Improving WiFi Ranging Through Frequency Diversity and Mobility

Indoor positioning with IEEE 802.11 Fine Timing Measurement (FTM) relies on precise estimation of the Round Trip Time (RTT) between a pair of WiFi devices. However, studies have demonstrated that multi-path propagation affect FTM, which tends to overestimate and only rarely underestimates the actual distance. We present HopFTM to enhances FTM ranging accuracy by leveraging frequency diversity and space diversity (mobility). Specifically, HopFTM performs ranging on multiple radio channels. Additionally, in a mobile scenario, ranging is performed at multiple locations. Our experimental results using low-cost ESP32-S3 hardware reveal that selecting the smallest RTT value from all measured channels significantly improves accuracy, especially in environments with strong multi-path effects or non-line-of-sight conditions. In a static scenario by using the smallest RTT from four channels, we were able to reduce the median ranging error by 4× from 120 cm to 30 cm while the worst-case error was decreased from 15 m to 10 m, compared to a traditional single-channel approach. In a mobile scenario, ranging measurements taken at different locations outside the decorrelation distance can also be fused. Our simulation shows that taking the smallest RTT value over multiple positions can improve FTM ranging error from 1.8 m to 0.76 m.