Recent Advances in Doppler Radar Sensors for Pervasive Healthcare

Home , Doppler radar

Proceedings of Asia-Pacific Microwave Conference 2010 WE4C-5 Recent Advances in Doppler Radar Sensors for Pervasive Healthcare Monitoring Changzhi Li 1, Jenshan Lin 2 1Department of Electrical and Computer Engineering, Texas Tech University Electrical and Computer Engineering, Box 43102, Lubbock, Texas, 79409, USA [email protected] 2 Department of Electrical and Computer Engineering, University of Florida 559 Engineering Building, Gainesville, Florida, 32611, USA [email protected]

Abstract — This paper reviews recent advances in alarm goes off. With growing interests in health and life technologies using Doppler radar to detect heartbeat and sciences in engineering community, many researchers have respiration of a human subject. With contributions from many been contributing to the technology advancement in this researchers in this field, new detection methods and system architectures have been proposed to improve the detection field. This paper reviews the achievements reported in recent accuracy and robustness. The advantage of noncontact/covert years, especially the last three years from 2008 to 2010 [18]- detection has drawn interests on various applications. While [56], and discusses several architectures with respect to many of the reported systems are bench-top prototypes for monolithic integration. Most of the results referenced in this concept demonstration, several portable systems and integrated paper were published on IEEE journals and conference radar chips have been demonstrated. This paper reviews different architectures and discusses their potentials for proceedings. Although many results were demonstrated using integrated circuit implementation. Integrating the radar sensor bench-top prototypes or board-level integration, their on a chip allows the function of noncontact vital sign and architectures still show the potential of being implemented on vibration detection to be embedded in portable electronic chip. In addition, there have been several reports of vital sign equipment, like many other radio frequency (RF) devices. A radar sensor chips using some of the architectures radar sensor network is then feasible for pervasive monitoring in healthcare applications. [21][33][52]. Index Terms — Doppler radar, noncontact measurement, vital The paper starts with a comparison to UWB vital sign sign, heartbeat, respiration, cardiopulmonary, sensor, radar, which uses a fundamentally different principle for healthcare, vibration, integrated circuit. detection, in Section II. The paper then focuses on CW Doppler radar approach and discusses the various architectures of RF front-end in Section III and different I. INTRODUCTION baseband demodulation and signal processing methods in Doppler radar has been widely used in a number of Section IV. A conclusion with discussion on future applications including vehicle speed measurement and storm applications and challenges is given in Section V. tracking. The same principle, detecting the frequency or phase shift in a reflected radar signal, can be used to detect II. CW DOPPLER RADAR VS. UWB PULSE RADAR tiny body movements induced by breathing and heartbeat, without any sensor attached to the body. The noncontact There are two main categories of noncontact vital sign remote detection of vital signs lead to several potential detection radar: continuous wave (CW) Doppler radar and applications such as searching survivors after earthquake and ultra-wideband (UWB) pulse radar. monitoring sleeping infants or adults to detect abnormal A. CW Doppler Radar breathing condition. While the concept of noncontact detection of vital signs has been successfully demonstrated by pioneers in this field before 2000 [1]-[4], research efforts in the first decade of this century have been moving the ADC technology development toward lower power, lighter weight, smaller form factor, better accuracy, longer detection range, λ DSP and more robust operation for portable and handheld Ant x()tm=⋅ sin(ω t )

applications. Among many possible applications this d0 technology can be used for, the applications in healthcare

seems to be drawing most of the interests [17]. As an Fig. 1. Scenario of CW Doppler radar vital sign detection. example, a baby monitor using this technology was recently

demonstrated [30]. The baby monitor integrates a low power The scenario of CW Doppler radar vital sign detection is Doppler radar to detect tiny baby movements induced by the shown in Fig. 1. An un-modulated signal T(t) = cos(2πft + Φ ) breathing. If no movement is detected within 20 seconds, the 1

