Installation guide

7
Wi-Fi Location-Based Services—Design and Deployment Considerations
OL-11612-01
Location Tracking Approaches
to hear arguments supporting the case that a fifth category should exist to include those RTLS systems
that sense and measure position using a combination of at least two of the four techniques mentioned
above.
Keep in mind that regardless of the underlying positioning technology, the “real-time” nature of an RTLS
is only as real-time as the most current timestamps, signal strengths, or angle-of-incidence
measurements. The timing of probe responses, beaconing rates, and location server polling intervals can
influence discrepancies seen between actual and reported device position from reporting interval to
reporting interval.
Cell of Origin
One of the simplest mechanisms of estimating approximate location in any system based on RF “cells”
is the concept of cell of origin (or “nearest access point” in Wi-Fi 802.11 systems), as shown in Figure 1.
Figure 1 Cell of Origin
In its simplest form, this technique makes no explicit attempt to resolve the position of the mobile device
beyond indicating the cell with which the mobile device is (or has been) registered. When applied to
802.11 systems, this technique tracks each cell to which a mobile device associates. The primary
advantage of this technique is ease of implementation. Cell of origin does not require the implementation
of complicated algorithms and thus positioning performance is very fast. Almost all cell-based WLANS
and other cellular-based RF systems can be easily and very cost-effectively adapted to provide cell of
origin positioning capability. However, the overwhelming drawback of pure cell of origin positioning
approaches continues to be coarse granularity. For various reasons, mobile devices can be associated to
cells that are not in close physical proximity, despite the fact that other nearby cells would be better
candidates. This coarse granularity can be especially frustrating when attempting to resolve the actual
location of a mobile device in a multi-story structure where there is considerable floor-to-floor cell
overlap.
To better determine which areas of the cell possess the highest probability of containing the mobile
device, some additional method of resolving location within the cell is usually required. This can either
be a manual method (such as a human searching the entire cell for the device) or an computer-assisted
method. When receiving cells provide received signal strength indication (RSSI) for mobile devices, the
use of the highest signal strength technique can improve location granularity over the cell of origin. In
this approach, the localization of the mobile device is performed based on the cell that detects the mobile
device with the highest signal strength. This is shown in Figure 2, where the blue rectangular client
device icon is placed nearest the cell that has detected it with the highest signal strength.
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