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This paper presents the novel design of a printed, low-cost, dual-port, and dual-polarized slot antenna for microwave biomedical radars. The butterfly shape of the radiating element, with orthogonally positioned arms, enables simultaneous radiation of both vertically and horizontally polarized waves. The antenna is intended for full-duplex in-band applications using two mutually isolated antenna ports, with the CPW port on the same side of the substrate as the slot antenna and the microstrip port positioned orthogonally on the other side of the substrate. Those two ports can be used as transmit and receive ports in a radar transceiver, with a port isolation of 25 dB. Thanks to the bow-tie shape of the slots and an additional coupling region between the butterfly arms, there is more flexibility in simultaneous optimization of the resonant frequency and input impedance at both ports, avoiding the need for a complicated matching network that introduces the attenuation and increases antenna dimensions. The advantage of this design is demonstrated through the modeling of an eight-element dual-port linear array with an extremely simple feed network for high-gain biosensing applications. To validate the simulation results, prototypes of the proposed antenna were fabricated and tested. The measured operating band of the antennas spans from 2.35 GHz to 2.55 GHz, with reflection coefficients of less than—10 dB, a maximum gain of 8.5 dBi, and a front-to-back gain ratio that is greater than 15 dB, which is comparable with other published single dual-port slot antennas. This is the simplest proposed dual-port, dual-polarization antenna that enables straightforward scaling to other frequency bands. 

Doppler radar node occupancy sensors are promising for applications in smart buildings due to their simple circuits and price advantage compared to quadrature radar sensors. However, single-channel sensitivity limitations may result in low sensitivity and misinterpreted motion rates if the detected subject is at or close to “null” points. We designed and tested a novel method to eliminate such limits, demonstrating that passive nodes can be used to detect a sedentary person regardless of position. This method is based on characteristics of chest motion due to respiration, found via both simulations and experiments based on a sinusoidal model and a more realistic model of cardiorespiratory motion. In addition, respiratory rate variability is considered to distinguish a true human presence from a mechanical target. Sensor node data were collected simultaneously with an infrared camera system, which provided a respiration signal reference, to test the algorithm with 19 human subjects and a mechanical target. The results indicate that a human presence was detected with 100% accuracy and successfully differentiated from a mechanical target in a controlled environment. The developed method can greatly improve the occupancy detection accuracy of single-channel radar-based occupancy sensors and facilitate their adoption in smart building applications. 

Physiological Doppler radar measurements for respiration do not typically take into consideration polarimetric effects caused by scattering from the irregular surface of the respiring torso. This study investigated the performance of a physiological Doppler radar system using both vertically and horizontally polarized transmit antennas, each in combination with both vertically and horizontally polarized receive antennas. Return signatures were analyzed for robotic phantom targets with varied arrangements of curvature, and for a representative group of human subjects. The results showed that while cylindrical targets generated significant amounts of cross-polarized return signal, typical human subject return signals produced an even larger proportion of cross-polarized signal. 

The quality of human respiratory motion measurements made with Doppler radar depends on the amount of reflected signal received and the overall signal to noise ratio (SNR) of the measurement. The non-uniform characteristics of the human torso and its motion impact both the amount of signal returned toward the radar and its polarization. This study used a 2.4 GHz continuous wave Doppler radar system to compare the respiratory motion measurement performance for circular polarized antennas and linear polarized antennas, using mechanical respiratory phantom measurements at a nominal distance of one meter. While the different surfaces examined produced varied levels of signal at the original and other polarizations, the measurements using circular polarized antennas consistently provided less overall received signal and no significant improvement of SNR. 

This paper proposes a spectral binning method for the classification of locomotion and extraneous body motion (EBM) that may occur during Continuous Wave (CW) Doppler radar motion sensing of human subjects. The method analyzes the spectral content of the arctangent demodulated displacement signature, generating an activity classification based on the magnitude of the spectral content for each of several frequency bins. The choice and number of bins used for the overall classification of data were determined by analyzing experimental data. The method successfully classified sedentary, EBM, and locomotion states for 5 subjects. The method can be used both for determining the presence and type of activity, and for recognizing when data segments are not suitable for monitoring sedentary vital signs.