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ESE680: Wireless Sensor Networks Special Topics in Embedded Systems Localization - II Lecture #12 - PowerPoint PPT Presentation

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ESE680: Wireless Sensor Networks Special Topics in Embedded Systems Localization - II Lecture #12 Prof. Rahul Mangharam Previous Lecture What is Localization? Taxonomy Applications Basic Approaches Coarse localization Fine-Grained Localization Ranging techniques Trilateration Outline

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ESE680: Wireless Sensor Networks

Special Topics in Embedded Systems

Localization - II

Lecture #12

Prof. Rahul Mangharam

Wireless Sensor Networks

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Previous Lecture

  • What is Localization?

    • Taxonomy

    • Applications Basic Approaches

  • Coarse localization

  • Fine-Grained Localization

  • Ranging techniques

  • Trilateration

Wireless Sensor Networks

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  • Interferometric localization (Vanderbilt)

  • Cricket (MIT)

  • Motetrack (Harvard)

Wireless Sensor Networks

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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

Wireless Sensor Networks

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  • 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

Wireless Sensor Networks

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Where am I?(Active map)

Wireless Sensor Networks

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What’s near me? Find this for me(Resource discovery)

Wireless Sensor Networks

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What’s in this direction?(Viewfinder)

Wireless Sensor Networks

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How do I get to Paja’s office?How do I get to Compaq’s booth at Comdex?

Wireless Sensor Networks

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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”

Wireless Sensor Networks

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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

Wireless Sensor Networks

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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

Wireless Sensor Networks

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Cricket Architecture

No central beacon control or location database

Passive listeners + active beacons preserves privacy

Straightforward deployment and programmability

Wireless Sensor Networks

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  • Obtain linear distance estimates

  • Pick nearest to infer “space”

  • Solve for mobile’s (x, y, z)

  • Determine θ w.r.t. each beacon and deduce orientation vector

Wireless Sensor Networks

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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

Wireless Sensor Networks

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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

Wireless Sensor Networks

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  • 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

Wireless Sensor Networks

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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

Wireless Sensor Networks

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Bounding Stray Signal Interference

No “naked” ultrasonic signal can be valid!

Wireless Sensor Networks

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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...

Wireless Sensor Networks

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Preventing repeated interactions

  • Randomize beacon transmissions:


    pick r ~ Uniform[T1, T2];



  • 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

Wireless Sensor Networks

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Estimation AlgorithmWindowed MinMode

Wireless Sensor Networks

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Wireless Sensor Networks

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Trigonometry 101

Wireless Sensor Networks

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Differential Distance Estimation

  • Problem: for reasonable values of parameters (d, z), (d2 - d1) must have 5mm accuracy

Wireless Sensor Networks

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Making This Idea Work

Wireless Sensor Networks

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Coordinate Estimation

Wireless Sensor Networks

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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 θ < 40 degrees

Wireless Sensor Networks

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Problem: Closest Beacon May NotReflect Correct Space

Wireless Sensor Networks

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Correct Beacon Placement

  • Position beacons to detect the boundary

  • Multiple beacons per space are possible

Wireless Sensor Networks

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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

Wireless Sensor Networks

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Cricket v1 Prototype

Wireless Sensor Networks

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Wireless Sensor Networks

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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

Wireless Sensor Networks

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Alternative Architecture(Active Badge, Bat Systems)


Privacy; administration; scalability; deployment cost

Wireless Sensor Networks

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Wireless Sensor Networks

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Cricket - Summary

  • Cricket provides location information for mobile, pervasive computing applications

    • Space

    • Position

    • Orientation

  • Flexible and programmable infrastructure

  • Deployment and management facilities

Wireless Sensor Networks

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Summary of Lecture

  • Several localization implementations

  • They greatly vary with respect to:

    • Accuracy

    • Ease of computation/implementation

    • Coordination

  • Application specific

  • What’s your favorite?

Wireless Sensor Networks