University at Buffalo, The State University of New York University at Buffalo - The State University of New York

Motion Detection in WLANs using PHY Layer Information

In this project, we demonstrate that it is possible to enable fine-grained human motion detection on commodity WiFi devices by exploiting PHY layer information available from today's WiFi chipsets.

In the first part of this project, we demonstrate how PHY layer information -- Channel State Information (CSI) and Time-of-Flight (ToF) values -- available from commodity APs can be leveraged to detect and classify different client mobility modes without any software modifications on the client side. We identify four broad categories of client mobility: static clients, environmental mobility (the client is static but the channel changes due to external movements), micro-mobility (the client's location is confined within a small area), and macro-mobility (the client moves towards or away from the AP). We show that the temporal changes in the CSI can be used to detect static, environmental, and device mobility. However the changes in CSI are similar for micro- and macro-mobility scenarios. To distinguish between these two mobility modes, we observe that the client's distance from the AP, which which is reflected in the ToF, changes significantly under macro-mobility. The increasing vs. decreasing trend of the client's distance can further indicate its relative heading with respect to the AP.

In addition, we demonstrate how fine-grained mobility classification can improve the performance of client roaming, rate control, frame aggregation, and MIMO beamforming. Our testbed experiments show that our mobility classification algorithm achieves more than 92% accuracy in a variety of scenarios, and the combined throughput gain of all four mobility-aware protocols over their mobility-oblivious counterparts can be more than 100%.

Fig. 1: Client mobility classification. Fig. 2: WiDraw: Core idea and examples.

In the second part of the project, we introduce WiDraw, the first hand motion tracking solution leveraging wireless signals that can be enabled on existing mobile devices using only a software patch, without requiring prior learning or the use of any wearable. WiDraw utilizes the presence of a large number of WiFi devices in today's WLANs. It leverages the Angle-of-Arrival (AoA) values of incoming wireless signals at the mobile device to track the detailed trajectory of the user's hand in both Line-of-Sight and Non-Line-of-Sight scenarios. The intuition behind WiDraw is that whenever the user's hand blocks a signal coming from a certain direction, the signal strength of the AoA representing the same direction will experience a sharp drop. Therefore, by tracking the signal strength of the AoAs, it is feasible to determine when and where such occlusions happen and further determine a set of horizontal and vertical coordinates along the hand's trajectory, given a depth value. The depth of the user's hand can also be approximated using the drop in the overall signal strength. By estimating the hand's depth, along with horizontal and vertical coordinates, WiDraw is able to track the user's hand in the 3-D space w.r.t the WiFi antennas of the receiver.

We demonstrate the feasibility of WiDraw by building a software prototype on HP Envy laptops, using Atheros AR9590 chipsets and 3 antennas. We show that by utilizing the AoAs from up to 25 WiFi transmitters, a WiDraw-enabled laptop can track the user's hand with a median error lower than 5 cm. WiDraw's rate of false positives -- motion detection in the absence of one -- is less than 0.04 events per minute over a 360 minute period in a busy office environment. Experiments across 10 different users also demonstrate that WiDraw can be used to write words and sentences in the air, achieving a mean word recognition accuracy of 91%.

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Li Sun worked at HP Labs as an intern for the duration of the project.