Human Activity Recognition by Using Enhanced Radar Point Cloud 2D Histograms and Doppler Feature Fusion

Human activity recognition (HAR) based on millimeter wave (mmWave) radar has recently attracted significant interest due to its diverse applications in intelligent robots and human-computer interaction (HCI), including the healthcare monitoring robot. 2-dimensional (2D) histogram features of radar point clouds have demonstrated high accuracy in HAR. But further expansion and refinement of this technique is needed. This paper presents a new precise non-invasive HAR framework based on radar point cloud 2D histograms. Our method enhances conventional 2D histograms by integrating fixed radar sensing boundaries into the histograms, which shows the relative spatial position changes of the target points detected by radar. Additionally, we have concatenated Doppler features (i.e., range-Doppler and angle-Doppler histograms) with the point cloud histograms, resulting in a more comprehensive feature representation than conventional point cloud histograms. We investigated the overfitting issue in stacked hybrid networks and established a multi-layer hybrid network with an optimal number of stacked layers for HAR. In the evaluation, our approach achieves state-of-the-art accuracy, with 99.72% on mmWaveRadarWalking dataset and 98.67% on CI4R-Human-Activity-Recognition dataset, respectively. The proposed method can be applied in the fields of robotics and HCI.