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60-GHz Millimeter-Wave Life Detection System (MLDS) for Noncontact Human Vital-Signal Monitoring Huey-Ru Chuang, Senior Member, IEEE, Hsin-Chih Kuo, Student Member, IEEE, Fu-Ling Lin, Member, IEEE, Tzuen-Hsi Huang, Member, IEEE, Chi-Shin Kuo, Student Member, IEEE, and Ya-Wen Ou, Student Member, IEEE

Abstract—A first reported experimental study of a 60 GHz millimeter-wave life detection system (MLDS) for noncontact human vital-signal monitoring is presented. This detection system is constructed by using V-band millimeter-wave waveguide components. A clutter canceller is implemented in the system with an adjustable attenuator and phase shifter. It performs clutter cancellation for the transmitting power leakage from the circulator and background reflection to enhance the detecting sensitivity of weak vital signals. The noncontact vital signal measurements have been conducted on a human subject in four different physical orientations from distances of 1 and 2 m. The time-domain and spectrum waveforms of the measured breathing and heartbeat are presented. This prototype system will be useful for the development of the 60-GHz CMOS MLDS detector chip design.

Fig. 1. Illustration of a noncontact life detection system for human vital signal monitoring.

Index Terms—60-GHz, clutter, life detection system, millimeterwave (MM-wave), millimeter-wave life detection system (MLDS), noncontact, V-band, vital signal.

I. INTRODUCTION

I

T has been decades since microwave remote detection systems for human vital signals, including the heartbeat and breathing signals, have been reported [1]–[4]. Recently, this research is becoming more popular for its application to noncontact human vital signal monitoring for health care [5]. In this application, most reported works have used a 2.4- or 5-GHz industrial, scientific, and medical (ISM) band frequency [6], [7]. The Ka-band system has also been studied [8], [9]. Recently, the 60-GHz millimeter-wave (MM-wave) frequency band has been extensively studied for short-range Gigabit communication applications [10]. It will be interested to investigate the use of a 60-GHz MM-wave for life-detection application.

Manuscript received November 01, 2010; revised February 01, 2011; accepted February 03, 2011. Date of publication February 22, 2011; date of current version February 01, 2012. The associate editor coordinating the review of this paper and approving it for publication was Prof. Aime Lay-Ekua Kille. H.-R. Chuang, H.-C. Kuo, T.-H. Huang, C.-S. Kuo, and Y.-W. Ou are with the Institute of Computer and Communication Engineering, Department of Electrical Engineering, National Cheng Kung University, Tainan 70101, Taiwan (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; q36984545@mail. ncku.edu.tw; [email protected]). F.-L. Lin is with the Department of Electronics Engineering, Southern Taiwan University of Technology, Tainan 71005, Taiwan (e-mail: [email protected]. tw). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2011.2118198

Fig. 2. Illustration of the measurement of the human-body reflected clutter power (at 60 GHz) by using a second V-band horn antenna connected to a spectrum analyzer (through a V-band harmonic mixer).

Since the basic principle of vital-signal detection system is that human breathing and heartbeat vibration will phase-modulate the backscattered EM wave signal (as illustrated in Figs. 1 and is the phase modulation factor), the use of an EM 3, where wave with higher frequency (and smaller wavelength) may have better phase-modulation sensitivity. This increased sensitivity may allow for better breathing and heartbeat signals to be extracted from the MM-wave life detection system than from the lower frequency systems. In addition, at the higher MM-wave frequency range, a smaller chip size could be developed for the CMOS MLDS detector. In this paper, a 60-GHz prototype MLDS is constructed by using V-band MM-wave waveguide components. A clutter canceller is implemented in the system with an attenuator and phase shifter. The clutter canceller performs the cancellation for the transmitted power leakage (to the receiver) and background reflection clutter to enhance the detecting sensitivity of the small vital signals. Noncontact vital-signal measurements have been taken on a human subject in four physical orientations from a distance of 2 m. The time-domain and spectrum waveforms of the measured breathing and heartbeat signals (by holding the

