Installation guide

9
Wi-Fi Location-Based Services—Design and Deployment Considerations
OL-11612-01
Location Tracking Approaches
• c = propagation speed of ~ 300 meters / microsecond
• t = time in microseconds
From distance
ρ used as radii, a circular representation of the area around the receiving sensor can be
constructed for which the location of the mobile device is highly probable. ToA information from two
sensors resolves a mobile device position to two equally probable points. ToA tri-lateration makes use
of three sensors to allow the mobile device location to be resolved with improved accuracy.
Figure 3 illustrates the concept of ToA tri-lateration. The amount of time required for a message
transmitted from station X to arrive at receiving sensors A, B, and C is precisely measured as t
A
, t
B
, and
t
C
. Given a known propagation velocity (stated as c), the mobile device distance ρ from each of these
three receiving sensors can then be calculated as
ρ
A
, ρ
B
, and ρ
C
respectively. Each calculated distance
value is used to construct a circular plot around the respective receiving sensor. From the individual
perspective of each receiver, station X is believed to reside somewhere along this plot. The intersection
of the three circular plots resolves the location of station X as illustrated in Figure 3. In some cases, there
may be more than one possible solution for the location of mobile device station X, even when using
three remote sensors to perform tri-lateration. In these cases, four or more receiving sensors are
employed to perform ToA multi-lateration.
Figure 3 Time of Arrival (ToA)
ToA techniques are capable of resolving location in two-dimensional as well as three-dimensional
planes. 3D resolution can be performed by constructing spherical instead of circular models.
A drawback of the ToA approach is the requirement for precise time synchronization of all stations,
especially the mobile device (which can be a daunting challenge for some 802.11 client device
implementations). Given the high propagation speeds, very small discrepancies in time synchronization
can result in very large errors in location accuracy. In fact, a time measurement error as small as 100
nanoseconds can result in a localization error of 30 meters. ToA-based positioning solutions are typically
challenged in environments where a large amount of multipath, interference, or noise may exist.
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A
A
C
B
B
C
X