Low-Cost WLAN based. Dr. Christian Hoene. Computer Science Department, University of Tübingen, Germany

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1 Low-Cos WLAN based Time-of-fligh fligh Trilaeraion Precision Indoor Personnel Locaion and Tracking for Emergency Responders Third Annual Technology Workshop, Augus 5, 2008 Worceser Polyechnic Insiue, Worceser, MA Dr. Chrisian Hoene Compuer Science Deparmen, Universiy of Tübingen, Germany

2 Ouline Moivaion Four-way TOA Quanizaion Issues Implemenaion Evaluaion Summary Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 2

3 Moivaion for Indoor Locaing wih WLAN Sophisicaed locaion sysems migh no always be available because hey are oo expensive especially for developing counries because hey are no available if needed. d Insead, why no use WLAN i is cheap and virually everywhere available. In addiion, WLAN can ransmi various daa, such as speech ho command and conrol lfirs responders, video sreams from firs responders, and physiological saus monioring Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 3

4 Locaing Tracking wih Wireless LAN Sae of he Ar: Using Received Signal Srengh h Indicaions (RSSI) Alernaive: Time of arrival (TOA) using he wo-way ime of fligh of WLAN packages beween sender and receiver [McCrady2000] Advanage: TOA measuremens scale linearly wih openair propagaion disances Challenge: Can we use cheap, off-he-shelf hardware? Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 4

5 Accurae Measuremen wih Four-way TOA Sender Receiver of = 1 4 ( 3 ) 3S 0S DATA CTS RTS SIFS Every IEEE card suppors RTS/CTS. TOA measuremen conforming o IEEE proocol using 4 ransmission seps [Hoene2008] Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 5

6 Indirec and Cooperaive TOA Assume a hird node moniors he ransmission Then i also can calculae he ime of fligh: 0M of.sm of = = 1 2 2M of.sm 2 SIFS 2 of CTS ( 2 ) 2M 0M SIFS CTS RTS RTS Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 6

7 Overcoming Quanizaion 1/2 WLAN clocks have low resoluion. Time is quanized. For example, WLAN cards provide a clock resoluion of 1µs o he device drivers. Example of ypical wo-way TOA measuremen wih WLAN cards (righ) Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 7

8 Overcoming Quanisaion 2/2 Problem WLAN cards max. resoluion = 1 µs (1µs 300m) ime-of-fligh canno be measured direcly Soluion Remoe and local delay vary real coninuous disribuionas assumed discree disribuionas measured local node remoe node d remoe 323µs d local 323µs 322µs 323µs 324µs 325µs due o Thermal Gaussian noise? Muli pah? Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 8

9 Bea Frequencies 1/4 Two-way TOA delay measuremens are no random no Gaussian noise! Block paern resuls in an alernaing auocorrelaion funcion (frequency 3.5 Hz) 0,2 Auocorrelaion funcion - 40m 0,1 remoe delay local delay auocorr relaion 00 0,0-0,1-0, ime lag expressed in packes (20ms per packe) Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 9

10 Bea Frequencies 2/4 local monior 60m remoe remoe node 0.00µs propagaion 0.20 µs wai o receive 0.80 µs remoe delay µs propagaion 0.20 µs µs µs arrival recorded 0.00µs Phase Offse 0.20µs 1.00µs µs remoe delay µs µs Assumpions : MAC proocol works in discree ime seps (1µs) Local and remoe clocks have same speed and phase offse Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 10

11 Bea Frequencies 3/4 local monior 60m remoe remoe node 0.00µs propagaion 0.20 µs remoe delay µs propagaion 0.20 µs µs µs arrival recorded -0.20µs Phase Offse 0.00µs µs remoe delay µs µs Assumpions : MAC proocol works in discree ime seps (1µs) Local and remoe clocks have same speed bu differen phase offse Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 11

12 Bea Frequencies 4/4 324µs 323µs 324µs [ [ ] ] phase offse -1.00µs -0.80µs -0.20µs 0.00µs00µs Mean remoe delay over all phase offses is = *0.20 Crysal oscillaors have Frequency olerances Relaive clock drif beween he wo wireless LAN card clocks Assumpion no clock drif during a round rip ime period bu phase offse changes for he nex RTT observaion Phase offse changes slowly l over ime and repeas Phase change resuls in a frequency ha equals he bea frequency. Bea Frequency: f bea = flocal f remoe f Relaive clock drif: f bea MACclocking 3.5Hz = = 3.5ppm 1MHz Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 12

13 Implemenaion named Goodry Goodry implemens he above menioned algorihms. Uses off-he-shelf h WLAN cards Open-source available under BSD license Bu, does i works? Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 13

14 Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 14

15 Experimenal Seup 1/2 Wha is he accuracy? WLAN anennas Mobile service robo (RWI B21) 2WLANPCIcards+ anennas (for pings and TOA measuremens) 240 laser scanner (reference posiioning) Laser scanner accurae reference posiioning Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 15

16 Experimenal Seup 2/2 Laboraory wih landmarks of known posiions 6 WLAN access poins Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 16

17 Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 17

18 Summary The IEEE MAC proocol is inherenly ime synchronous, why no use his feaure for TOA measuremen? Ineroperabiliy is given. An inerface for TOA and RSSI racking has been included ino IEEE draf v (2006) Muliple research groups have verified hese resuls. This mehod is sill in research Is applicaion for indoor locaing seems promising! Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 18

19 Thank you for your ineres! Acknowledgmens This work was funded by he Landessifung Baden-Würemberg in he scope of he BW-FIT projec AmbiSense. References [McCrady2000] D. McCrady, L. Doyle, H. Forsrom, T. Dempsey, and M. Marorana, "Mobile ranging using low-accuracy clocks," IEEE Transacions on Microwave Theory and Techniques, vol. 48, pp. 951, [Hoene2008] Chrisian Hoene and Jörg Willmann. Four-way TOA and sofwarebased rilaeraion of IEEE devices. In IEEE PIMRC, Cannes, Sepember [Günher2005] A. Günher and Chrisian Hoene. Measuring round rip imes o deermine he disance beween WLAN nodes. In Neworking 2005, Waerloo, Canada, May Dr. Chrisian Hoene (Universiy of Tübingen, Germany) 19