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

2. 2SAWN 2005 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

3. 3SAWN 2005 Problem Statement Positioning based on RSSI b1b1 b2b2 b3b3 b 1 b 2 b 3

4. 4SAWN 2005 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, 3 rd 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

5. 5SAWN 2005 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

6. 6SAWN 2005 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

7. 7SAWN 2005 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

8. 8SAWN 2005 Signal Distribution Over Time weak signal strong signal 3 min1 h1 h

9. 9SAWN 2005 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

10. 10SAWN 2005 System Model Calibration phase Operation phase

11. 11SAWN 2005 Calibration Phase b2b2 b3b3 b4b4 b1b1 b5b5 (s 1, v 1 ), (s 2, v 2 )(s 2, v 2 ), (s 3, v 3 ), (s 4, v 4 ) (s 1, v 1 ), (s 3, v 3 ), (s 5, v 5 ) (s 4, v 4 ), (s 5, v 5 ) average variance

12. 12SAWN 2005 Operation Phase Recorded average Recorded variance Currently measured average Location with minimum Z i All locations with Z i ·  Confidence interval p if Maximum likelihood estimator

13. 13SAWN 2005 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

14. 14SAWN 2005 Practical Evaluation 10 measurements over 1 second 20 measurements over 20 secondsBest matching location Calibration: 60 measurements over 6 seconds 80% confidence interval

15. 15SAWN 2005 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

16. 16SAWN 2005 Problems and Limitations Factor ¼ 100! Factor ¼ 10 4 ! Same room but different position Orientation Base station constellation and position granularity Moving objects Inherent issues

17. 17SAWN 2005 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

18. 18SAWN 2005 Conclusion Coordinates logical location Explicitly address high signal fluctuation Proposed prototype system forming “first step” –Certain accuracy –But also problems

19. Distributed Computing Group Questions? Comments?