MFPL: Multi-Frequency Phase Difference Combination Based Device-Free Localization

Abstract

With the popularity of indoor wireless network, device-free indoor localization has attracted more and more attention. Unlike device-based localization where the target is required to carry an active transmitter, their frequent signal scanning consumes a large amount of energy, which is inconvenient for devices with limited energy. In this work, we propose the MFPL, device-free localization (DFL) system based on WiFi distance measurement. First, we combine multi-subcarrier characteristic of Channel State Information (CSI) with classical Fresnel reflection model to get the linear relationship between the change of the length of reflection path and the subcarrier phase difference. Then we calculate the Fresnel phase difference between subcarrier pairs with different spacing from CSI amplitude time series. Finally, we get the change of the length of the reflection path caused by target moving to achieve distance measurement and localization. Using a combination of subcarriers with different spacing to achieve distance measurement effectively broadens the maximum unambiguous distance of the system. To solve the complex non-linear problem of the intersection of two elliptic equations, we introduce Newton's method to transform the non-linear problem into a linear one. The effectiveness of our approach is verified using commodity WiFi infrastructures. The experimental results show our method achieves a median error of 0.87 m in actual indoor environment.