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Fig. 3. Block diagram of a 60-GHz MLDS.

breath) are presented. The prototype system will be useful for the development of the 60-GHz CMOS MLDS detector chip. II. MLDS SYSTEM DESIGN The block diagram of a noncontact life detection system is illustrated in Fig. 1. An RF signal source produces a continuous-wave (CW) carrier, and it is fed into a directional coupler. One output of the directional coupler is amplified by a power amplifier (PA) and fed through a circulator to the antenna. The other output of the directional coupler provides a local oscillator (LO) signal for the receiver. As shown in the figure, the leakage of transmitting power through the circulator and the environmental background (and the human body) reflection clutter will be much higher than the weak phase-modulated backscattered signal from the breathing and heartbeat vibration. These strong clutter signals may saturate the receiver and the detection sensitivity of the weak vital signal will be reduced (as demonstrated in the following measurement experiments). Hence, a clutter canceller in front of the receiver, as shown in Fig. 1, is very important for the vital signal detection [3]–[5]. The clutter canceller consists of an adjustable attenuator and phase shifter (see Fig. 3) with an input signal from the RF signal source branched through directional couplers. The signal power levels have been carefully measured at each output port of the detection components to ensure the clutter cancellation capability. The reflected signal power from the human body is measured by using the experimental setup as shown in Fig. 2. At a distance of 1 m, the measured receiving power is approximately 50 dBm when the transmitting power is set to be 0 dBm and the horn antenna gain is 25 dBi. The simulated backscattered receiving power from a

modeled muscle heart sphere is approximately 55 dBm at a distance of 1 m (see the appendix). These values will be useful for the power level analysis in the receiving blocks. The block diagram of the 60-GHz MLDS is depicted in Fig. 3. An MM-wave signal source provides a 60-GHz CW signal. This CW signal is fed through a 20-dB directional coupler, a V-band PA, and a circulator to a V-band horn antenna. The main output signal of the 20-dB directional coupler is then divided by a 3-dB hybrid-T coupler to provide a reference signal for clutter cancellation and a local signal (LO) for the mixer operation. The V-band horn antenna with a gain of 25 dBi radiates a 7-degree beam is aimed at the human subject. The transmitted power is approximately 8 dBm. The signal received by the antenna consists of clutter reflected from the background environment and a backscattered signal from the human body. It should also be noted that because the V-band circulator has only 18-dB isolation, there will be a large leakage of power from the PA output to the receiving circuit (as shown in Fig. 3). The measured leakage power is approximately 8 dBm. To be able to detect the weak vital signal modulated by breathing and heartbeat, the background clutter and the circulator leakage power have to be canceled before they saturate the receiving low-noise amplifier (LNA). The clutter canceller consists of an adjustable attenuator and phase shifter with an input signal from the RF signal source branched through directional couplers. The output signal of the clutter canceller serves to generate a signal which has the same amplitude and 180 phase difference of the clutter signal, and is combined with the receiving signal (mixed with the clutter) in a 3-dB hybrid-T coupler. The output of the coupler then mainly contains the

