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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/ Motivation Emergence of pervasive computing environments Context-aware applications Location-dependent behavior

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the cricket location support system

The Cricket Location-Support System

Nissanka B. Priyantha Anit Chakraborty

Hari Balakrishnan

MIT Lab for Computer Science

http://nms.lcs.mit.edu/

motivation
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

design goals
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
traditional approach

Controller/

Location database

ID= u ?

Base stations

ID = u ?

ID = u ?

ID = u ?

ID = u

Transceivers

Traditional Approach

Centralized architecture

User-privacy issues

High deployment cost

cricket architecture

Beacon

Space

C

Space

A

Space

B

I am at

C

Listener

Cricket Architecture

Decentralized, no tracking, low cost

Think of it as an “inverted BAT”!

determining distance

RF data

(locationname)

Determining Distance

Beacon

  • A beacon transmits an RF and an ultrasonic signal simultaneously
    • RF carries location data, ultrasound is a narrow pulse
    • Velocity of ultra sound << velocity of RF

Ultrasound

(pulse)

Listener

  • 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
uncoordinated beacons
Uncoordinated Beacons

Beacon A

Beacon B

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

Incorrect distance

RF B

RF A

US B

time

US A

handling spurious interactions
Handling Spurious Interactions
  • Combination of three different techniques:
    • Bounding stray signal interference
    • Preventing repeated interactions via randomization
    • Listener inference algorithms
bounding stray signal interference

RF A

US A

t

Bounding Stray Signal Interference
  • RF range > ultrasonic range
    • Ensures an accompanied RF signal with ultrasound
bounding stray signal interference10

S/b

t

r/v (max)

S r

b v

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)

bounding stray signal interference11
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
preventing repeated interactions
Preventing Repeated Interactions
  • Randomize beacon transmissions:

loop:

pick r ~ Uniform[T1, T2];

delay(r);

xmit_beacon(RF,US);

  • Erroneous estimates do not repeat
  • Optimal choice of T1 and T2can be calculated analytically
    • Trade-off between latency and collision probability
inference algorithms
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
inference algorithms14
Inference Algorithms

A

Frequency

B

5

Distance

(feet)

5

10

correct beacon positioning
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

implementation
Implementation
  • Cricket beacon and listener

LocationManager provides an API to applications

Integrated with intentional naming system for resource discovery

implementation18
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

static listener performance

Interference

L2

L1

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 A

Room B

% readings due to interference of RF from I1 and I2 with ultrasound from beacons

I1

I2

Room C

mobile listener performance
Mobile listener performance

Room A

Room B

Room C

comparisons
Comparisons

System

Attribute

summary
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!
slide28
Preserves user privacy
  • Good granularity
  • Component cost U.S. $10
beacon positioning
Beacon positioning

Location X

Imaginary

Boundary

X1

X2

X3

Imaginary boundaries

Multiple beacons per location

future work
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.
inference algorithms31
Inference algorithms
  • Compared three algorithms
    • Minimum mode
    • Minimum arithmetic mean
    • Majority
minimizing errors
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 < 49 ft
    • Signal separation < 49 ms
minimizing errors33
Minimizing errors.
  • Interfering ultrasound causes RF signals to collide
  • Listener does a block parity error check
    • The reading is discarded