  Motion tracking systems based on optical sensors typically often suffer from
issues, such as poor lighting conditions, occlusion, limited coverage, and may
raise privacy concerns. More recently, radio frequency (RF)-based approaches
using commercial WiFi devices have emerged which offer low-cost ubiquitous
sensing whilst preserving privacy. However, the output of an RF sensing system,
such as Range-Doppler spectrograms, cannot represent human motion intuitively
and usually requires further processing. In this study, MDPose, a novel
framework for human skeletal motion reconstruction based on WiFi micro-Doppler
signatures, is proposed. It provides an effective solution to track human
activities by reconstructing a skeleton model with 17 key points, which can
assist with the interpretation of conventional RF sensing outputs in a more
understandable way. Specifically, MDPose has various incremental stages to
gradually address a series of challenges: First, a denoising algorithm is
implemented to remove any unwanted noise that may affect the feature extraction
and enhance weak Doppler signatures. Secondly, the convolutional neural network
(CNN)-recurrent neural network (RNN) architecture is applied to learn
temporal-spatial dependency from clean micro-Doppler signatures and restore key
points' velocity information. Finally, a pose optimising mechanism is employed
to estimate the initial state of the skeleton and to limit the increase of
error. We have conducted comprehensive tests in a variety of environments using
numerous subjects with a single receiver radar system to demonstrate the
performance of MDPose, and report 29.4mm mean absolute error over all key
points positions, which outperforms state-of-the-art RF-based pose estimation
systems.
