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WALRUS: Wireless Acoustic Location with Room-Level Resolution Using Ultrasound

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WALRUS: Wireless Acoustic Location with Room-Level Resolution Using Ultrasound WALRUS: Wireless Acoustic Location with Room-Level Resolution using Ultrasound Gaetano Borriello1,2, Alan Liu1, Tony Offer1, Christopher Palistrant1, Richard Sharp3 1Department of Computer Science & Engineering, University of Washington, Seattle, WA [USA] 2Intel Research Seattle, Seattle, WA [USA] 3Intel Research Cambridge, Cambridge [UK] {[email protected]} Abstract browser can be set up to automatically render web pages associated with the user’s current location. In this paper, we propose a system that uses the The Global Positioning System (GPS) is by far the most wireless networking and microphone interfaces of prevalent example of a location system. It uses signals mobile devices to determine location to room-level from synchronized orbiting satellites to calculate a accuracy. The wireless network provides a three-dimensional position relative to Earth’s synchronizing pulse along with information about the coordinate system. There are two issues with GPS that room. This is accompanied by an ultrasound beacon limit its utility in ubiquitous and mobile computing that allows us to resolve locations to the confines of a scenarios. First, it requires line-of-sight to at least three physical room (since audio is mostly bounded by walls). satellites for 2-D location resolution (four for 3-D). We generate the wireless data and ultrasound pulses Unfortunately, it is difficult to obtain line-of-sight in from the existing PCs in each room; a PDA carried by most environments where users spend most of their a user listens for both signals. Thus, our approach does time (i.e., indoors) and in places where most users live not require special hardware. We do not use ultrasound and work (i.e., urban centers). Second, and more to send data. As a result we dramatically reduce the importantly, a 3-D coordinate does not help a user computational burden on the mobile device while also locate what they need as that coordinate must be decreasing the latency of location resolution. Our translated to a form that is understandable to a person. results indicate that (i) ultrasound detection is robust For example, knowing that someone is 100 meters even in noisy environments with many reflective above sea level at 47°N and 122°W is much less useful surfaces; and (ii) that we can determine the correct than knowing they are in room 572 of the Allen Center room within a couple of seconds with high probability on the campus of the University of Washington. even when the ultrasound emitting PCs are not Clearly, the information in the latter is much more synchronized. useful in finding people and services. 1. INTRODUCTION Many systems have been designed to provide mobile devices with the capability to monitor their location Future mobile devices will need the ability to determine indoors (some of these will be discussed in detail in the their location and, thus, enable location-enhanced next section and a more complete bibliography can be computing. Location is a major part of a user’s context found at: http://binary.engin.brown.edu/publication/ and applications can be constructed that adapt to the Positioning_Ref.pdf). Designers of these location user’s current location. For example, a calendar systems need to make several key tradeoffs that affect reminder system can adapt by adjusting the time of an the system’s usability [5, 7], among these are: alarm based on traffic conditions or public x Affordability. A location system should be a transportation options between the user’s current minute fraction of the total cost of a mobile device. location and their next destination. Applications can be Cost includes not only the final monetary cost to designed that record the current location so as to better individual users, but also the cost associated with classify data for future retrieval. For example, a digital installation, management, and maintenance of the camera can record the location at which each picture infrastructure portion of the system. was taken. Location can also be used to modify the x behavior of existing applications. For example, a web Resource Requirements. Mobile devices have limited memory, computational capabilities, and USENIX Association MobiSys ’05: The Third International Conference on Mobile Systems, Applications, and Services 191 power; the need to accommodate expensive unique criteria in the available design space. This paper computations not only adds extra cost to the system describes, WALRUS, a location system that but also makes it less usable if it shortens battery emphasizes: low cost, high privacy, high portability, life. room-level precision, and high accuracy. x Privacy. A system that requires a user or mobile A key design feature of WALRUS is that it leaverages device to query a server or host for a location will existing hardware. The WALRUS client can run on any need to reveal the user’s identity in exchange for device that can receive WiFi packets and listen to this information. This may be considered an ultrasound (at approximately 