**4. Proposed system: design and preliminary results**

The most important factors in a Doppler radar using a CW signal are the operational frequency and the radiated power. The former governs the penetration of clothing, the reflection at the air/skin interface, and the signal-to-noise ratio. The latter must be taken into consideration as both patients and medical staff are exposed to the radiations. Our proposed system shows the ability of tuning the operational frequency, as well as the transmitted power. The choice of the operational frequency and the radiated power are subject to international standards.

This section introduces the limitations in terms of power and frequency, presents the proposed system, and shows the preliminary results obtained at different operational frequencies: 2.4, 5.8, 10, 16, and 60 GHz.

#### **4.1. Frequency and power limitations**

The touchless cardiopulmonary detection is based on a reflected signal off the person's chest. Hence, the most important feature is that the RF signal penetrates clothing with minimal reflection and has a higher reflection at the air/skin interface. These properties depend on the operational frequency. For most microwave frequencies, the reflection at the air/skin interface is high. However, the signal reflects at the air/skin interface less as the frequency decreases, and it has more significant reflections from clothing or bedding as the frequency increases. As shown in Eq. (3), the amount of the phase variation is indirectly proportional to wavelength of the carrier. Hence, the signal-to-noise ratio is directly proportional to the operational frequency (*f* = *c*/*λ*). The higher the frequency, the shorter the wavelength, and then the greater the phase variation. For a constant phase noise at different frequencies, increasing the fre‐ quency increases the signal-to-noise ratio. Another feature related to the operational frequency is the antenna size. As the frequency increases, the same antenna gain can be obtained with a physically smaller antenna. The maximum directivity that can be obtained from an antenna with aperture area *A* is

$$D\_{\text{max}} = \frac{4\pi A}{\lambda^2} \tag{2}$$

where *λ* is the wavelength of the transmitted signal. When *λ* decreases, the area decreases for constant directivity. All these factors should be taken into consideration in order to specify the operational frequency.

As the Doppler radar results in a transmitted power, it is necessary to decrease the transmitted power in order to decrease the radiated energy the patient is exposed to during measurements. Various organizations and countries have developed exposure standards for radio frequency energy. These standards recommend safe levels of exposure for both the general public and for workers. Since 1985, In the USA, the Federal Communications Commission (FCC) has accepted and used approved safety guidelines for the exposure of the RF environmental. Several Federal health organizations such as the US Food and Drug Administration (FDA), the Environmental Protection Agency (EPA), the Occupational Safety and Health Adminis‐ tration (OSHA), and the National Institute for Occupational Safety and Health (NIOSH) have also been concerned in issues related to monitoring and RF exposure [46]. The FCC guidelines for human exposure to RF electromagnetic fields were derived from the recommendations of two expert organizations: the National Council on Radiation Protection and Measurements (NCRP) and the Institute of Electrical and Electronics Engineers (IEEE). Both the NCRP exposure criteria and the IEEE standard were developed by expert scientists and engineers after extensive reviews of the scientific literature related to RF biological effects.

The exposure guidelines are based on thresholds for known adverse effects, and they incor‐ porate prudent margins of safety. In adopting the most recent RF exposure guidelines, the FCC consulted with the EPA, FDA, OSHA, and NIOSH and obtained their support for the guide‐ lines that the FCC is using. The International Commission on Non-Ionizing Radiation Protec‐ tion (ICNIRP) has developed exposure guidelines that are applicable in many countries including Europe. The safety limits stated by the ICNIRP limits are similar to those stated by the NCRP and IEEE, with few exceptions. For example, ICNIRP recommend different exposure thresholds in both the lower and the upper frequency ranges and for localized exposure due to some devices as cellular phones. The NCRP, IEEE, and ICNIRP exposure guidelines identify the same threshold level at which harmful biological effects may occur, and the values for maximum permissible exposure (MPE) recommended for electric and magnetic field strength and power density are based on this level. The American National Standards Institute (ANSI) standard was developed over a period of several years by scientists and engineers with considerable experience and knowledge in the area of RF biological effects and related issues. The recommendations were based on a determination that the threshold of hazardous biological effects was approximately 4 W/kg [47]. The Watts per kilogram unit is an expression for the rate of energy absorption in the body given in terms of the specific absorption rate (SAR). A safety factor of 10 was then incorporated to arrive at the final recommended protec‐ tion guidelines. In other words, the protection guides can be correlated with an SAR threshold of about 0.4 W/kg [29]. In addition, the guidelines stated by NCRP, IEEE, and ICNIRP for the maximum permissible exposure depend on the transmitting frequencies. This is caused by the fact that the human body absorption of RF energy varies accordingly with the RF signal frequency. The highest RH energy absorbed by the human body lies in the frequency range 30–300 MHz; thus, the most restrictive limits are applied. Other exposure limits are stated for devices that expose only part of the body such as mobile phones [48].