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weak backscattered phase-modulated signal (by the breathing and heartbeat) from the human body. It is then amplified by the LNA and mixed with the LO signal in a V-band mixer. Between the LNA and the mixer, a 20-dB directional coupler provides an output signal to be sent to a spectrum analyzer (through a V-band harmonic mixer) to monitor the cancellation degree of the 60-GHz clutter signal power. The mixer output provides the low-frequency breathing and heartbeat vital signals from the human body. The V-band mixer output is amplified by an operational amplifier, and it then passes through a bandpass filter (0.1–5 Hz) before being applied to a recorder. A digital scope with the analog-to-digital (A/D) converter is employed as the recorder. The scope records the detected time-domain vital signal waveforms that are converted to data files by the A/D converter and then processed by the FFT program to obtain the vital-signal spectrums. To demonstrate the importance of the clutter cancellation, two experimental measurement results of the breathing signal are shown in Fig. 4. Fig. 4(a) is the measured result with a clutter power level of 5 dBm shown in the monitoring spectrum analyzer and Fig. 4(b) is that with a value of 35 dBm. Significant improvement of the detection sensitivity is observed when the clutter power is highly reduced III. EXPERIMENTAL MEASUREMENT As shown in Fig. 3, the constructed 60-GHz MLDS is used to measure the vital signals of the human body in four orientations. The transmitting power is approximately 8 dBm and the horn antenna gain is 25 dBi. Fig. 5 shows the measured time- and frequency-domain of the breathing signal (superimposed with the heartbeat) of a human body at 1-m distance with a 20-s recording time. The clutter power level shown in the monitoring spectrum analyzer is 35 dBm. In each of these figures, the breathing signals are clearly recorded. The frequency-domain results show that the measured timedomain signal has a breathing signal at about 0.3 Hz. The other peaks shown are the harmonics of the breathing signal. Other small peaks are due to noise or harmonics of the breathing and heartbeat. It is also noted that the detected breathing signal is quite strong when the subject is in the orientation with the face to the right. This result may be due to the location of the heart inside the left chest. Fig. 6 shows the measured time- and frequency-domain of the heartbeat signal (obtained while the subject was holding the breath) of a human body at 1-m distance with a 20-s recording time. The reason to measure the pure heartbeat signal by holding the breath is to simulate a situation that a human subject may fall in a faint (due to injury or illness) and have no breath for a while. In this case, the acquiring heartbeat signal becomes very important for the monitoring or diagnosis of the physiological condition of the human subject. Again, in each of these figures, the heartbeat signals are clearly recorded. The frequency domain result in Fig. 6(a) shows that there is a dominant peak at about 1.4 Hz, which is the heartbeat rate. It is also observed that there are second and third heartbeat harmonic peaks at about 2.8 and 4.2 Hz. It is noted that when the measured signals in Figs. 5 and 6 are

Fig. 4. Experimental measurement results of the time-domain breathing signal of a human body at 1-m distance by the 60-GHz MLDS with clutter power levels: (a) 5 dBm and (b) 35 dBm.

0

0

compared, the amplitude of the breathing signal is significantly higher than that of the heartbeat signal, as expected. Finally, Fig. 7 shows the measured time- and frequency-domain vital signals of a human body (face front) at 2-m distance. The human vital signals maintain to be clearly recorded when compared with those at 1-m distance. Table I lists the specifications of V-band waveguide components used to construct the 60-GHz MLDS. The MM-wave signal source is an Agilent 67-GHz signal generator. Fig. 8 shows the photograph of the 60-GHz MLDS prototype. IV. DETECTION SENSITIVITY ANALYSIS In [14], the sensitivity for the noncontact viral-signal detection has been extensively discussed. The formula of the receiver sensitivity in communication systems is applied for analysis Sensitivity dBm

(1)

J/K) is Boltzmann’s constant, where ( K in room temperature, ( 174 dBm/Hz) is the input thermal noise floor per unit bandwidth, is the bandwidth of the receiver, NF is the noise figure of the receiver, and SNR is the signal-to-noise ratio required for baseband (vital-signal) signal

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Fig. 5. Measured time- and frequency-domain breathing signal (superimposed with the heartbeat signal) of a human body in four orientations at 1-m distance by the 60-GHz MLDS: (a) face front; (b) face back; (c) face left; and (d) face right.

