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Wireless Sensor Networks

Wireless Sensor Networks. Previous Lecture. What is Localization?TaxonomyApplications Basic ApproachesCoarse localizationFine-Grained LocalizationRanging techniquesTrilateration. Wireless Sensor Networks. Outline. Interferometric localization (Vanderbilt)Cricket (MIT)Motetrack (Harvard). Wi

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Wireless Sensor Networks

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    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 Whats near me? Find this for me (Resource discovery)

    8. Wireless Sensor Networks Whats in this direction? (Viewfinder)

    9. Wireless Sensor Networks How do I get to Pajas office? How do I get to Compaqs booth at Comdex?

    10. Wireless Sensor Networks Desired Functionality What space am I in? Room 317, reception area, Compaqs booth, How do I learn more about whats 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: dont 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 mobiles (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 Cause Complications 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 Algorithm Windowed 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 Placement Considerations 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 Not Reflect 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 dont 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 Whats your favorite?

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