with carrier frequency f and residual phase Φ1 is transmitted significantly alleviated by the range-correlation effect at short toward a human body, where it is phase-modulated by the detection distance [9]. By using the same transmitted signal physiological movement x(t). The reflected signal captured as the LO of the down-converter, the LO phase noise is by the radar receiver is represented as R(t) = cos[2πft - effectively cancelled out, making high sensitivity vital sign 4πx(t)/λ + Φ2], where Φ2 is a constant phase shift due to detection radar possible. nominal detection distance d0 and the phase noise. Using the The choice of carrier frequency is very important. [15] has same transmitted signal T(t) as the local oscillator (LO) signal, demonstrated that there exists an optimal carrier frequency the radar receiver down-converts the received signal R(t) into for people with different physiological movement strength. baseband signal B(t) = cos[4πx(t)/λ + ΔΦ], where ΔΦ is Carrier frequencies ranging from hundreds of MHz [6] to determined by the nominal detection distance and oscillator millimeter wave frequency [31] has been tested for phase noise. noncontact vital sign detection. In [31], a 228 GHz carrier After analog-to-digital conversion (ADC), the information was used for noncontact vital sign detection for three reasons. x(t) related to physiological movement (i.e. heartbeat and First, shorter wavelengths provide a greater sensitivity to respiration) can be identified by proper digital signal small displacement. Second, this frequency is in an processing (DSP). Fig. 2 shows an example of the baseband atmospheric window with at least 50% single-pass signal and spectrum detected using a CW Doppler radar transmission [12]. Finally, higher frequency can maintain a integrated on a single CMOS chip. The radar chip has a collimated beam over much greater distances for reasonable homodyne quadrature architecture, and thus has two output aperture sizes, and the radar cross section of the vital sign channels (I/Q) as will be discussed in Section III-A. area may also increase as frequency increases. This 228 GHz system has successfully extended the respiration and heart rate measurement to a range of 50 m. However, using very 20 high carrier frequency makes it difficult to measure 0 respiration and heartbeat together. The results in [31] only -20 showed the detection of heartbeat while the subject was

I/Q Signal [mV] -40 holding the breadth. Nevertheless, the encouraging result 0 5 10 15 20 25 verified that long range detection is feasible by using higher Time (Second) 1 carrier frequency. Respiration B. UWB Pulse Radar 0.5 Heartbeat In a UWB pulse radar, the transmitter sends very short

CSD Spectrum electromagnetic pulses toward the target. The typical pulse 0 0 20 40 60 80 100 120 duration for vital sign detection is around 200~300 ps, and Beats/Min the pulse repetition frequency is in the range of 1~10 MHz.

Fig. 2. Baseband I/Q channel signal (a) and spectrum (b) When the transmitted pulse reaches the chest wall, part of the detected from the front of a human subject at 1.5m away. energy is reflected and captured by the receiver. The nominal From [33]. round-trip travelling time of the pulse is t = 2d/C, where d is the nominal detection distance and C is the speed of Because the same transmitted signal is used as the LO to electromagnetic wave. If a local replica of the transmitted down-convert the received signal which is phase-modulated pulse with a delay close to the nominal round-trip travelling by the physiological movement, there is no frequency offset time is correlated with the received echo, the output in the baseband. The timing delay does not affect the correlation function will have the same frequency as the detection either. Therefore, no synchronization mechanism is physiological movement. required for the system. It should be noted that the radar There are two ways to build such a UWB pulse radar. In block diagram in Fig. 1 is simplified. For the implementation one approach as shown in Fig. 3 (a) [23], the pulse generator of robust CW Doppler vital sign detection radar, building is activated by the negative edge of a digital control signal. blocks such as low noise amplifier (LNA), baseband The delay generator provides a digitally programmable delay amplifier, and filter are needed. Different front-end time of the aforementioned control signal in the range of 1-3 architectures can be used for the radar, as will be discussed in ns. The 5-bit programmable delay extends the flexibility and Section III. the ranging of the radar system. The integrator output is In radar applications, the phase noise of the LO can mix proportional to the correlation between the delayed pulse and with the received echo signal and disrupt the desired the echo pulse. As a result, the signal at the output of the baseband signal if the phase noise of the two signals entering integrator is time-variant with the frequency of the the mixer is not correlated. Mathematically, it means the ΔΦ physiological movement monitored, and thus it contains the term in the baseband signal B(t) may disrupt accurate information of heart beat and respiration. detection of x(t) due to phase noise. This challenge is

