Installation guide

16
Wi-Fi Location-Based Services—Design and Deployment Considerations
OL-11612-01
Location Tracking Approaches
Calibration Phase
During the calibration phase, data is accumulated by performing a walk-around of the target environment
with a mobile device and allowing multiple receiving sensors (access points in the case of 802.11
WLANs) to sample the signal strength of the mobile device (this refers to a “network-side”
implementation of location patterning).
A graphical representation of the area to be calibrated is typically overlaid with a set of grid points or
notations to guide the operator in determining precisely where sample data should be acquired. At each
sample location, the array (or location vector) of RSS values associated with the calibration device is
recorded into a database known as a radio map or training set. The size of the vector for this sample
location is determined by the number of receiving stations that can detect the mobile device. Figure 6
provides a simplified illustration of this approach, showing two sample points and how their respective
location vectors might be formed from detected client RSSI.
Figure 6 Location Patterning Calibration
Because of fading and other phenomena, the observed signal strength of a mobile device at a particular
location is not static but is seen to vary over time. Because of this, calibration phase software typically
records many samples of signal strength for a mobile device during the actual sampling process.
Depending on technique, the actual vector array element recorded may account for this variation via one
or more creative approaches. A popular, simple-to-implement method is to represent the array element
associated with any specific receiver as the mean signal strength of all measurements of that mobile
device made by that receiver sensor for the reported sample coordinates. The location vector therefore
becomes a vector array of mean signal strength elements as shown in the following equation, where x
and y represent the reported coordinates of the sample and r represents the reported RSSI:
0
0
1
2
3
4
5
6
7
8
9
123456789
190539
Loc 7,6(-49,-43,-58,-57)
Loc 3,2(-61,-55,-46,-46)
AP1
AP4
AP2
AP2
),,,()(
AP4AP3AP2AP1
rrrrx,y
=