Wireless Local Area Network Positioning - Ambient Intelligence for Scientific Discovery - page 248

and employment of a variety of methodologies and algorithms, location tracking
can be performed to varying degrees of precision. Context-aware applications
rely on swift and accurate location information to operate effectively and de-
velopers must put extensive emphasis on keeping location data private [6, 4, 22].
AWLANpositioningsystem,ifproperlyconstructed, creates a data layer that
developers can interface with without the need for network-specific knowledge.
This interface provides developers with the data they need to produce context-
aware applications and provides users with security. This chapter will survey
various models and implementations for generating location data and discuss
related topics such as privacy, visualization and user movement and mobility.

2 Algorithms

2.1 Radio Waves
A basic understanding of the characteristics of a radio wave is quite helpful in
understanding the challenges involved with WLAN positioning and making sense
of the algorithms employed. A radio wave is an electromagnetic wave propagated
by an antenna and travels at the speed of light. Radio waves have the ability
to travel through obstructions such as people or walls, but this results in a loss
of energy and interference, also known as attenuation and refraction. For exam-
ple, lead walls dissipate signal strength rapidly while microwave ovens generate
substantial interference when in use. Lastly, in free space, all electromagnetic
waves obey the inverse-square law. This means that an electromagnetic wave's
strength is proportional to

1

d 2 ,where d represents the distance from the source,
or access point in the case of wireless networks. Hence, doubling the distance
from a transmitter reduces the strength to a quarter, and so on.

2.2 Association Method
The simplest and most straightforward means of determining a wireless device's
position in a wireless network is the association method [10]. The association
method simply entails associating a mobile device with the access point it is
connected to. As each access point holds a unique identifier for every device that
is connected to it, retrieving this information is simply a matter of querying
the access points. This information places the mobile device's position within a
certain radial distance of an access point.
While associating a device to an access point is rather easy to accomplish, it
lacks granularity. Additionally, the area that a given access point covers varies
[24]. The average area covered by an access point can be represented by a sphere
that is 75 feet in diameter centered on the access point [19], limiting the use of the
location data retrieved by this method for certain context-aware applications.
Still, depending on the intended use, this method can prove sucient.

2.3 Propagation Method
The propagation method involves using triangulation to calculate user locations.
Recall that the strength of a wireless signal degrades as distance increases. This