21KHz). These undesirable feature for users who wish to remain capabilities are found in most modern laptops, tablets, anonymous. Moreover, the infrastructure-based and PDAs that usually include integrated WiFi and supplier of this information may charge the user for microphones/speakers. Furthermore, they are likely to this service thereby limiting the number of make their way into even more devices, such as cell applications and/or their frequency of location phones and wrist-watches, in the near future with the updates. With careful design, a system can be advent of low-power radio protocols such as 802.15.4 devised where a user receives information that (Zigbee) and ultra-wide-band (UWB). helps determine their location without potentially Section 2 of this paper discusses several other location revealing confidential information or even their systems that share similarities with the WALRUS presence. system, but as will be seen later, exhibit important x Portability. Mobile systems are an evolving differences as well. Section 3 describes the technology and some consideration should be implementation of WALRUS. Section 4 details the given to ensure that a system can be easily results of our experiments and evaluates how well the maintained during upgrades and across most system worked. Finally, Section 5 outlines future work platforms. To ensure that a system is readily that can be done to improve the WALRUS systems and adopted and maintained across several generations how it may evolve. of hardware, a location system should consider how and if the system will be able to adapt to 2. RELATED WORK future technologies. The problem of determining a device’s location has x Precision. Designers must decide what degree of been the topic of countless research endeavors, all of precision a location system will provide. Precision which have had to balance the various tradeoffs is defined as the granularity that a system is between affordability, privacy, portability, precision, capable of measuring. Many location systems have and accuracy. As in all engineering disciplines, been developed with precisions ranging from tradeoffs have to be made; in order to improve one centimeters to kilometers. For many ubiquitous aspect of a project, another aspect must be computing applications, room-level accuracy is an compromised to some degree. There are no location- important grain size as it closely relates to the sensing technologies that excel at everything. This places people often think about. Usually, higher section describes the strengths and weaknesses of levels of precision correlate strongly with increased several existing location-sensing technologies in terms cost of the location system. of the following attributes: cost, privacy, precision, and x Accuracy. Designers of location aware systems operational scope. We will compare the WALRUS consider accuracy to be the percentage of the time system to each of the technologies described in Table 1. a known level of precision is reached. For GPS uses time-of-flight calculations from orbiting example, a GPS receiver that has a precision level satellites to triangulate the position of mobile receivers of 15 meters might be accurate 95% of the time in near the surface of the Earth. GPS is similar to an open field; however, system accuracy will WALRUS in that there is no centralized system that diminish within an office building. Accuracy at the tracks the location of the mobile devices. However, room-level is very important as, for example, it is GPS operates at a much larger and much more not acceptable for a context-aware location system expensive scale than WALRUS. It costs billions of to inadvertently connect a user’s laptop to the data dollars to establish the infrastructure for GPS and the projector that is in the conference room next door. mobile receivers usually cost on the order of Based on these design factors, many different types of USD100.GPS can determine location with a precision systems can be developed that will meet a variety of of 1 to 5 meters [5]. User privacy is respected since 192 MobiSys ’05: The Third International Conference on Mobile Systems, Applications, and Services USENIX Association System Privacy Client cost Infrastructure cost Precision Operational Scope Client devices No additional WALRUS PC per room + WiFi room-level Indoor compute location hardware Client devices Approx. USD100 GPS Satellite network 1-5m Outdoor compute location receiver Client devices No additional WiFi, GSM, and/or Bluetooth Place Lab 15-30m Outdoor/Indoor compute location hardware beacons Client devices No additional Radar WiFi coverage 5m Indoor compute location hardware Central server ActiveBadge Low-cost badge Badge IR receivers room-level Indoor tracks clients Client devices IR transmitter on all objects of CoolTown IR transceiver 3-5m IR Indoor talk to appliances interest Central server Ultrasound receivers in ceiling Active Bat Low-cost bat 5-10cm Indoor tracks clients + RF link Client devices Low-cost client Cricket Ultrasound transmitters 1 m2 region Indoor compute location ultrasound rcvr Central server No additional E911/E112 Cellular network 100m Outdoor/Indoor tracks clients hardware Table 1.
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