Wireless Sensor Networks

1. Wireless Sensor Networks

2. Wireless Sensor Networks Previous Lecture What is Localization? Taxonomy Applications Basic Approaches Coarse localization Fine-Grained Localization Ranging techniques Trilateration

3. Wireless Sensor Networks Outline Interferometric localization (Vanderbilt) Cricket (MIT) Motetrack (Harvard)

4. Wireless Sensor Networks The Cricket Indoor Location System Courtesy of: Hari Balakrishnan Bodhi Priyantha, Allen Miu, Jorge Nogueras, John Ankcorn, Kalpak Kothari, Steve Garland, Seth Teller MIT Wireless Sensor Networks Laboratory for Computer Science http://nms.lcs.mit.edu/

5. Wireless Sensor Networks Motivation Location-awareness will be a key feature of many future mobile applications Many scenarios in pervasive computing Active maps Resource discovery and interaction Way-finding & navigation Stream redirectors Cricket focuses mainly on indoor deployment and applications

6. Wireless Sensor Networks Where am I?(Active map)

7. Wireless Sensor Networks What�s near me? Find this for me(Resource discovery)

8. Wireless Sensor Networks What�s in this direction?(Viewfinder)

9. Wireless Sensor Networks How do I get to Paja�s office?How do I get to Compaq�s booth at Comdex?

10. Wireless Sensor Networks Desired Functionality What space am I in? Room 317, reception area, Compaq�s booth,� How do I learn more about what�s in this space? An application-dependent notion What are my (x,y,z) coordinates? �Cricket GPS� Which way am I pointing? �Cricket compass�

11. Wireless Sensor Networks Design Goals for Cricket Must determine: Spaces: Good boundary detection is important Position: With respect to arbitrary inertial frame Orientation: Relative to fixed-point in frame Must operate well indoors Preserve user privacy: don�t track users Must be easy to deploy and administer Must facilitate innovation in applications Low energy consumption

12. Wireless Sensor Networks System Components Location inference modules Hardware, software, algorithms for space, position coordinates, orientation Programming (using) Cricket API; language-independent �RPC� Customized beaconing Deploying and managing a Cricket deployment Configuration, security, data management

13. Wireless Sensor Networks Cricket Architecture No central beacon control or location database Passive listeners + active beacons preserves privacy Straightforward deployment and programmability

14. Wireless Sensor Networks Machinery Obtain linear distance estimates Pick nearest to infer �space� Solve for mobile�s (x, y, z) Determine ? w.r.t. each beacon and deduce orientation vector

15. Wireless Sensor Networks Determining Distance A beacon transmits an RF and an ultrasonic signal simultaneously RF carries location data, ultrasound is a narrow pulse The listener measures the time gap between the receipt of RF and ultrasonic signals A time gap of x ms roughly corresponds to a distance of x feet from beacon Velocity of ultra sound << velocity of RF

16. Wireless Sensor Networks Multiple Beacons CauseComplications Beacon transmissions are uncoordinated Ultrasonic signals reflect heavily Ultrasonic signals are pulses (no data) These make the correlation problem hard and can lead to incorrect distance estimates

17. Wireless Sensor Networks Solution Carrier-sense + randomized transmission Reduce chances of concurrent beaconing Bounding stray signal interference Envelop all ultrasonic signals with RF Listener inference algorithm Processing distance samples to estimate location

18. Wireless Sensor Networks Bounding Stray Signal Interference Engineer RF range to be larger than ultrasonic range Ensures that if listener can hear ultrasound, corresponding RF will also be heard

19. Wireless Sensor Networks Bounding Stray Signal Interference

20. Wireless Sensor Networks Bounding stray signal interference Envelop ultrasound by RF Interfering ultrasound causes RF signals to collide Listener does a block parity error check The reading is discarded...

21. Wireless Sensor Networks Preventing repeated interactions Randomize beacon transmissions: loop: pick r ~ Uniform[T1, T2]; delay(r); xmit_beacon(RF,US); Optimal choice of T1 and T2 can be calculated analytically Trade-off between latency and collision probability Erroneous do Wireless Sensor Networks estimates not repeat

22. Wireless Sensor Networks Estimation AlgorithmWindowed MinMode

23. Wireless Sensor Networks Orientation

24. Wireless Sensor Networks Trigonometry 101

25. Wireless Sensor Networks Differential Distance Estimation Problem: for reasonable values of parameters (d, z), (d2 - d1) must have 5mm accuracy

26. Wireless Sensor Networks Making This Idea Work

27. Wireless Sensor Networks Coordinate Estimation

28. Wireless Sensor Networks Deployment: Beacon PlacementConsiderations Placement should allow correct inference of space Boundaries between spaces need to be detected Placement should provide enough information for coordinate estimation No 4 beacons on same circle on a ceiling At least one beacon must have ? < 40 degrees

29. Wireless Sensor Networks Problem: Closest Beacon May NotReflect Correct Space

30. Wireless Sensor Networks Correct Beacon Placement Position beacons to detect the boundary Multiple beacons per space are possible

31. Wireless Sensor Networks System Administration Password-based authentication for configuration Currently, coordinates manually entered Working on algorithm to deduce this from other beacons MOREINFO database centrally managed with Web front-end Relational DBMS Challenge: queries that don�t divulge device location, but yet are powerful

32. Wireless Sensor Networks Cricket v1 Prototype

33. Wireless Sensor Networks Deployment

34. Wireless Sensor Networks Some Results Linear distances to within 6cm precision Spatial resolution of about 30cm Coordinate estimation to within 6cm in each dimension Orientation to within 3-5 degrees when angle to some beacon < 45 degrees Several applications (built, or being built) Stream redirection, active maps, Viewfinder, Wayfinder, people locater, smart meeting notifier,� Probably no single killer app, but a whole suite of apps that might change the way we do things

35. Wireless Sensor Networks Alternative Architecture(Active Badge, Bat Systems) Problems: Privacy; administration; scalability; deployment cost

36. Wireless Sensor Networks Comparisons

37. Wireless Sensor Networks Cricket - Summary Cricket provides location information for mobile, pervasive computing applications Space Position Orientation Flexible and programmable infrastructure Deployment and management facilities

38. Wireless Sensor Networks Summary of Lecture Several localization implementations They greatly vary with respect to: Accuracy Ease of computation/implementation Coordination Application specific What�s your favorite?