DiFS: Wi-Fi 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 article, we introduce a device-free localization scheme requiring low human effort, i.e., directed Fresnel signature (DiFS), which utilizes an onboard off-the-shelf Wi-Fi infrastructure. An intuitive idea of DiFS is that because the channel state information (CSI) is sensitive to the target location within the Wi-Fi Fresnel zone, the target location can be determined by extracting the Fresnel signature and coordinates of the Wi-Fi 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 Wi-Fi APs are not deployed perfectly. We observed that the power delay 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 DiFSs 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 (LoS) and non-LoS (NLoS) scenarios, respectively.