Installation guide

8
Wi-Fi Location-Based Services—Design and Deployment Considerations
OL-11612-01
Location Tracking Approaches
Figure 2 Highest Signal Strength Technique
Using this technique, the probability of selecting the true “nearest cell” is increased over that seen with
pure cell of origin. Depending on the accuracy requirements of the underlying business application,
performance may be more than sufficient for casual location of mobile clients using the highest signal
strength technique. For instance, users intending to use location-based services only when necessary to
help them find misplaced client devices in non-mission critical situations may be very comfortable with
the combination of price and performance afforded by solutions using the highest signal strength
approach. However, users requiring more precise location would find the inability of the highest signal
strength technique to isolate the location of a mobile device with finer granularity than that of an entire
coverage cell to be a serious limitation. These users are better served by those approaches using the
techniques of lateration, angulation, and location patterning that provide finer resolution and improved
accuracy. These techniques are discussed in the subsequent sections.
Distance-Based (Lateration) Techniques
Time of Arrival
Time of Arrival (ToA) systems are based on the precise measurement of the arrival time of a signal
transmitted from a mobile device to several receiving sensors. Because signals travel with a known
velocity (approximately the speed of light (c) or ~300 meters per microsecond), the distance between the
mobile device and each receiving sensor can be determined from the elapsed propagation time of the
signal traveling between them. The ToA technique requires very precise knowledge of the transmission
start time(s), and must ensure that all receiving sensors as well as the mobile device are accurately
synchronized with a precise time source.
From the knowledge of both propagation speed and measured time, it is possible to calculate the distance
ρ between the mobile device and the receiving station:
ρ = c (t)
where
• ρ = distance (meters)