The heartbeat detection using the scaling function of Wavelet transform is proposed for a Doppler radar sensor. The conventional methods such as the fast‐Fourier transform and the autocorrelation show the respiration rate and the heartbeat from the raw data of the radar sensors acquiring for a sufficient sampling time. The methods have the limit to detect the biometric information that varies with real‐time because they only show the overall statistical information of the sampled data. In the proposed method, the scaling function in the Daubechies wavelet transform can be used to accurately find out the periodicity of radar signals for detecting heartbeat varying in real‐time. The results of the signal processing using the radar signals acquired for 3 min results show that the proposed method lowered a mean error rate of 2.5% and a SD of 2.0% compared with the method using the wavelet function. The proposed method in the measurement for 1 minute using the radar sensor also showed the lowest mean error rate of 3.8% and the low SD of 3.2% using the contact sensor as the reference among various signal processing methods including auto‐correlation and peak detection with filtering.

A continuous‐wave (CW) Doppler radar using two‐tone frequencies is presented for the measurement of short‐range distance. The envelope detection method is proposed to discriminate each frequency signal in the output waveform of the two‐tone CW Doppler radar sensor. The phase difference in each frequency is obtained from the nonlinearly demodulated I/Q baseband signals, following calibration of the I/Q imbalance and DC offset voltages. The absolute distance is derived from the phase differences at the two‐tone frequencies with a 100 MHz spacing. The measurement results in the two‐tone radar system show a maximum error of 2.5 cm in the 20 to 50 cm distance range.

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In this letter, a high‐order composite right/left‐handed (CRLH)‐coupled line bandpass filter using the first negative resonant mode is presented. The unit‐cell structure of a half wavelength resonator is composed of an interdigital capacitor for a series capacitance and a shorted stub for a shunt inductance, respectively. The design equations are derived from those of a conventional coupled line filter. The even‐mode and odd‐mode impedances of a CRLH‐coupled line section are obtained using full wave simulation to complete the design equations. An insertion loss of a CRLH filter is suppressed from 2.1 to 1.1 dB by adding via hole in the end of resonators. The size of a CRLH filter is reduced by 53%, compared with that of conventional coupled line bandpass filter. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:943–947, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25856

A detrending technique is proposed for continuous-wave (CW) radar to remove the effects of direct current (DC) offset, including DC drift, which is a very slow noise that appears near DC. DC drift is mainly caused by unwanted vibrations (generated by the radar itself, target objects, or surroundings) or characteristic changes in components in the radar owing to internal heating. It reduces the accuracy of the circle fitting method required for I/Q imbalance calibration and DC offset removal. The proposed technique effectively removes DC drift from the time-domain waveform of the baseband signals obtained for a certain time using polynomial fitting. The accuracy improvement in the circle fitting by the proposed technique using a 5.8 GHz CW radar decreases the error in the displacement measurement and increases the signal-to-noise ratio (SNR) in vital signal detection. The measurement results using a 5.8 GHz radar show that the proposed technique using a fifth-order polynomial fitting decreased the displacement error from 1.34 mm to 0.62 mm on average when the target was at a distance of 1 m. For a subject at a distance of 0.8 m, the measured SNR improved by 7.2 dB for respiration and 6.6 dB for heartbeat.