HLoc: Exploiting Height Difference for WiFi Indoor Localization With Single Commercial AP

WiFi indoor localization, as a fundamental task to many real-world applications, has attracted widespread attention from both academia and industry over the past decade. While existing works have already achieved decimeter-level accuracy, they either require multiple access points (APs) or rely on the assumption that the AP and the client are at the same height, and their performance will degrade dramatically when there exists a height difference. In this article, we explore the neglected elevation angle dimension and propose HLoc, the first elevation angle-based WiFi indoor localization system, which can achieve decimeter-level accuracy with a single commercial AP. Inspired by existing 4-D mmwave radar, HLoc transforms the horizontal distance between the client and the AP into an elevation angle estimation problem and efficiently resolves it through a modified sparse recovery algorithm. Moreover, existing commercial APs are usually equipped with nonuniform planar arrays to enhance communication performance, which brings us the opportunity to jointly estimate the azimuth and elevation angles. After obtaining the horizontal distance, the azimuth angle is used to determine the orientation, and thus a single AP can achieve localization. The impact of the client height error on system localization performance is also analyzed theoretically and experimentally. We evaluate HLoc under a variety of complex environments, and the experimental results show that HLoc can achieve median errors of 9.4° and 13.6° for azimuth and elevation angle estimation, respectively, and a 98-cm median localization error with only a single commercial AP. We believe that this additional height information can be combined with other existing systems and inspire researchers to further push indoor localization from laboratory to the wild.