WALRUS: Wireless Active Location Resolver with Ultrasound

Tony Offer, Christopher Palistrant

Basic Concept

Function Make mobile device aware of its location within building Location awareness can then be used to provide location-

dependent services

Usefulness Many applications for devices that know where they are Provision of services based on the context of locality

Target Users Businesses with large office buildings Maintenance workers

Related Work Cricket (MIT)

Similarities Uses a combination of radio waves and ultrasound to determine location No central management

Differences Uses specialized $10 beacons and receivers Determines 4x4 ft region within a room

Active Bat (AT&T) Similarities

Uses a combination of radio waves and ultrasound to determine location Differences

Uses a central management server to perform computations Uses specially made hardware for tags and sensors Determines location within a room to an accuracy of 9 cm

Related Work (continued)

Context-Aware Computing With Sound (Intel Research, Cambridge) Similarities

Uses standard computer speakers and microphones to generate and detect ultrasound

Differences Modulates ultrasonic sound waves to carry data Does not provide positioning capabilities

Demonstration

Two servers, two rooms, one client User will carry iPAQ between rooms and

watch message updates on screen Shows plausibility and basic functionality Original scenario demonstrated future

potential applications Scalability is easy with good design

Implementation

System Components Server-sided beacon software Mobile client software

Datagram signal detection Ultrasonic signal detection

Current Status Working model demonstrates basic functionality Testing overall reliability of system Increasing range of ultrasonic reception Tweaking various parameters

Server-sided beacon software

Mobile client signal detection

Ultrasonic signal detection

Evaluation

Performance Reliability and speed System quality depends on ultrasound attenuation

Metrics Percentages and distances

Ultrasound detection when present or not present Detection of correct room when 802.11 is from other rooms

Time Delays 1st ultrasound detection with Goertzel until actually deciding

on positive beacon signal Entering a room and seeing an update in the room location

Evaluation (continued) Data Collection

Reception varies on mobile platforms iPAQ h3870 receives signal 10’ - 15’ away Dell 8100 Inspirion with external mic.

receives signals closer to 15’-20’ Does this increase with foil ear?

Rough tests show 1.2 secs. between 802.11 reception and positive ultrasonic decision.

Conclusions Multiple avenues for performance

improvement Alternate DSP methods may give better

results Fixed point arithmetic may give better

speed

Future Work

Add a windowing function to Goertzel Large scale application experiment

How does WALRUS respond with many offices broadcasting their locations?

Modulate frequencies in different rooms Sacrifice accuracy in exchange for precision

Can we increase the accuracy but retain ultimate cost metrics?

Develop applications to maximize benefit Are users willing to sacrifice identity in exchange

for more information?