Breaking the Resolution Barriers of mmWave Arrays via Null Steering for Sleep Monitoring in Multi-Person Scenarios

Couples sharing a bed is a common practice in households, making sleep monitoring in multi-person scenarios a significant area of research. The high sensitivity of mmWave makes it a promising tool for monitoring vital signs during sleep. However, implementing mmWave-based solutions for such settings poses two major challenges. First, the range and angular resolution of commercial mmWave radar are still inadequate for handling such close-proximity scenarios where individuals share the same bed. Second, even with sufficient angular resolution, aligning the main lobe with one target usually faces strong interference from other targets located in the sidelobes. To address these challenges, we leverage antenna pattern nulls---directions with zero gain---to mitigate interference and overcome resolution limitations. Implementing null steering on practical mmWave radar, however, is challenging due to the human body being an extended target that spans a range of angles, as well as hardware imperfections that render traditional algorithms ineffective. In this paper, we present mmNull, the first null steering system designed for commercial mmWave radars. mmNull models the human body as an extended target rather than a single point, derives a closed-form solution to maximize target gain while suppressing interferences, and employs a heuristic algorithm to address hardware imperfections. Comprehensive experiments show that mmNull achieves a more than 5x improvement in accuracy compared to current state-of-the-art (SOTA) methods for monitoring respiration rate, heart rate, and heart rate variability (HRV) in multi-person scenarios.