The Cricket Indoor Location System

1
The Cricket Indoor Location System

• Hari Balakrishnan
• Bodhi Priyantha, Allen Miu,
• Jorge Nogueras, John Ankcorn, Kalpak Kothari,
• Steve Garland, Seth Teller
• MIT Laboratory for Computer Science
• http//nms.lcs.mit.edu/

2
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

3
Where am I?(Active map)
4
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
5
Whats in this direction?(Viewfinder)
Point-and-use UIs
6
How do I get to Satyas office?How do I get to
Compaqs booth at Comdex?
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Desired Functionality

• What space am I in?
• Room 510, 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

8
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

9
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

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Cricket Architecture
Beacon
Estimate distances to infer location
Listener
No central beacon control or location
database Passive listeners active beacons
preserves privacy Straightforward deployment and
programmability
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Machinery
B
Beacons on ceiling
ltSPACEgt NE43-510 ltIDgt34lt/IDgt lt/SPACEgt ltCOORDgt1
46 272 0lt/COORDgt ltMOREINFOgt
http//cricket.lcs.mit.edu/ lt/MOREINFOgt
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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MOREINFO Database
ltSPACEgt NE43-510 ltIDgt34lt/IDgt lt/SPACEgt ltCOORDgt1
46 272 0lt/COORDgt ltMOREINFOgt
http//cricinfo.lcs.mit.edu/ lt/MOREINFOgt
Centralized DB key to simple administration
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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 ultra sound 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
5
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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Problem Closest Beacon May Not Reflect Correct
Space
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 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

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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, smart meeting
  notifier,
• Probably no single killer app, but a whole suite
  of apps that might change the way we do things

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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
Active Badge Bat RADAR Cricket
Tracking? Yes Yes Depends No
Deployment Central controller wired IR sensors Central controller wired RF /USsensors RF signal map great radios Beacon placement wireless
Spatial resolution Room ? (linear few cm) Room 30cm (linear 5cm)
Orientation No No No Yes 3-5 degree prec.
Scalability All devices transmit periodically All devices transmit periodically All devices must use same RF net Devices passive distributed scheduling
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Summary

• Cricket provides location information for mobile,
  pervasive computing applications
• Space
• Position
• Orientation
• Flexible and programmable infrastructure
• Deployment and management facilities
• Starting to be used by other research groups
• http//nms.lcs.mit.edu/