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Recent Advances in Doppler Radar Sensors for Pervasive Healthcare

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Recent Advances in Doppler Radar Sensors for Pervasive Healthcare 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 Copyright 2010 IEICE 283 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 284 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.
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