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In the other approach as shown in Fig. 3 (b) [25], the pulse focus on recent development of CW radar for noncontact generator forms two short UWB pulses in every pulse motion detection in healthcare applications. repetition period. The first short pulse from the generator enters the transmit chain then is transmitted toward the target III. RF FRONT-END ARCHITECTURES through the antenna. The second short pulse from the generator is directed into a reference channel of the phase Various RF front-end architectures for noncontact motion detector. When the transmitted pulse is reflected by the detection have been reported. In addition to previously used target, it is captured by the receiving antenna, amplified, and homodyne, heterodyne, and double-sideband architectures, then fed into the other channel of the phase detector. As a direct intermediate frequency (IF) sampling and self-injection result, the phase detector plays the role of correlation and locking were recently reported. generates in-phase and quadrature (I/Q) outputs containing A. Homodyne vital sign information. It is known that a single-channel direct conversion Doppler (a) radar has a null detection point problem [9], which means the detected signal is very weak at certain detection distances. To eliminate this problem, quadrature radar architecture can be used. A block diagram and a chip microphotograph of homodyne motion detector are shown in Fig. 4. It has been implemented and demonstrated in [9] that since there are in-phase and quadrature (I/Q) baseband outputs,

(b) there is always one channel not at the null detection point, thus eliminating the null detection point problem. Moreover, since the vital sign signal has low bandwidth, the two output channels can be combined in software to perform complex signal demodulation [18] or arctangent demodulation [16] as low-cost baseband solutions, which will be discussed in Section III.

(a)

0 TxA 90 Fig. 3. Block diagram of UWB vital sign detection radar of: Power LO (a) delay cell, multiplier, and integrator solution [23]; (b) I Splitter RxA phase detector and double-pulse-generator solution [25]. Q C. CW vs. UWB Radar LNA By controlling the delay between the two inputs of the (b) correlation function block, the detection range of UWB radar can be changed since the delay corresponds to the signal round-trip travelling time. This mechanism makes it possible for UWB radar to eliminate interference caused by reflection from other objects (clutter) and multi-path reflection. This is the main advantage of UWB vital sign detection radar. However, the disadvantage of UWB radar is, the delay need to be calibrated once the detection distance changes, thus increasing the system complexity and cost. Also, since the correlation function may be nonlinear, it may be nontrivial for UWB radar to recover the original movement pattern even though frequency information can be easily obtained. CW radar has the advantages of low power consumption Fig. 4. (a) Simplified block diagram of homodyne motion and simple radio architecture. Moreover, CW radar can also detector; (b) chip microphotograph of a homodyne motion cancel out clutter noise by proper adjustment of the radio detector fabricated in 0.13 µm CMOS process. front-end architecture [37][53]. Also, MIMO/SIMO B. Heterodyne [11][14][20] technologies can be easily implemented with CW radar for the detection of multiple targets and multiple Heterodyne radio architecture [4][6] has been the only movements. Therefore, the remaining part of this paper will solution for noncontact vital sign detection until the homodyne noncontact vital sign detector was first

