DiFS: WiFi-based Directed Fresnel Signature Localization for Mobile Ship Environment

The device-free localization of individuals not equipped with a radio device plays a critical role in cruise ships, particularly during an emergency. In this paper, we introduce a device-free localization scheme requiring low human effort, i.e., DiFS, which utilizes an onboard off-the-shelf WiFi infrastructure. An intuitive idea of DiFS is that because the channel state information (CSI) is sensitive to the target location within the WiFi Fresnel zone, the target location can be determined by extracting the Fresnel signature and coordinates of the WiFi access points (APs). However, due to the skin effect of signal propagation onboard a ship, CSI cannot reflect a target with precision. Furthermore, an undirected Fresnel signature may lead to misinterpretation if the WiFi APs are not deployed perfectly. We observed that the power delayed profiles (PDPs) can accurately reflect the target shadowing within the Fresnel zone in a rich multipath environment. In addition, we leverage the specific skin effect in metal ships and infer directed Fresnel signatures using a set of power fading models. Extensive experimental results on an actual ship demonstrate that DiFS outperforms the state-of-the-art methods and achieves an accuracy of 0.9 and 1.2 m in the line-of-sight and non-line-of-sight scenarios, respectively.