**2. Theory and methods**

Here, we describe a system that employs microwaves to remotely measure vital signs by detecting vibrations on the body surface induced by cardiac and respiratory activity. Vibrations induced by heartbeat are particularly small with amplitudes of about 0.1–0.2 mm on average. This section discusses approaches using continuous-wave (CW) Doppler radar and ultra-wideband (UWB) pulse radar, which are generally used for measuring vital signs, and their mechanisms.

### **2.1 Mechanisms of measurement**

While frequency-modulated continuous wave (FMCW) radar is used to identify the exact location of a subject in some reports, UWB or CW Doppler radar are generally used for monitoring vital signs. (Saunders, 1990, Immoreev & Tao, 2008, Li & Lin, 2010)

In a UWB pulse radar, the transmitter sends very short electromagnetic pulses toward the target. A pulse duration of about 200–300 ps and a pulse repetition frequency in the range of 1–10 MHz are typically used for vital sign detection. When the transmitted pulse reaches the chest wall, some of the energy is reflected and captured by the receiver. The nominal round-

Remote Sensing for Medical and Health Care Applications 483

characteristic response for vital sign measurement. Some studies used extremely highfrequency waves (228 GHz (Petkie et al., 2009)), which have shorter wavelengths and are more sensitive to small displacements. Moreover, a 228 GHz frequency prototype has been extended to perform heart rate and respiration measurements at a distance of 50 m. However, such high frequency waves are not realistic for monitoring vital signs in everyday applications. In many cases, a carrier frequency that does not require a license is often chosen. However, carrier frequencies that do not require a license vary from country to country and some frequency bands are allocated to amateur radio stations. For example, the laws regulating radio frequency use in Japan allow band frequencies 10.525 and 24.15 GHz to be used for detecting moving objects. These devices are marketed as sensors for measuring the speeds of vehicles. Although there are limitations on how they are used (e.g., limited to indoor use), these frequency bands can be used by low-power radio stations without a license provided the output is less than 10 mW. They have been increasing studies on frequency bands for vital sign monitoring. Regarding safety, different countries have

The World Health Organization (WHO) and the Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR) define exposure as *the subjection of a person to electric, magnetic, or electromagnetic fields or to contact currents other than those originating from physiological processes in the body and other natural phenomena* (WHO, 2003, SCENIHR, 2006). The intended frequency band of electromagnetic field intensity differ slightly in different guidelines. For example, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines (ICNIRP, 1998) specify the frequency band from 300 Hz to 300 GHz, SCENIHR specifies 100 Hz to 300 GHz (SCENIHR, 2006), and the IEEE Standard is from 3 kHz to 300 GHz (IEEE Standard Committee, 1998). Each country employs different methods for determining their criteria. Consequently, it is important consider device

The carrier frequency of medical applications of ultrasound is a low frequency of about 3 to 10 MHz. Such applications acquire information by penetrating the human body. In comparison, monitoring using microwave frequencies in the range 10.525 to 24.15 GHz is considered to be less invasive and safer. It is not easy to make simple comparisons, but wireless local area networks (WLANs) use 2.4 GHz radio waves and microwaves in the range 10.525 and 24.15 GHz are considered to be safer. Moreover, safety can be further

It seems appropriate to use high frequencies for sensing to ensure a high resolution while considering invasiveness. However, high frequencies are not necessarily ideal for actual applications as increased sensitivity results in increased susceptibility to artifacts; the target motion induced by heartbeat on the body surface is much smaller than the artifacts generated by general movement of the body and arm. Furthermore, people being monitored move freely as they conduct everyday activities, which makes artifacts a significant

While remote sensing is not currently used for medical and health care applications in everyday life, several studies have been conducted. This section discusses the following

different guidelines regarding radio-frequency electromagnetic fields.

development and intended usage.

