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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/ Motivation

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the cricket indoor location system

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/

motivation
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
what s near me find this for me resource discovery
What’s 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”

desired functionality
Desired Functionality
  • What space am I in?
    • Room 510, reception area, Compaq’s booth,…
    • How do I learn more about what’s in this space?
    • An application-dependent notion
  • What are my (x,y,z) coordinates?
    • “Cricket GPS”
  • Which way am I pointing?
    • “Cricket compass”
design goals for cricket
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: don’t track users
  • Must be easy to deploy and administer
  • Must facilitate innovation in applications
  • Low energy consumption
system components
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
cricket architecture

info = “a2”

info = “a1”

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

slide11

Machinery

B

Beacons on

ceiling

<SPACE> NE43-510

<ID>34</ID>

</SPACE>

<COORD>146 272 0</COORD>

<MOREINFO>

http://cricket.lcs.mit.edu/

</MOREINFO>

Cricket

listener

Mobile device

Mobile device

Obtain linear distance estimates

Pick nearest to infer “space”

Solve for mobile’s (x, y, z)

Determine  w.r.t. each beacon and deduce

orientation vector

moreinfo database

Space ID INSName Aura MOTD etc.

(Oxygen)

Server

DB Query

App

MOREINFO Database

<SPACE> NE43-510

<ID>34</ID>

</SPACE>

<COORD>146 272 0</COORD>

<MOREINFO>

http://cricinfo.lcs.mit.edu/

</MOREINFO>

Centralized DB key to simple administration

determining distance

RF data

(spacename)

Determining Distance

Beacon

  • A beacon transmits an RF and an ultrasonic signal simultaneously
    • RF carries location data, ultrasound is a narrow pulse

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
    • Velocity of ultra sound << velocity of RF
multiple beacons cause complications
Multiple Beacons Cause Complications

Beacon A

Beacon B

  • 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

Incorrect distance

Listener

t

RF B

RF A

US B

US A

solution
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
bounding stray signal interference

RF A

US A

t

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
bounding stray signal interference17

S/b

t

r/v (max)

S r

b v

Bounding Stray Signal Interference
  • No “naked” ultrasonic signal can be valid!

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)

estimation algorithm windowed minmode

A

B

Actual distance (feet)

6

8

Mode (feet)

6

8

Mean (feet)

7.2

6.4

Majority

9

10

Estimation AlgorithmWindowed MinMode

A

Frequency

B

5

Distance

(feet)

5

10

slide19

Orientation

B

Beacons on

ceiling

Orientation relative to B

on horizontal plane

Cricket listener with

compass hardware

Mobile device

(parallel to horizontal plane)

trigonometry 101

d1

d2

z

Cricket

Compass

Trigonometry 101

Beacon

Idea: Use multiple ultrasonic sensors

and estimate differential distances

sin  = (d2 - d1) / sqrt (1 - z2/d2)

where d = (d1+d2)/2

Two terms need to be estimated:

1. d2 – d1

2. z/d (by estimating

coordinates)

Heading

differential distance estimation

Beacon

d1

d2

L

t

f = 2p (d2 – d1)/l

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!

making this idea work
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

slide23

vt1

vt2

vt3

vt4

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!)

deployment beacon placement considerations
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  < 40 degrees
    • sin  = (d2 - d1) / sqrt (1 - z2/d2), so Dq goes as tan q
correct beacon placement
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
system administration
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 don’t divulge device location, but yet are powerful
slide28

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…

some results
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 < 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
summary
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/