The Cricket Location-Support System N. Priyantha, A. Chakraborty, and H. Balakrishnan MIT Lab for Computer Science MOBICOM 2000 Presenter: Kideok Cho

Contents Introduction Design Goals Cricket System and Operation Some Problems and Solutions System Performance and Comparision Summary

Why Location Support? The emergence of ubiquitous or pervasive computing Technology trends are – Context-aware, Location-dependent Global Positioning System (GPS) – Outdoor We need indoor location support system

What is Cricket? A location-support system for in-building, mobile, location-dependent applications Cricket consists of – Beacons spread throughout the building – Listeners attached to mobile or static nodes to learn their physical location

Cricket Application The Floorplan is an active map navigation utility that uses Cricket and a map server to present a location-dependent “active” map to the user, highlighting her location on it as she moves. Floorplan

Design Goals Preserve user privacy – Tracking vs. Support Operate inside buildings Easy to administer and deploy – Decentralized architecture and control Low cost Recognize spaces, not just physical position – Room-sized granularity – Good boundary detection is important

How does it work? Cricket Architecture Beacon Listener Space A Space B Space C I am at C Decentralized, no tracking, low cost

Determining Distance RF data (location name) Beacon Listener Ultrasound (pulse) A beacon transmits an RF and an ultrasonic signal simultaneously – RF carries location data, ultrasound is a narrow pulse – Velocity of ultrasound << Velocity of RF

Determining the Location The listener measures the time gap between the receipt of RF and ultrasonic signals Decide which is the closest beacon using estimated distance RF data (location name) Beacon Listener Ultrasound (pulse)

Does it have any problem? Multiple beacon transmissions are uncoordinated Different beacon transmissions can interfere – Causing inaccurate distance measurement Beacon A Beacon B timeRF BRF AUS B US A Incorrect distance

Solution to this problem Combination of three different techniques: – Preventing repeated interactions via randomization – Using proper listener inference algorithm – Bounding stray signal interference

Bounding Stray Signal Interference t S/b r/v (max) S - size of space string b - RF bit rate r - ultrasound range v - velocity of ultrasound (RF transmission time) (Max. propagation time for an ultrasonic signal) S r b v Envelop ultrasound by RF Without interfering beacon transmission – Correct correlation

Beacon positioning I am at B Room ARoom B

Correct Beacon Positioning Room ARoom B xx I am at A - Position beacons to detect the boundary - Multiple beacons per space are possible

Static listener performance Interference L2 L1 I1I2 L10.0% L20.3%0.4% I1 I2 % readings due to interference of RF from I1 and I2 with ultrasound from beacons Room B Room C Room A

Comparisons Bat Active badge RADARCricket Track user location? Yes No, if client has signal map No Deployment considerations Centralized controller + matrix of sensors Centralized database + wired IR sensors RF signal mapping and good radios Space naming convention Position accuracyFew cmRoom-wide ~2 feet for spatial resolution Attribute System

Summary Cricket provides information about geographic spaces to applications – Location-support, not tracking: privacy concern – Decentralized operation and administration Passive listeners and no explicit beacon coordination – Requires distributed algorithms for beacon transmission and listener inference It ’ s a working system!!

Any Question?