Pervasive LocationAware Computing

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Pervasive Location-Aware Computing

• Hari Balakrishnan
• Networks and Mobile Systems Group
• MIT Laboratory for Computer Science
• http//nms.lcs.mit.edu/

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Why you should care

• Location-awareness will be a key feature of many
  future mobile applications
• Many scenarios in pervasive computing
• Navigation
• Resource discovery
• Embedded applications, sensor systems
• Monitoring and control applications
• The design of good location-aware computing
  systems cuts across many areas of CS/EE
• E.g., sensors, signal processing, networking,
  mobility, data management, graphics/visualization,
  planning, HCI,
• Most of the exciting stuff will happen in the
  next few years!

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Computing
Input
Output
Processing
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Networked Computing
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Networked, Context-Aware Computing
Environmental Context
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Location-Aware Applications

• Human-centric
• Finding applications
• Embedded
• Sensors actuators
• Devices
• Monitoring and control
• System should support both forms

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This Talk

• Cricket location infrastructure
• Some applications
• System architecture
• Challenges for the future

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Cricket

• Architecture for ubiquitous location-sensing
• No single location-sensing technology works
  everywhere today, particularly indoors
• Integrates variety of sensory information
• GPS wide-open outdoors
• Wireless access info coarse-grained info
• RF ultrasonic trilateration indoors and in
  urban areas
• Sensor-independent location API

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Desired Functionality

• What space am I in?
• Room 510, reception area, seminar room,
• 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
• Goal Linear precision of a few centimeters,
  angular precision of a few degrees

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Design Goals

• 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

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Cricket Architecture
Beacon
Estimate distances to infer location
Listener
Autonomous No central beacon control or
tracking Passive listeners active beacons
facilitates privacy Straightforward deployment
and programmability
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Machinery
B
Beacons on ceiling
SPACENE43-510 ID34 COORD146 272
0 MOREINFO http//cricket.lcs.mit.edu/
Cricket listener
Mobile device
Mobile device
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
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Determining Distance
Beacon
Ultrasound (pulse)
Listener

• 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 ultrasound ltlt velocity of RF

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Multiple Beacons Cause Complications
Beacon A
Beacon B
Incorrect distance
t
Listener
RF B
RF A
US B
US A

• 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

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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

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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

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Bounding Stray Signal Interference

S size of space advertisement b RF bit
rate r ultrasound range v velocity of
ultrasound
(RF transmission time) (Max. RF-US
separation
at the listener)

• No naked ultrasonic signal can be valid!

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Bounding stray signal interference

RF B
US B
RF A
US A
t

• Envelop ultrasound by RF
• Interfering ultrasound causes RF signals to
  collide
• Listener does a block parity error check
• The reading is discarded...

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Preventing repeated interactions

• Randomize beacon transmissions
• loop
• pick r UniformT1, 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 estimates do not repeat

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Estimation AlgorithmWindowed MinMode
A
Frequency
B
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Distance (feet)
5
10
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Orientation
B
Beacons on ceiling
Orientation relative to B on horizontal plane
?
Cricket listener with compass hardware
Mobile device (parallel to horizontal plane)
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Trigonometry 101
Beacon
Idea Use multiple ultrasonic sensors and
estimate differential distances
sin ? (d2 - d1) / sqrt (1 - z2/d2) where d
(d1d2)/2
Two terms need to be estimated 1. d2 d1 2.
z/d (by estimating coordinates)
Heading
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Differential Distance Estimation

• Problem for reasonable values of parameters (d,
  z), (d2 - d1) must have 5mm accuracy
• Well beyond all current technologies!

Estimate phase difference between ultrasonic
waveforms!
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Making This Idea Work
Beacon
d2
d3
d1
L23
L12
t
3l/2
4l/2
Estimate 2 phase differences to uniquely estimate
d2-d1 Can do this when L12 and L23 are
relatively-prime multiples of l/2
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Coordinate Estimation
B
Beacons on ceiling at known coordinates
?
(x,y,z)
Four equations, four unknowns Velocity of sound
varies with temperature, humidity Can be
eliminated (or calculated!)
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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 ? lt 40 degrees
• sin ? (d2 - d1) / sqrt (1 - z2/d2), so Dq
  goes as tan q

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Beacon Placement
Totally arbitrary beacon placement wont
demarcate spaces correctly
Room A
Room B
I am at B
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Correct Beacon Placement
Room A
Room B
x
x
I am at A

• Position beacons to detect the boundary
• Multiple beacons per space are possible

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System Configuration Administration

• Password-based authentication for configuration
• Currently, coordinates manually entered
• Auto-configuration algorithm being developed
• MOREINFO database centrally managed with Web
  front-end
• Relational DBMS
• Challenge queries that dont divulge device
  location, but yet are powerful

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Cricket v1 Prototype
RF module (rcv)
RF module (xmit)
Ultrasonic sensor
Ultrasonic sensor
RF antenna
Listener
Beacon
Atmel processor
RS232 i/f
Host software libraries in Java Linux daemon
(in C) for Oxygen BackPaq handhelds Several apps
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Deployment
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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 lt 45 degrees
• Several applications (built, or being built)
• Stream redirection, active maps, Viewfinder,
  Wayfinder, people-locater
• Scalable location-aware monitoring (SLAM) apps
  MIT library book tracking, asset management, MIT
  physical plant maintenance

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Where am I?(Active map)
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Whats near me? Find this for me(Resource
discovery)
Print map on a color printer, and system
sends data to nearest available free color
printer and tells you how to get there
Location by intent
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Whats in this direction?(Viewfinder)
Point-and-use UIs
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How do I get to Jorgs office?
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Large-Scale Monitoring
Scale ( sensors)
Power, thermal Monitoring control
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Asset tracking
Fire detection Assisted evacuation
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Library usage
Motion detection Leaks, floods
Lab equipment monitoring
Cricket network auto-configuration
Physical plant Repair orders
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HazMat response Local navigation
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Personal safety Traffic, parking
Irrigation
Days/Hours
Minutes
Seconds
Response time
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Requirements

• Ubiquitous location-sensing
• Heterogeneous sensor networking/comm. protocols
• Resource discovery
• Event handling
• Query processing
• Spatial databases
• Mapping and representation
• Navigation
• User interfaces

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Strawman Architecture
Fixed sensor proxy (sensor integration, pruning)
Mobile sensor proxy
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Alternative Architecture (Active Badge, Bat
Systems)
Location DB
ID u?
Networked sensor grid
ID u
Responder
Problems privacy administration scalability
deployment cost
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Comparisons
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Summary

• Location-aware computing poses numerous
  interesting challenges for CS
• An important component of pervasive computing
• Integrating real-world information
• App spectrum from HCI ? Embedded apps
• Cricket provides location information for mobile,
  pervasive computing applications
• Space, position, orientation
• Flexible and programmable infrastructure
• Deployment and management facilities

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Collaborators

• Bodhi Priyantha
• Allen Miu
• Ken Steele
• Rafael Nogueras
• Seth Teller
• Steve Garland
• Dorothy Curtis
• Omar Aftab
• Erik Demaine
• Mike Stonebraker
• http//nms.lcs.mit.edu/