detection. To estimate the SNR value for the vital-signal detection, the detected vital signals shown in Fig. 9 are used for reference. For the vital-signal detection in the MLDS, if the waveform magnitude ratio of the detected vital signal to the noise floor in more than 5 (or 14 dB), it should be sufficient for clear identification of the vital-signal waveform. This value also corresponds to that (10 to 20 dB) indicated in [14]. Hence the SNR is reasonably set to be 14 dB for the sensitivity analysis of the 60-GHz MLDS. As the heartbeat signal frequency is around 1 Hz and at 60-GHz the receiver will have a higher LO phase-noise than that of the lower frequency system, the receiver could have a high NF due to LO flicker noise contribution. Also, since the clutter signal (including the circulator leakage to the Rx port) may not be cancelled completely in the receiving system, the residual clutter signal will mix with the LO signal and result in much more flicker noise at 1 Hz. This effect will deteriorate

the noise figure performance again. Hence, compare with the estimated NF value of 40 dB in [14], the 60-GHz MLDS receiver could have a high NF as to 60 dB. From the above arguments and 5-Hz the bandwidth of the baseband bandpass filter, the sensitivity can be determined as Sensitivity

93 dBm (2)

25 As for the receiving power to the horn antenna ( dBi) from reflected wave of the simulated heart muscle sphere with different distances at 60 GHz, it can be determined from the ) and radar (A2) and is listed in Table II. calculated RCS ( Fig. 10 shows the plot of the estimated detection sensitivity and the simulated receiving power from the reflected wave of the heart muscle sphere with different distances. Basically, the detecting distance should be able to be more than 5 m (based on the estimated sensitivity of 93 dBm).

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Fig. 6. Measured time- and frequency-domain heartbeat signal (obtained while the subject was holding the breath) of a human body in four orientations at 1-m distance by the 60-GHz MLDS: (a) face front; (b) face back; (c) face left; and (d) face right.

V. CONCLUSION This paper presented the first reported experimental measurements of a 60-GHz MLDS with a clutter canceller for noncontact human vital signal monitoring. This 60-GHz MLDS system is constructed by using V-band MM-wave waveguide components. The clutter canceller uses an adjustable attenuator and phase shifter to cancel the transmitting power leakage from the circulator and background reflection clutter to enhance the detecting sensitivity of the weak vital signals. The noncontact vital signal measurements have been conducted for four different orientations of the human test subjects at 1- and 2-m distances. The time-domain and spectrum waveforms of the measured breathing and heart-

beat (obtained while the subjects was holding the breath) are presented. The experimental measurements show clearly recorded waveforms of the breathing and heartbeat signals when the clutter is cancelled to a low power level by properly adjusting the attenuator and phase shifter. The theoretical plot and discussion of MLDS detection sensitivity with respect to the distance are also presented. Based on the estimated detection sensitivity of 93 dBm, the detection distance up to 5 m (or beyond) is expected by using the current 60-GHz MLDS system. The prototype system will be useful for the development of the 60-GHz CMOS MLDS detector chip, which can be incorporated in a hand held device (such as a cellular handset) for wireless healthcare applications.

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Fig. 8. Photograph of a 60-GHz MLDS prototype constructed by using V-band waveguide components.

TABLE I SPECIFICATIONS OF V-BAND WAVEGUIDE COMPONENTS IN 60-GHz MLDS

Fig. 7. Measured time- and frequency-domain vital signals of a human body (face front) at 2-m distance by the 60-GHz MLDS: (a) breathing and (b) heartbeat. Fig. 9. Measured waveforms of the breathing and heartbeat signals (at 1-m distance) with the background noise displayed on the recorded scope.

APPENDIX In order to access the receiving power reflected from the heart for the MLDS system design, the human heart is modeled as a 5 cm. The relative permittivity muscle sphere with a radius ( ) is 12.856 and the conductivity ( ) is 52.826 S/m for the muscle at 60 GHz [11]. As shown in Fig. 11, the backscat) from the muscle heart sphere can be determined tered filed ( by using the vector spherical wave function [12]

(A1) is the amplitude of the incident plane wave, is where the complex permittivity of the muscle sphere, is its radius, and represent wavenumbers of free space and the muscle sphere, respectively. Also typical notations for Bessel functions and their derivatives are employed in the formula. can The radar cross section (RCS) of the heart sphere m at 60 GHz. then be computed, which is If a V-band horn antenna with a gain of 25 dBi and a transmitting

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Fig. 10. Estimated 60-GHz MLDS detection sensitivity and the simulated receiving power from the reflected wave of the heart muscle sphere with different distances (transmitting power P t 8 dBm and antenna gain G 25 dBi).

