Location in Pervasive Computing

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Location in Pervasive Computing
Shwetak N. PatelUniversity of Washington More
info shwetak.com Special thanks to Alex
Varshavsky and Gaetano Borriello for their
contribution to this content

university of washington
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Location

• A form of contextual information
• Persons physical position
• Location of a device
• Device is a proxy of a persons location
• Used to help derive activity information

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Location

• Well studied topic (3,000 PhD theses??)
• Application dependent
• Research areas
• Technology
• Algorithms and data analysis
• Visualization
• Evaluation

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Location Tracking
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Representing Location Information

• Absolute
• Geographic coordinates (Lat 33.98333, Long
  -86.22444)
• Relative
• 1 block north of the main building
• Symbolic
• High-level description
• Home, bedroom, work

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No one size fits all!

• Accurate
• Low-cost
• Easy-to-deploy
• Ubiquitous
• Application needs determine technology

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Consider for example

• Motion capture
• Car navigation system
• Finding a lost object
• Weather information
• Printing a document

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Others aspects of location information

• Indoor vs. outdoor
• Absolute vs. relative
• Representation of uncertainty
• Privacy model

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Lots of technologies!
Ultrasound
Floor pressure
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Some outdoor applications
E-911
Bus view
Car Navigation
Child tracking
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Some indoor applications
Elder care
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Outline

• Defining location
• Methods for determining location
• Ex. Triangulation, trilateration, etc.
• Systems
• Challenges and Design Decisions
• Considerations

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Approaches for determining location

• Localization algorithms
• Proximity
• Lateration
• Hyperbolic Lateration
• Angulation
• Fingerprinting
• Distance estimates
• Time of Flight
• Signal Strength Attenuation

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Proximity

• Simplest positioning technique
• Closeness to a reference point
• Based on loudness, physical contact, etc

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Lateration

• Measure distance between device and reference
  points
• 3 reference points needed for 2D and 4 for 3D

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Hyperbolic Lateration

• Time difference of arrival (TDOA)
• Signal restricted to a hyperbola

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Angulation

• Angle of the signals
• Directional antennas are usually needed

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Determining Distance

• Time of flight
• Speed of light or sound
• Signal strength
• Known drop off characteristics 1/r2-1/r6
• Problems Multipath

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Fingerprinting

• Mapping solution
• Address problems with multipath
• Better than modeling complex RF propagation
  pattern

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Fingerprinting
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Fingerprinting

• Easier than modeling
• Requires a dense site survey
• Usually better for symbolic localization
• Spatial differentiability
• Temporal stability

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Reporting Error

• Precision vs. Accuracy

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Reporting Error

• Cumulative distribution function (CDF)
• Absolute location tracking systems
• Accuracy value and/or confusion matrix
• Symbolic systems

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Location Systems

• Distinguished by their underlying signaling
  system
• IR, RF, Ultrasonic, Vision, Audio, etc

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GPS

• Use 24 satellites
• TDOA
• Hyperbolic lateration
• Civilian GPS
• L1 (1575 MHZ)
• 10 meter acc.

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Active Badge

• IR-based
• Proximity

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Active Bat

• Ultrasonic
• Time of flight of ultrasonic pings
• 3cm resolution

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Cricket

• Similar to Active Bat
• Decentralized compared to Active Bat

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Cricket vs Active Bat

• Privacy preserving
• Scaling
• Client costs

Active Bat Cricket
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Ubisense

• Ultra-wideband (UWB) 6-8 GHz
• Time difference of arrival (TDOA) and Angle of
  arrival (AOA)
• 15-30 cm

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RADAR

• WiFi-based localization
• Reduce need for new infrastructure
• Fingerprinting

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Place Lab

• Beacons in the wild
• WiFi, Bluetooth, GSM, etc
• Community authored databases
• API for a variety of platforms
• RightSPOT (MSR) FM towers

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ROSUM

• Digital TV signals
• Much stronger signals, well-placed cell towers,
  coverage over large range
• Requires TV signal receiver in each device
• Trilateration, 10-20m (worse where there are
  fewer transmitters)

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Comparing Approaches

• Many types of solutions (both research and
  commercial)
• Install custom beacons in the environment
• Ultra-wideband (Ubisense), Ultrasonic (MIT
  Cricket, Active Bat), Bluetooth
• Use existing infrastructure
• GSM (Intel, Toronto), WiFi (RADAR, Ekahau, Place
  Lab), FM (MSR)

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Limitations

• Beacon-based solutions
• Requires the deployment of many devices
  (typically at least one per room)
• Maintenance
• Using existing infrastructure
• WiFi and GSM
• Not always dense near some residential areas
• Little control over infrastructure (especially
  GSM)

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• Beacon-based localization

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• Wifi localization (ex. Ekahau)

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Tower IDs and signals change over time!

• GSM localization

Coverage?
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PowerLine Positioning

• Indoor localization using standard household
  power lines

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Signal Detection

• A tag detects these signals radiating from the
  electrical wiring at a given location

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Signal Map

1st Floor 2nd
Floor
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Example
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Passive location tracking

• No need to carry a tag or device
• Hard to determine the identity of the person
• Requires more infrastructure (potentially)

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Active Floor

• Instrument floor with load sensors
• Footsteps and gait detection

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Motion Detectors

• Low-cost
• Low-resolution

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Computer Vision

• Leverage existing infrastructure
• Requires significant communication and
  computational resources
• CCTV

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Other systems?

• Inertial sensing
• HVACs
• Ambient RF
• etc.

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Considerations

• Location type
• Resolution/Accuracy
• Infrastructure requirements
• Data storage (local or central)
• System type (active, passive)
• Signaling system