285 demonstrated at the beginning of this century [7][8]. The double-sideband transmission method with indirect- Compared with the homodyne architecture, the heterodyne conversion architecture eliminates the need for the image- architecture has the advantage of robust against DC offset. reject filter and IF filter, thus can be monolithically integrated. However, single-channel heterodyne radar has the null A double-sideband vital sign detector integrated in 0.18 µm detection point problem. In order to overcome the null CMOS process has been demonstrated in [21]. The double- detection point problem, quadrature architecture or sideband transmission method also eliminates the need of frequency-tuning on a single channel has to be used. In 2005, generating quadrature LO signals. a double-sideband transmission heterodyne architecture was D. Direct IF Sampling proposed to eliminate the need for generating quadrature LO and several filtering requirements in traditional heterodyne For the conventional quadrature direct-conversion architecture [10]. architecture, I/Q imbalance and DC offset are two inevitable challenges that degrade the demodulation accuracy and the C. Double-Sideband output signal-to-noise ratio (SNR). To ensure I/Q amplitude Fig. 5 shows a simplified block diagram and a chip and phase balance, complicated calibration procedures such microphotograph of the double-sideband radar architecture. as the Gram-Schmidt procedure have to be applied [5]. By In a double-sideband radar [13], both upper and lower using a digital I/Q demodulation technique, the two output sidebands are transmitted by mixing signals from two VCOs. channels are in perfect quadrature phase relationship, making In the receiver, the two sidebands are automatically combined calibration procedures unnecessary. To overcome the DC by two-stage down-conversion. The signal detected by either offset problem, subsystems such as the DC offset the upper or the lower sideband has null detection and compensation unit [16] are needed. However, these methods optimal detection points separated by 1/8 of the signal add to the system complexity and increase the hardware cost. wavelength. When the two sidebands are combined together, If configured with a heterodyne architecture, there will be no the distance between optimal and null detection points is significant amount of DC offset. Therefore, direct IF changed to λIF/16 [13], where λIF is the wavelength at the IF sampling with heterodyne architecture has been proposed for and thus resulting in a much longer separation. Moreover, noncontact vital sign detection [26]. Fig. 6 shows the block when the IF is tuned, the location of optimal/null detection diagram of direct IF sampling architecture. In the receiver points can be changed [13]. Therefore the null detection point chain, the mixer after LNA down-converts the received problem can be eliminated by IF tuning. signal into an intermediate frequency. Then high speed ADC is used to convert the analog IF signal to digital. After that, (a) quadrature demodulation is performed using a high speed DSP.

Power Power TxA Splitter LO1Splitter LO2 Digital I out IF TxA f1 f2 900 IF_AMP LNA RxA LNA ADC RxA Out Q (b) out

Fig. 6. Direct IF sampling architecture.

Extensive experimental verifications have been carried out in [56] to demonstrate the direct IF sampling architecture’s advantages in alleviating I/Q imbalance and eliminating the complicated DC offset calibration. Since the information bandwidth of vital sign signals is very small, IF can be chosen at a low frequency such that the demand on ADC sampling speed can be relaxed. Therefore, this architecture is

suitable for integrated circuit implementation. Fig. 5. (a) Block diagram of the double-sideband radar architecture; (b) chip microphotograph of a double-sideband E. Self-Injection Locking radar detector fabricated in 0.18 µm CMOS process. Unlike all previous architectures that were adopted from

RF front-end architectures used in wireless communications, a new approach of using self-injection locking to detect vital

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signs was proposed in 2010 [54]. The architecture is based on Another way to eliminate the optimum/null detection point an injection-locked oscillator that was previously used for problem in the quadrature demodulation system is to use spectrum sensing [35]. By transmitting the oscillator signal to arctangent demodulation [16] by calculating the total Doppler a human subject through an antenna and using the reflected phase shift as Ψ = arctang(Q/I). Since Ψ is directly signal as the injection signal to the oscillator itself, the vital proportional to the movement x(t), desired motion sign movement modulating on the frequency (or phase) of the information can be recovered reliably. However, DC offset injection signal due to Doppler effect can be demodulated calibration is required before performing the arctangent through this self-injection locking mechanism [54]. In this demodulation. architecture (Fig. 7), the spectrum sensing and vital sign sensing can be performed concurrently as long as the radar (a) I − Re FFT round-trip time delay is much smaller than the oscillator DC St() 1 frequency scan time. The analysis of modulation and noise 0.5

Q 0 transfer functions showed that this self-injection locking 0 50 100 − Im Complex method has higher signal gain at low frequency modulation DC and better noise attenuation with increased delay time (longer (b) distance), which is desirable for vital sign detection. Its I FFT 1 measurement result demonstrated successful detection of an DC Q ψ ()t arctan 0.5 Calibration Q I + 0 adult subject's vital signs at 50 cm away, with about 0 dBm 0 50 100 of VCO output power. Even though the demonstrated system Fn=⋅1800 was not an integrated circuit solution, the proposed architecture can potentially be monolithically integrated. Fig. 8. Block diagram of complex signal demodulation (a) and arctangent demodulation (b) realized in software.