=

=

TABLE II RECEIVING POWER OF THE HORN ANTENNA FROM THE REFLECTED WAVE OF THE SIMULATED HEART MUSCLE SPHERE WITH DIFFERENT DISTANCES AT 60 GHz

[4] K. M. Chen, Y. H. Huang, J. Zhang, and A. Norman, “Microwave life-detection systems for searching human subjects under earthquake rubble or behind barrier,” IEEE Trans. Biomed. Eng., vol. 27, no. 1, pp. 105–114, Jan. 2000. [5] C. Li, J. Cummings, J. Lam, E. Graves, and W. Wu, “Radar remote monitoring of vital signs,” IEEE Microw. Mag., vol. 10, no. 1, pp. 47–56, Feb. 2009. [6] B. Park, O. Boric-Lubecke, and V. M. Lubecke, “Arctangent demodulation with DC offset compensation in quadrature Doppler radar receiver systems,” IEEE Trans. Microw. Theory Tech., vol. 55, no. 5, pp. 1073–1079, May 2007. [7] Y. Xiao, C. Li, and J. Lin, “A portable noncontact heartbeat and respiration monitoring system using 5-GHz radar,” IEEE Sensors J., vol. 7, no. 7, pp. 1042–1043, Jul. 2007. [8] Y. Xiao, J. Lin, O. Boric-Lubecke, and V. M. Lubecke, “Frequency tuning technique for remote detection of heartbeat and respiration using low-power double-sideband transmission in Ka-band,” IEEE Trans. Microw. Theory Tech., vol. 54, no. 5, pp. 2023–2032, May 2006. [9] C. Li, Y. Xiao, and J. Lin, “Experiment and spectral snalysis of a low-power Ka-band heartbeat detector measuring from four dides of a human body,” IEEE Trans. Microw. Theory Tech., vol. 54, no. 12, pp. 4464–4471, Dec. 2006. [10] P. Smulders, “Exploiting the 60 GHz band for local wireless multimediaaccess: Prospects and future directions,” IEEE Commun. Mag., vol. 40, no. 1, pp. 140–147, Jan. 2002. [11] Italian National Research Council, Florence, Italy, “Dielectric properties of body tissues: HTML clients,” 2007. [Online]. Available: http:// niremf.ifac.cnr.it/tissprop/htmlclie/htmlclie.htm#atsftag [12] R. F. Harrinton, Time Harmonic Electromagnetic Fields. New York: McGraw-Hill, 1961. [13] D. M. Pozar, Microwave and RF Wireless System Design. New York: Wiley, 2001. [14] C. Li, X. Yu, C.-M. Lee, D. Li, L. Ran, and J. Lin, “High-sensitivity software-configurable 5.8-GHz radar sensor receiver chip in 0.13- m CMOS for noncontact vital sign detection,” IEEE Trans. Microw. Theory Tech., vol. 58, no. 5, pp. 1410–1418, May 2010.

55 dBm at

Huey-Ru Chuang (SM’06) received the B.S.E.E. and M.S.E.E. degrees from the National Taiwan University, Taipei, Taiwan, R.O.C., in 1977 and 1980, respectively, and Ph.D. degree in electrical engineering from Michigan State University, East Lansing, Michigan, in 1987. From 1987 to 1988, he was a Post-Doctoral Research Associate with the Engineering Research Center, Michigan State University. From 1988 to 1990, he was with the Portable Communication Division, Motorola Inc., Ft. Lauderdale, FL. Since 1991, he has been with the Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, R.O.C., and currently he is a Professor in the Institute of Computer and Communication Engineering, Department of Electrical Engineering, National Cheng Kung University. His research interests include microwave/millimeter-wave circuits and systems, RF integrated circuits (RFICs) and antenna design for wireless communications, electromagnetic computation and applications, and microwave/MM-wave communication/detection systems.

