The Cricket Indoor Location System Hari Balakrishnan Bodhi

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 • 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

Where am I? (Active map)

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”

What’s in this direction? (Viewfinder) Point-and-use UIs

How do I get to Satya’s office? How do I get to Compaq’s booth at Comdex?

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 • 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 • 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 = “a 2” Beacon info = “a 1” Listener Estimate distances to infer location No central beacon control or location database Passive listeners + active beacons preserves privacy Straightforward deployment and programmability

Machinery Beacons on ceiling <SPACE> NE 43 -510 <ID>34</ID> </SPACE> <COORD>146 272 0</COORD> <MOREINFO> http: //cricket. lcs. mit. edu/ Cricket </MOREINFO> listener 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 B

MOREINFO Database <SPACE> NE 43 -510 <ID>34</ID> </SPACE> <COORD>146 272 0</COORD> <MOREINFO> http: //cricinfo. lcs. mit. edu/ </MOREINFO> App Space ID INSName Aura MOTD etc. (Oxygen) Server DB Query Centralized DB key to simple administration

Determining Distance Beacon RF data (space name) Ultrasound (pulse) Listener • A beacon transmits an the and an ultrasonic The listener measures RF time gap between the receipt of RF and signal simultaneously ultrasonic signals A time gap of x ms data, ultrasound is a narrow – RF carries location roughly corresponds to a distance of x feet from beacon pulse – Velocity of ultra sound << velocity of RF

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

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 • 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 Interference S b r v = size of space advertisement = RF bit rate = ultrasound range = velocity of ultrasound S b (RF transmission time) S/b r/v (max) r v (Max. RF-US separation at the listener) • No “naked” ultrasonic signal can be valid! t

Estimation Algorithm Windowed Min. Mode A B Frequency 5 5 Distance (feet) 10 A B Actual distance (feet) 6 8 Mode (feet) 6 8 Mean (feet) 7. 2 6. 4 9 10 Majority

Orientation Beacons on ceiling Orientation relative to B on horizontal plane Cricket listener with compass hardware Mobile device (parallel to horizontal plane) B

Trigonometry 101 Beacon Idea: Use multiple ultrasonic sensors and estimate differential distances sin = (d 2 - d 1) / sqrt (1 - z 2/d 2) where d = (d 1+d 2)/2 d 1 Two terms need to be estimated: 1. d 2 – d 1 2. z/d (by estimating coordinates) H Cr Co icke mp t as s g din ea d 2 z

Differential Distance Estimation • Problem: for reasonable values of parameters (d, z), (d 2 - d 1) must have 5 mm accuracy – Well beyond all current technologies! Beacon d 2 d 1 L t f = 2 p (d 2 – d 1)/l Estimate phase difference between ultrasonic waveforms!

Making This Idea Work Beacon d 1 L 12 3 l/2 d 3 L 23 4 l/2 Estimate 2 phase differences to uniquely estimate d 2 -d 1 Can do this when L 12 and L 23 are relatively-prime multiples of l/2 t

Coordinate Estimation Beacons on ceiling at known coordinates vt 1 vt 2 vt 3 (x, y, z) vt 4 Four equations, four unknowns Velocity of sound varies with temperature, humidity Can be “eliminated” (or calculated!) B

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 = (d 2 - d 1) / sqrt (1 - z 2/d 2), so D goes as tan

Problem: Closest Beacon May Not Reflect Correct Space Room A Room B I am at B

Correct Beacon Placement Room A Room B x I am at A • Position beacons to detect the boundary • Multiple beacons per space are possible x

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

Cricket v 1 Prototype RF module (rcv) Ultrasonic sensor Listener RF module (xmit) RF antenna Beacon Atmel processor RS 232 i/f Host software libraries in Java; Linux daemon (in C) for Oxygen Back. Paq handhelds Several apps…

Deployment

Some Results • Linear distances to within 6 cm precision • Spatial resolution of about 30 cm • Coordinate estimation to within 6 cm 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 • 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/