From User Recognition to Activity Counting: An Identity-Agnostic Approach to Multi-User WiFi Sensing

TL;DR

This paper introduces an identity-agnostic activity counting approach using Wi-Fi CSI data, improving generalization to new users and environments compared to traditional identity-dependent methods.

Contribution

It reformulates multi-user activity recognition as activity counting, enabling user-invariant predictions without prior user identity knowledge.

Findings

Identity-agnostic model achieves a mean absolute error of 0.1081 in activity counting.

Under unseen-user conditions, the identity-agnostic model's error remains stable, unlike the identity-dependent model.

Feature analysis shows identity-agnostic representations are more user-invariant, enhancing generalization.

Abstract

Wi-Fi Channel State Information (CSI) enables device-free human activity recognition, but existing multi-user approaches assume a fixed set of known users during both training and inference. This closed-set assumption limits deployment, as models trained on a specific user set degrade when applied to new individuals or environments. We reformulate multi-user activity recognition as activity counting, estimating how many users perform each activity type at a given time, without associating actions with specific individuals. We propose a pipeline that converts CSI measurements into spatial projections and extracts features using a pretrained convolutional backbone. Two formulations are evaluated on the WiMANS dataset: a conventional identity-dependent model that assigns activities to fixed user slots, and an identity-agnostic model that estimates scene-level activity composition through regression. Under standard evaluation, the identity-agnostic model achieves a mean absolute error of 0.1081 on a 0-5 count scale. Under unseen-user evaluation, the identity-dependent model's macro-F1 drops from 80.38 to 32.61, while the identity-agnostic model's counting error remains stable. Feature space analysis confirms that identity-agnostic representations are more user-invariant, which explains their stronger generalization. These results suggest that activity counting provides a more practical and generalizable alternative to identity-dependent formulations for multi-user WiFi sensing.