**3. Examples of applications** 

problem.

increased by using a lower power than a WLAN.

trip travel time of the pulse is defined as *t dC* 2 , where d is the nominal detection distance and *C* is the speed of the electromagnetic wave. If a local replica of the transmitted pulse with a delay close to the nominal round-trip travel time correlates with the received echo, the output correlation function will have the same frequency as the physiological movement.

On the other hand, the CW Doppler radar mechanism is based on following (1);

$$T(t) = \cos\left[2\pi ft + \varphi(t)\right] \tag{1}$$

where an unmodulated signal *T*(*t*) with a carrier frequency *f* and a residual phase *t* , is transmitted toward a human body where it is phase-modulated by the physiological movement *x*(*t*). The reflected signal *R*(*t*) detected by the radar receiver is given by following (2);

$$R(t) \approx \cos\left[2\pi ft - \frac{4\pi d\_0}{\lambda} - \frac{4\pi \chi\left(t\right)}{\lambda} + \varphi\left(t - \frac{2d\_0}{c}\right)\right] \tag{2}$$

where 4 *d*<sup>0</sup> is a constant phase shift due to the nominal detection distance *d0* and the 2 <sup>0</sup> *t dc* is phase noise. Using the same transmitted signal *T*(*t*) as the local oscillator signal, the radar receiver down-converts the received signal *R*(*t*) to the baseband signal *B*(*t*) as following (3);

$$B(t) \approx \cos\left[\frac{4\pi d\_0}{\lambda} + \frac{4\pi\chi\left(t\right)}{\lambda} + \theta\_0 + \Delta\varphi\right] \tag{3}$$

where is determined by the nominal detection distance and the oscillator phase noise.

Since the delay corresponds to the signal round-trip travel time, the detection range of a UWB radar can be varied by controlling the delay between the two inputs of the correlation function block. This makes it possible to eliminate interference caused by reflection from other objects (clutter) and multipath reflection. However, one disadvantage of UWB radar is that the delay needs to be recalibrated when the detection distance is changed; this increases the system complexity and cost. Furthermore, since the correlation function is nonlinear, it is not simple to recover the original movement pattern, even though frequency information can be easily obtained. On the other hand, CW Doppler radar has a low power consumption and a simple radio architecture. These characteristics make it suitable for home-based systems. Moreover, proper adjustment of the radio front-end architecture of a CW radar can cancel clutter (Li & Lin, 2008a, 2008b). In addition, single-input multi-output and multiinput multi-output techniques can be easily implemented with CW radar, enabling the movements of multiple targets to be detected (Boric-Lubecke et al., 2005, Zhou et al., 2006).

#### **2.2 Carrier frequency and output power**

The carrier frequency and output power employed must be safe for use on people. Carrier frequencies ranging from hundreds of megahertz to millimeter wave frequencies have been tested for remote vital sign detection using a variety of physiological movements. The carrier frequency should be carefully selected to ensure suitable sensitivities and

trip travel time of the pulse is defined as *t dC* 2 , where d is the nominal detection distance and *C* is the speed of the electromagnetic wave. If a local replica of the transmitted pulse with a delay close to the nominal round-trip travel time correlates with the received echo, the output correlation function will have the same frequency as the physiological

> cos

transmitted toward a human body where it is phase-modulated by the physiological movement *x*(*t*). The reflected signal *R*(*t*) detected by the radar receiver is given by following

 

0 0 4 2 4

is a constant phase shift due to the nominal detection distance *d0* and the

0

*B t* (3)

   

*T t ft t* 2 (1)

*c*

*t* , is

(2)

On the other hand, the CW Doppler radar mechanism is based on following (1);

where an unmodulated signal *T*(*t*) with a carrier frequency *f* and a residual phase

 cos 

> cos

0

4 *d* 4 *x t*

Since the delay corresponds to the signal round-trip travel time, the detection range of a UWB radar can be varied by controlling the delay between the two inputs of the correlation function block. This makes it possible to eliminate interference caused by reflection from other objects (clutter) and multipath reflection. However, one disadvantage of UWB radar is that the delay needs to be recalibrated when the detection distance is changed; this increases the system complexity and cost. Furthermore, since the correlation function is nonlinear, it is not simple to recover the original movement pattern, even though frequency information can be easily obtained. On the other hand, CW Doppler radar has a low power consumption and a simple radio architecture. These characteristics make it suitable for home-based systems. Moreover, proper adjustment of the radio front-end architecture of a CW radar can cancel clutter (Li & Lin, 2008a, 2008b). In addition, single-input multi-output and multiinput multi-output techniques can be easily implemented with CW radar, enabling the movements of multiple targets to be detected (Boric-Lubecke et al., 2005, Zhou et al., 2006).

