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Received-Signal- Strength-Based Logical Positioning Resilient to Signal Fluctuation Thomas Locher Roger Wattenhofer Aaron Zollinger
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?
