  We developed a noncontact measurement system for monitoring the respiration
of multiple people using millimeter-wave array radar. To separate the radar
echoes of multiple people, conventional techniques cluster the radar echoes in
the time, frequency, or spatial domain. Focusing on the measurement of the
respiratory signals of multiple people, we propose a method called
respiratory-space clustering, in which individual differences in the
respiratory rate are effectively exploited to accurately resolve the echoes
from human bodies. The proposed respiratory-space clustering can separate
echoes, even when people are located close to each other. In addition, the
proposed method can be applied when the number of targets is unknown and can
accurately estimate the number and positions of people. We perform multiple
experiments involving five or seven participants to verify the performance of
the proposed method, and quantitatively evaluate the estimation accuracy for
the number of people and the respiratory intervals. The experimental results
show that the average root-mean-square error in estimating the respiratory
interval is 196 ms using the proposed method. The use of the proposed method,
rather the conventional method, improves the accuracy of the estimation of the
number of people by 85.0%, which indicates the effectiveness of the proposed
method for the measurement of the respiration of multiple people.
