  This study proposes an accurate method to estimate human body orientation
using a millimeter-wave radar system. Body displacement is measured from the
phase of the radar echo, which is analyzed to obtain features associated with
the fundamental and higher-order harmonic components of the quasi-periodic
respiratory motion. These features are used in body-orientation estimation
invoking a novel hierarchical regression model in which a logistic regression
model is adopted in the first step to determine whether the target person is
facing forwards or backwards; a pair of ridge regression models are employed in
the second step to estimate body-orientation angle. To evaluate the performance
of the proposed method, respiratory motions of five participants were recorded
using three millimeter-wave radar systems; cross-validation was also performed.
The average error in estimating body orientation angle was 38.3$^\circ$ and
23.1$^\circ$ using respectively a conventional method with only the fundamental
frequency component and our proposed method, indicating an improvement in
accuracy by factor 1.7 when using the proposed method. In addition, the
coefficient of correlation between the actual and estimated body-orientation
angles using the conventional and proposed methods are 0.74 and 0.91,
respectively. These results show that by combining the characteristic features
of the fundamental and higher-order harmonics from the respiratory motion, the
proposed method offers better accuracy.
