Normalization of Signal Strength Measurements for WLAN Based Indoor Positioning

In this paper, we study the normalization of WLAN signal strength measurements to enable the use of WLAN radio maps generated using one device for position estimation with signal strength measurements by another device. In the field experiments, we used different devices: one laptop PC and two handheld devices. The data for the analysis was collected using these devices simultaneously in the same locations inside an office building. The tests demonstrated that using a simple conversion, the measurements of two different devices can be used to generate histogram based radio maps for positioning on a third device. It was found that the optimal conversion is different for different devices. The experiments also show that the positioning with one device and radio map based on measurements obtained with another device can be as accurate as positioning with radio map obtained with measurements from the device itself; the difference in mean errors between these setups is some tens of centimeters, the relative difference in mean errors being less than 10%, while the mean errors are approximately 3 m.