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]
{gaetano@cs.washington.edu}


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