ASSAFormer: a sensor data-based approach to human activity recognition.

Human Activity Recognition (HAR) plays a significant role in the field of health monitoring. By accurately identifying common activities such as walking, go up/down stairs, sitting, standing, and lying down, continuous tracking, analysis of an individual's behavioral state can be achieved. This is of great importance for health monitoring and intelligent healthcare. However, due to the noise in data collected by wearable sensors and the variability in data distribution, existing HAR methods still face limitations in accuracy and generalization ability, making it difficult to maintain stable and reliable early-warning performance across diverse motion scenarios and individual differences. To address these challenges, a human activity recognition method ASSAFormer is proposed in this paper. ASSAFormer integrates mode decomposition, heuristic optimization algorithms, and an improved Transformer for health monitoring. During data preprocessing, variational mode decomposition (VMD) is employed to filter noise from sensor data sequences, while the Whale Optimization Algorithm (WOA) optimizes the number of decomposition modes and the penalty factor, thereby mitigating the parameter sensitivity issue in mode decomposition. In terms of network architecture design, Adaptive Sparse Self-Attention (ASSA) and Contrastive Normalization (ContraNorm) are introduced into the vanilla Transformer. Firstly, the self-attention mechanism in Transformer is prone to introducing low correlation interference information and leading to overfitting. Therefore, an Adaptive Sparse Self-Attention (ASSA) mechanism is proposed. The Sparse Self-Attention (SSA) branch of ASSA filters query-key matching scores, allowing only highly relevant information to pass through, thereby reducing noise interference. Meanwhile, the Dense Self-Attention (DSA) of ASSA branch retains weakly relevant yet useful information that might be overlooked due to excessive sparsification. Secondly, Contrastive Normalization (ContraNorm) is introduced to alleviate dimensional collapse, enabling better implicit dispersion of representations in the feature space. The comparative experiments demonstrate that the proposed method achieves the best performance on both the UCI dataset and URFD dataset, respectively. And the ablation studies further validate the effectiveness of the improved modules.