[1] J. C. Lin, “Microwave sensing of physiological movement and volume change: A review,” Bioelectromagn., vol. 13, pp. 557–565, 1992. [2] K. M. Chen, D. Misra, H. Wang, H.-R. Chuang, and E. Postow, “An X-band microwave life-detection system,” IEEE Trans. Biomed. Eng., vol. 33, no. 7, pp. 697–702, Jul. 1986. [3] H. R. Chuang, Y. F. Chen, and K. M. Chen, “Automatic clutter-canceller for microwave life-detection system,” IEEE Trans. Instrum. Meas., vol. 40, no. 4, pp. 747–750, Aug. 1991.

Hsin-Chih Kuo (S’10) received the B.S.E.E. degree from Feng Chia University, Taichung, Taiwan, in 2006, the M.S.E.E degree from National Cheng Kung University, Tainan, Taiwan, in 2008, and is currently working toward his Ph.D. degree in the Institute of Computer and Communication Engineering, Department of Electrical Engineering, National Cheng Kung University. His research interests include millimeter-wave RFIC design and RF system design.

Fig. 11. Illustration of EM wave scattering from the muscle heart sphere.

power ( ) of 0 dBm is used to radiate the muscle heart sphere 1 m, since the wavelength at 60 GHz is 5 mm at a distance ( 1 m), the radar equation [13] can then be applied to calculate the receiving power ( ) (A2) The calculated receiving power 1 m.

at 60 GHz is

REFERENCES

CHUANG et al.: 60-GHZ MLDS FOR NONCONTACT HUMAN VITAL-SIGNAL MONITORING

Fu-Ling Lin (M’00) received the B.S.E.E., M.S.E.E., and Ph.D. degree in electrical engineering from National Central University, Chung-Li, National Tsing Hua University, Hsinchu, and National Cheng Kung University, Tainan, Taiwan, in 1984, 1990 and 2000, respectively. From 1984 to 1988 and 1990 to 1995, he was a Wireless Communication Engineer at the Chung Shan Institute of Science and Technology (CSIST), Taiwan. Currently he is an assistant Professor in the Department of Electronic Engineering at Southern Taiwan University, Tainan, Taiwan. His research interests include digital wireless communication and RF system design.

Tzuen-Hsi Huang (M’96) received the B.S. degree in electrical engineering from National Cheng Kung University, Tainan, in 1988, and the Ph.D. degree in solid-state electronics from National Chiao Tung University, Hsinchu, Taiwan, in 1995. From 1995 to 2001, he worked at ERSO/ITRI, Hsinchu, Taiwan. He was involved in high-speed poly-emitter bipolar technology development, device characterization/modeling, and RF circuit design. From 2001 to 2004, he was with AIROHA Technology, Hsinchu, where he was involved in GSM/WLAN front-end chip design. In August 2004, he joined the faculty of the Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, where he is currently an Associate Professor. His research interests include CMOS RF and MM-wave integrated circuit designs.

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Chi-Shin Kuo (S’11) received the B.S.E.E degree from University of Tainan, Tainan, Taiwan, in 2009. He is currently working toward his M.S. degree in the Institute of Computer and Communication Engineering, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan. His research interests include millimeter-wave RFIC design and RF system design.

Ya-Wen Ou (S’11) received the B.S. degree in Communication Engineering from Feng Chia University, Taichung, Taiwan, in 2009. She is currently working toward the M.S. degree in the Institute of Computer and Communication Engineering, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan. Her research interests include millimeter-wave RFIC design and RF system design.