Exploiting User Mobility for WiFi RTT Positioning: A Geometric Approach
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Due to the massive deployment of WiFi APs and its accessibility to various positioning elements, WiFi positioning is a key enabler to provide seamless and ubiquitous location services to users. There are various kinds of WiFi positioning technologies, depending on the concerned positioning element. Among them, round-trip time (RTT) measured by a fine-timing measurement protocol has received great attention recently. It provides an acceptable ranging accuracy near the service requirements in favorable environments when a line-of-sight (LOS) path exists. Otherwise, a signal is detoured along with non-LOS paths, making the resultant ranging results different from the ground-truth. The difference between the two is called an RTT bias, which is the main reason for poor positioning performance. To address it, we aim at leveraging the history of user mobility detected by a smartphone's inertial measurement units, called pedestrian dead reckoning (PDR). Specifically, PDR provides the geographic relation among adjacent locations, guiding the resultant positioning estimates' sequence not to deviate from the user trajectory. To this end, we describe their relations as multiple geometric equations, enabling us to render a novel positioning algorithm with acceptable accuracy. The algorithm is designed into two phases. First, an RTT bias of each AP can be compensated by leveraging the geometric relation mentioned above. It provides a user's relative trajectory defined on the local coordinate system of the concerned AP. Second, the user's absolute trajectory can be found by rotating every relative trajectory to be aligned, called trajectory alignment. The proposed algorithm gives a unique position when the number of detected steps and APs is at least 4 and 3 for linear mobility and 5 and 2 for arbitrary mobility.
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