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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

previous lecture
Previous Lecture
  • What is Localization?
    • Taxonomy
    • Applications Basic Approaches
  • Coarse localization
  • Fine-Grained Localization
  • Ranging techniques
  • Trilateration

Wireless Sensor Networks

outline
Outline
  • Interferometric localization (Vanderbilt)
  • Cricket (MIT)
  • Motetrack (Harvard)

Wireless Sensor Networks

the cricket indoor location system
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/

Wireless Sensor Networks

motivation
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

Wireless Sensor Networks

where am i active map
Where am I?(Active map)

Wireless Sensor Networks

how do i get to paja s office how do i get to compaq s booth at comdex
How do I get to Paja’s office?How do I get to Compaq’s booth at Comdex?

Wireless Sensor Networks

desired functionality
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

design goals for cricket
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

system components
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

cricket architecture
Cricket Architecture

No central beacon control or location database

Passive listeners + active beacons preserves privacy

Straightforward deployment and programmability

Wireless Sensor Networks

machinery
Machinery
  • 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

determining distance
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

multiple beacons cause complications
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

solution
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

Wireless Sensor Networks

bounding stray signal interference
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

bounding stray signal interference19
Bounding Stray Signal Interference

No “naked” ultrasonic signal can be valid!

Wireless Sensor Networks

bounding stray signal interference20
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

preventing repeated interactions
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

Wireless Sensor Networks

estimation algorithm windowed minmode
Estimation AlgorithmWindowed MinMode

Wireless Sensor Networks

orientation
Orientation

Wireless Sensor Networks

trigonometry 101
Trigonometry 101

Wireless Sensor Networks

differential distance estimation
Differential Distance Estimation
  • Problem: for reasonable values of parameters (d, z), (d2 - d1) must have 5mm accuracy

Wireless Sensor Networks

making this idea work
Making This Idea Work

Wireless Sensor Networks

coordinate estimation
Coordinate Estimation

Wireless Sensor Networks

deployment beacon placement considerations
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

correct beacon placement
Correct Beacon Placement
  • Position beacons to detect the boundary
  • Multiple beacons per space are possible

Wireless Sensor Networks

system administration
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

cricket v1 prototype
Cricket v1 Prototype

Wireless Sensor Networks

deployment
Deployment

Wireless Sensor Networks

some results
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

alternative architecture active badge bat systems
Alternative Architecture(Active Badge, Bat Systems)

Problems:

Privacy; administration; scalability; deployment cost

Wireless Sensor Networks

comparisons
Comparisons

Wireless Sensor Networks

cricket summary
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

summary of lecture
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

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