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?