The carrier frequency and output power employed must be safe for use on people. Carrier frequencies ranging from hundreds of megahertz to millimeter wave frequencies have been tested for remote vital sign detection using a variety of physiological movements. The carrier frequency should be carefully selected to ensure suitable sensitivities and

<sup>2</sup> *d d x t R t ft <sup>t</sup>*

 2 <sup>0</sup> *t dc* is phase noise. Using the same transmitted signal *T*(*t*) as the local oscillator signal, the radar receiver down-converts the received signal *R*(*t*) to the baseband signal *B*(*t*)

 

is determined by the nominal detection distance and the oscillator phase noise.

movement.

(2);

where 4

where

 *d*<sup>0</sup> 

as following (3);

**2.2 Carrier frequency and output power** 

characteristic response for vital sign measurement. Some studies used extremely highfrequency waves (228 GHz (Petkie et al., 2009)), which have shorter wavelengths and are more sensitive to small displacements. Moreover, a 228 GHz frequency prototype has been extended to perform heart rate and respiration measurements at a distance of 50 m. However, such high frequency waves are not realistic for monitoring vital signs in everyday applications. In many cases, a carrier frequency that does not require a license is often chosen. However, carrier frequencies that do not require a license vary from country to country and some frequency bands are allocated to amateur radio stations. For example, the laws regulating radio frequency use in Japan allow band frequencies 10.525 and 24.15 GHz to be used for detecting moving objects. These devices are marketed as sensors for measuring the speeds of vehicles. Although there are limitations on how they are used (e.g., limited to indoor use), these frequency bands can be used by low-power radio stations without a license provided the output is less than 10 mW. They have been increasing studies on frequency bands for vital sign monitoring. Regarding safety, different countries have different guidelines regarding radio-frequency electromagnetic fields.

The World Health Organization (WHO) and the Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR) define exposure as *the subjection of a person to electric, magnetic, or electromagnetic fields or to contact currents other than those originating from physiological processes in the body and other natural phenomena* (WHO, 2003, SCENIHR, 2006). The intended frequency band of electromagnetic field intensity differ slightly in different guidelines. For example, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines (ICNIRP, 1998) specify the frequency band from 300 Hz to 300 GHz, SCENIHR specifies 100 Hz to 300 GHz (SCENIHR, 2006), and the IEEE Standard is from 3 kHz to 300 GHz (IEEE Standard Committee, 1998). Each country employs different methods for determining their criteria. Consequently, it is important consider device development and intended usage.

The carrier frequency of medical applications of ultrasound is a low frequency of about 3 to 10 MHz. Such applications acquire information by penetrating the human body. In comparison, monitoring using microwave frequencies in the range 10.525 to 24.15 GHz is considered to be less invasive and safer. It is not easy to make simple comparisons, but wireless local area networks (WLANs) use 2.4 GHz radio waves and microwaves in the range 10.525 and 24.15 GHz are considered to be safer. Moreover, safety can be further increased by using a lower power than a WLAN.

It seems appropriate to use high frequencies for sensing to ensure a high resolution while considering invasiveness. However, high frequencies are not necessarily ideal for actual applications as increased sensitivity results in increased susceptibility to artifacts; the target motion induced by heartbeat on the body surface is much smaller than the artifacts generated by general movement of the body and arm. Furthermore, people being monitored move freely as they conduct everyday activities, which makes artifacts a significant problem.
