SiFi: Pushing the Limit of Time-Based WiFi Localization Using a Single Commodity Access Point

There has been a booming interest in developing WiFi localization using multi-antenna (MIMO) access points (APs). Recent advances have demonstrated promising results that break the meter-accuracy barrier using commodity APs. Yet these state-of-the-art solutions require either multiple APs that are not necessarily available in practice, or multiple-channel measurements that disrupt normal data communication. In this paper, we present SiFi, a single AP-based indoor localization system that for the first time achieves sub-meter accuracy with a single channel only. The SiFi design is based on a key observation: with MIMO, the multiple (typically three) antennas of an AP are frequency-locked; although the accurate Time-of-Arrival (ToA) estimation on commodity APs is fundamentally limited by the imperfect time and frequency synchronization between the transmitter and receiver, there should be only one value for the ToA distortion that can cause three direct-path ToAs of the antennas to intersect at a single point, i.e., the position of the target. We develop the theoretical foundations of SiFi and demonstrate its realworld implementation with off-the-shelf WiFi cards. Our implementation introduces no hardware modification and is fully compatible with concurrent data transmission. It achieves a median accuracy of 0.93 m, which significantly outperforms the best known single AP single channel solution.