Tracking Keystrokes Using Wireless Signals

We implement a passive remote keystroke detection mechanism using only changes in the wireless channel. The detection algorithm does not require the user to wear any active devices nor does it require a change in the user's wireless transmission technique. The receiver system is implemented with five antennas. We cancel the signals received on multiple antennas. The key insight to realizing a fine-grained localization system is to exploit the extremely high sensitivity of cancellation performance (interference cancellation in full-duplex for example) to exact amplitude and phase matching. The receiver design introduces a delay mismatch between the received signal streams to guarantee imperfect cancellation across the transmission bandwidth except at one in-band frequency resulting in a trough in the cancellation spectrum. We detect keystrokes by forming an array of observed trough frequency across different antenna pairs.

We implement our receiver system on the NI-based SDR platform. With full-training the receiver detects a keystroke within one key offset with an accuracy of 91.8%. With a short ≈ 10 character training input, this accuracy drops to 85%.