Received signal strength calibration for handset localization in WLAN

Terminal localization for indoor Wireless Local Area Networks (WLANs) is critical for the deployment of location based services inside of buildings. A key challenge in localization arises when more than one mobile device is used for collecting received signal strength (RSS) measurements due to the existence of inconsistencies in the signal measurement hardware from different devices. This paper focuses on developing a calibration method to bring RSS measurements from different source devices to comparable levels by the means of an affine transformation. This calibration method is automated based on the Expectation-Maximization (EM) algorithm to find the effective transformation parameters in real time. The results show that the estimated locations are within 2m of the true locations for laptop and handset localization when an affine transformation is used and that measurements from only 11 locations were needed for the automated calibration algorithm to obtain the transformation parameters. The results indicate that this calibration technique yields high accuracy levels in localization and that automated RSS measurement calibration is feasible.