Nissanka B. PriyanthaAnit Chakraborty

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The Cricket Location-Support System

• Nissanka B. Priyantha Anit Chakraborty
• Hari Balakrishnan
• MIT Lab for Computer Science
• http//nms.lcs.mit.edu/

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Motivation

• Emergence of pervasive computing environments
• Context-aware applications
• Location-dependent behavior
• User and service mobility
• Navigation via active maps
• Resource discovery

Cricket provides applications information about
geographic spaces they are in
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Design Goals

• Preserve user privacy
• Operate inside buildings
• Recognize spaces, not just physical position
• Good boundary detection is important
• Easy to administer and deploy
• Decentralized architecture and control
• Low cost and power consumption

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Traditional Approach

• Centralized architecture
• User-privacy issues
• High deployment cost

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Cricket Architecture

• Decentralized, no tracking, low cost
• Think of it as an inverted BAT!

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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
• Velocity of ultra sound ltlt velocity of RF

• 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

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Uncoordinated Beacons
Beacon A
Beacon B
Incorrect distance
time
RF B
RF A
US B
US A

• Multiple beacon transmissions are uncoordinated
• Different beacon transmissions can interfere
• Causing inaccurate distance measurements at the
  listener

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Handling Spurious Interactions

• Combination of three different techniques
• Bounding stray signal interference
• Preventing repeated interactions via
  randomization
• Listener inference algorithms

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Bounding Stray Signal Interference

• RF range gt ultrasonic range
• Ensures an accompanied RF signal with ultrasound

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Bounding Stray Signal Interference

S - size of space string b - RF bit rate r -
ultrasound range v - velocity of ultrasound
(RF transmission time) (Max. RF US
separation
at the listener)
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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)
• Erroneous estimates do not repeat
• Optimal choice of T1 and T2 can be calculated
  analytically
• Trade-off between latency and collision
  probability

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Inference Algorithms

• MinMode
• Determine mode for each beacon
• Select the one with the minimum mode
• MinMean
• Calculate the mean distance for each beacon
• Select the one with the minimum value
• Majority (actually, plurality)
• Select the beacon with most number of readings
• Roughly corresponds to strongest radio signal

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Inference Algorithms
A
Frequency
B
5
Distance (feet)
5
10
A B
Actual distance (feet) 6 8
Mode (feet) 6 8
Mean (feet) 6.14 6.4
Number of samples 7 10
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Closest Beacon May Not Reflect Correct Space
Room A
Room B
I am at B
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Correct Beacon Positioning
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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Implementation

• Cricket beacon and listener

• LocationManager provides an API to applications
• Integrated with intentional naming system for
  resource discovery

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Implementation

• Cricket beacon and listener

RF
RF
Micro- controller
Micro- controller
RS232
US
US

• LocationManager provides an API to applications
• Integrated with intentional naming system for
  resource discovery

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Static listener performance

• Immunity to interference
• Four beacons within each others range
• Two RF interference sources
• Boundary detection ability
• L1 only two feet away from boundary

Room B
Room A
readings due to interference of RF from I1
and I2 with ultrasound from beacons
I1
I1 I2
L1 0.0 0.0
L2 0.3 0.4
I2
Room C
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Inference Algorithm Error Rates
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Mobile listener performance
Room A
Room B
Room C
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Comparisons
Bat Activebadge RADAR Cricket
Track user location? Yes Yes No, if client has signal map No
Deploymentconsiderations Centralized controller matrix ofsensors Centralized database wired IR sensors RF signal mapping and good radios Spacenamingconvention
Position accuracy Few cm Room-wide Room-wide 2 feet forspatialresolution
System
Attribute
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Summary

• Cricket provides information about geographic
  spaces to applications
• Location-support, not tracking
• Decentralized operation and administration
• Passive listeners and no explicit beacon
  coordination
• Requires distributed algorithms for beacon
  transmission and listener inference
• Implemented and works!

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• Decentralized

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• Preserves user privacy
• Good granularity
• Component cost U.S. 10

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Beacon positioning
Location X
Imaginary Boundary
X1
X2
X3

• Imaginary boundaries
• Multiple beacons per location

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Future work

• Dynamic transmission rate with carrier-sense for
  collision avoidance.
• Dynamic ultrasonic sensitivity.
• Improved location accuracy.
• Integration with other technologies such as Blue
  Tooth.

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Related work

• Bat
• Pinpoint
• Active badge
• Radar

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Inference algorithms

• Compared three algorithms
• Minimum mode
• Minimum arithmetic mean
• Majority

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Minimizing errors.

• Proper ultrasonic range ensures overlapping RF
  and ultrasonic signals
• RF data 7 bytes at 1 kb/s bit rate
• RF signal duration 49 ms
• Selected ultrasonic range 30ft lt 49 ft
• Signal separation lt 49 ms

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Minimizing errors.

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

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