10!• 1 day later:

Factor 214!

Explicitly address signal fluctuation

SAWN 2005 8

Signal Distribution Over Time

0

0.05

0.1

0.15

0.2

0.25

0.3

6.00E-10

9.00E-10

1.20E-09

1.50E-09

1.80E-09

2.10E-09

2.40E-09

2.70E-09

3.00E-09

3.30E-09

3.60E-09

3.90E-09

4.20E-09

Received Signal Strength [mW]

Fre

qu

ency

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

3.50E-10

4.50E-10

5.50E-10

6.50E-10

7.50E-10

8.50E-10

9.50E-10

1.05E-09

1.15E-09

1.25E-09

1.35E-09

Received Signal Strength [mW]

Fre

qu

ency

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

8.00E-07

1.60E-06

2.40E-06

3.20E-06

4.00E-06

4.80E-06

5.60E-06

6.40E-06

7.20E-06

8.00E-06

8.80E-06

9.60E-06

1.04E-05

Received Signal Strength [mW]

Fre

qu

ency

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

8.30E-06

1.03E-05

1.23E-05

1.43E-05

1.63E-05

1.83E-05

2.03E-05

2.23E-05

2.43E-05

2.63E-05

Received Signal Strength [mW]

Fre

qu

ency

weak signal

strong signal

3 min 1 h

SAWN 2005 9

Overview

• Problem statement– RSSI-based positioning– Related work– Logical positions instead of coordinates

• Experimental observations– signal fluctuation

• System– Calibration phase– Operation phase

• Practical evaluation

• Problems and limitations

• Conclusion

SAWN 2005 10

System Model

Calibration phase

Operation phase

SAWN 2005 11

Calibration Phase

b2

b3

b4

b1

b5

(s1, v1), (s2, v2)(s2, v2), (s3, v3), (s4,

v4)

(s1, v1), (s3, v3), (s5, v5)

(s4, v4), (s5, v5)

average

variance

SAWN 2005 12

Operation Phase

Recorded average

Recorded variance

Currently measured average

Location withminimum Zi

All locationswith Zi ·

Confidence interval p

if0

0.05

0.1

0.15

0.2

0.25

0.3

6.00E-10

9.00E-10

1.20E-09

1.50E-09

1.80E-09

2.10E-09

2.40E-09

2.70E-09

3.00E-09

3.30E-09

3.60E-09

3.90E-09

4.20E-09

Received Signal Strength [mW]

Fre

qu

ency

Maximum likelihood estimator

SAWN 2005 13

Overview

• Problem statement– RSSI-based positioning– Related work– Logical positions instead of coordinates

• Experimental observations– signal fluctuation

• System– Calibration phase– Operation phase

• Practical evaluation

• Problems and limitations

• Conclusion

SAWN 2005 14

Practical Evaluation

10 measurements over 1 second

20 measurements over 20 seconds Best matching location

Calibration: 60 measurements over 6 seconds

80% confidence interval

SAWN 2005 15

Overview

• Problem statement– RSSI-based positioning– Related work– Logical positions instead of coordinates

• Experimental observations– signal fluctuation

• System– Calibration phase– Operation phase

• Practical evaluation

• Problems and limitations

• Conclusion

SAWN 2005 16

Problems and Limitations

Factor ¼ 100!

Factor ¼ 104!

• Same room but different position• Orientation

• Base station constellation and position granularity

• Moving objects

Inherentissues

SAWN 2005 17

Overview

• Problem statement– RSSI-based positioning– Related work– Logical positions instead of coordinates

• Experimental observations– signal fluctuation

• System– Calibration phase– Operation phase

• Practical evaluation

• Problems and limitations

• Conclusion

SAWN 2005 18

Conclusion

• Coordinates logical location

• Explicitly address high signal fluctuation

• Proposed prototype system forming “first step”– Certain accuracy

– But also problems

DistributedComputing

Group

Questions?Comments?