RadSee: See Your Handwriting Through Walls Using FMCW Radar

This paper aims to design and implement a radio device capable of detecting a person's handwriting through a wall.Although there is extensive research on radio frequency (RF) based human activity recognition, this task is particularly challenging due to the through-wall requirement and the tinyscale handwriting movements.To address these challenges, we present RadSee-a 6 GHz frequency modulated continuous wave (FMCW) radar system designed for detecting handwriting content behind a wall.RadSee is realized through a joint hardware and software design.On the hardware side, RadSee features a 6 GHz FMCW radar device equipped with two customdesigned, high-gain patch antennas.These two antennas provide a sufficient link power budget, allowing RadSee to "see" through most walls with a small transmission power.On the software side, RadSee extracts effective phase features corresponding to the writer's hand movements and employs a bidirectional LSTM (BiLSTM) model with an attention mechanism to classify handwriting letters.As a result, RadSee can detect millimeterlevel handwriting movements and recognize most letters based on their unique phase patterns.Additionally, it is resilient to interference from other moving objects and in-band radio devices.We have built a prototype of RadSee and evaluated its performance in various scenarios.Extensive experimental results demonstrate that RadSee achieves 75% letter recognition accuracy when victims write 62 random letters, and 87% word recognition accuracy when they write articles.Chirp generator ADC Power amp (30dB) Custom-designed and optimized 6GHz FMCW radar circuit on PCB PA Two-stage LNA (38dB) Mixer TX RX Custom-designed and optimized patch antennas coupler