M3: Multipath Assisted Wi-Fi Localization with a Single Access Point

Owing to the ubiquitous penetration of Wi-Fi in our daily lives, Wi-Fi indoor localization has attracted intensive attentions in the last decade or so. Despite some significant progresses, the high accuracy of existing systems is still achieved at the cost of dense access point (AP) deployment. The more practical single AP localization is largely left as an open problem because the hardware-induced time delay "contaminates" the measurement of signal propagation time in the air. In this article, we design and implement M-3 to tackle this challenge with commodity Wi-Fi cards. M-3 exploits a multipath-assisted approach that turns the harmful multipath from foe to friend to enable single AP localization: a device can be pinpointed through the combination of azimuths and the relative time of flight (ToF) of Line-of-Sight (LoS) signal and reflection signals, eliminating the need for multiple APs along with their absolute ToF measurements. M-3 further utilizes frequency hopping to combine multiple channels to form a virtually wider-spectrum channel for higher ToF resolution. As a prominent feature of M-3, the channels do not need to be adjacent. Comprehensive experiments demonstrate that M-3 outperforms the state-of-the-art systems and achieves a median localization accuracy of 71 cm in three environments with a single AP.