Passive Handwriting Tracking via Weak mmWave Communication Signals

In this letter, a cooperative sensing framework based on millimeter wave (mmWave) communication systems is proposed to detect tiny motions with a millimeter-level resolution. Particularly, the cooperative sensing framework is facilitated with one transmitter and two receivers. There are two radio frequency (RF) chains at each receiver. Hence, the Doppler effect due to the tiny motions can be detected via passive sensing respectively at the receivers, and the velocities of the motions can be estimated by integrating the Doppler frequencies. It is demonstrated that the proposed cooperative sensing system is able to track the handwriting with 90% error below 6 mm. Moreover, the proposed cooperative sensing is robust to the strength of received signal. For example, it works even without the line-of-sight paths from the transmitter to the receivers or the sensing target, where the received signal strength is not sufficient for timing synchronization or demodulation.