B. Noise Cancellation and Multiple Subject Detection It was found in experiments that other major problems of the Doppler non-contact motion detection include the noise caused by other motion artifacts and the presence of multiple subjects. The multiple-input, multiple-output (MIMO) technique was then proposed to solve the problems. Another proposed approach is the single-input, multiple-output (SIMO) technique. In [11] and [14], the single and multiple antenna systems and SIMO/MIMO signal processing were Fig. 7. Block diagram of the concurrent spectrum and vital explored to isolate desired radar return signals from multiple sign sensing architecture. From [54]. subjects. A generalized likelihood ratio test (GLRT), based on a model of the heartbeat, has been developed to show that this technique can be used to distinguish between the IV. BASEBAND SIGNAL PROCESSING METHODS presence of 2, 1, or 0 subjects, even with a single antenna. Furthermore, this technique was extended to detect up to 2N- Various methods have been used to process the baseband 1 subjects using N antennas. signals for noncontact motion detection. They are In the meantime, multiple transceivers have been used to implemented in analog or digital domain, or a combination of cancel the noise caused by motion artifacts such as random both. body motion. It has been demonstrated that based on the A. Demodulation different movement patterns of physiological movement and random body movement, it is possible to cancel out the noise Complex signal demodulation and arctangent caused by random body movement using two transceivers demodulation are two popular demodulation methods for detecting from two sides of the human body [27]. noncontact motion detection. Their simplified block diagrams Another solution to improve the performance against are presented in Fig. 8. random body motion is the differential front-end Doppler The complex signal demodulation can eliminate the radar operating at two different frequencies. By using dual optimum/null detection point problem by combining the I and helical antennas each with a 40-degree beamwidth, it is Q signals in baseband to form a complex signal [18]. Since possible to illuminate the body in two adjacent locations to the amplitude of the complex signal is not affected by the perform a differential measurement. Since only one of the residual phase, desired signal components can be reliably beams illuminates the heart, the baseband signal from the identified from its spectrum. second radar is used for motion cancellation. A dual helical

287 antenna and simple direct-conversion radar were movements were demonstrated, more sophisticated signal implemented to demonstrate this approach [34]. processing methods are still needed to make it more robust to Similar approaches such as two-frequency radar sensor measure a subject's vital signs while in motion, e.g., walking [19] and multifrequency interferometric radar [32] have also or running. been reported recently. Combined with advanced signal Even without physical body movement artifacts, the processing, these technologies can improve the performance accurate measurement of respiration rate and heartbeat rate of noncontact vital sign detection. can be complicated when abnormal cardiopulmonary activities happen. The combination of tachypnea (rapid C. Spectral Estimation breathing) and bradycardia (slow heartbeat) that might Another challenge encountered in vital sign detection is the possibly happen to newborn infants is an example [39]. In presence of undesired harmonic terms and intermodulations this case the rates might be mistakenly measured. Advanced other than the sinusoids of interest. For example, the third- signal processing and hardware will be needed to accurately and forth- order harmonics of respiration signal are very diagnose this syndrome. close to the frequency of heartbeat, complicating the output spectrum [15]. A spectral estimation algorithm is needed to ACKNOWLEDGEMENT accurately estimate the sinusoidal frequencies before identifying the heartbeat and respiration rates. Sometimes the The authors would like to thank many researchers who found conventional periodogram obtained from Fourier transform this subject interesting and contributed to the advances in the cannot reliably separate the rich sinusoidal components since technology development. 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