#### **4.2. System operating at different frequencies**

**4.1. Frequency and power limitations**

38 Advanced Biosignal Processing and Diagnostic Methods

with aperture area *A* is

operational frequency.

The touchless cardiopulmonary detection is based on a reflected signal off the person's chest. Hence, the most important feature is that the RF signal penetrates clothing with minimal reflection and has a higher reflection at the air/skin interface. These properties depend on the operational frequency. For most microwave frequencies, the reflection at the air/skin interface is high. However, the signal reflects at the air/skin interface less as the frequency decreases, and it has more significant reflections from clothing or bedding as the frequency increases. As shown in Eq. (3), the amount of the phase variation is indirectly proportional to wavelength of the carrier. Hence, the signal-to-noise ratio is directly proportional to the operational frequency (*f* = *c*/*λ*). The higher the frequency, the shorter the wavelength, and then the greater the phase variation. For a constant phase noise at different frequencies, increasing the fre‐ quency increases the signal-to-noise ratio. Another feature related to the operational frequency is the antenna size. As the frequency increases, the same antenna gain can be obtained with a physically smaller antenna. The maximum directivity that can be obtained from an antenna

> max 2 <sup>4</sup> <sup>=</sup> *<sup>A</sup> <sup>D</sup>* p

l

where *λ* is the wavelength of the transmitted signal. When *λ* decreases, the area decreases for constant directivity. All these factors should be taken into consideration in order to specify the

As the Doppler radar results in a transmitted power, it is necessary to decrease the transmitted power in order to decrease the radiated energy the patient is exposed to during measurements. Various organizations and countries have developed exposure standards for radio frequency energy. These standards recommend safe levels of exposure for both the general public and for workers. Since 1985, In the USA, the Federal Communications Commission (FCC) has accepted and used approved safety guidelines for the exposure of the RF environmental. Several Federal health organizations such as the US Food and Drug Administration (FDA), the Environmental Protection Agency (EPA), the Occupational Safety and Health Adminis‐ tration (OSHA), and the National Institute for Occupational Safety and Health (NIOSH) have also been concerned in issues related to monitoring and RF exposure [46]. The FCC guidelines for human exposure to RF electromagnetic fields were derived from the recommendations of two expert organizations: the National Council on Radiation Protection and Measurements (NCRP) and the Institute of Electrical and Electronics Engineers (IEEE). Both the NCRP exposure criteria and the IEEE standard were developed by expert scientists and engineers

after extensive reviews of the scientific literature related to RF biological effects.

The exposure guidelines are based on thresholds for known adverse effects, and they incor‐ porate prudent margins of safety. In adopting the most recent RF exposure guidelines, the FCC consulted with the EPA, FDA, OSHA, and NIOSH and obtained their support for the guide‐ lines that the FCC is using. The International Commission on Non-Ionizing Radiation Protec‐

(2)

The proposed system consists of using only a vector network analyzer (VNA) and two antennas. This accommodates a quick and simple installation process. **Figure 1** represents the proposed system. Beside simplicity, many benefits are offered upon the use of a VNA. This section describes the characteristics of the proposed system including VNA and antennas and shows the benefits of this system.

**Figure 1.** Proposed system design: microwave system and ECG.

In order to provide a comparative study in terms of operational frequency, several frequencies were tested using the proposed system. The operational frequencies should cover as much radar band designations as possible, as well as industrial-scientific-medical (ISM) bands. The operational frequencies chosen in this work are 2.4 GHz (ISM S-band), 5.8 GHz (ISM C-band), 10 GHz (X-band), 16 GHz (Ku-band), and 60 GHz (ISM V-band). Choosing these frequencies allows providing a comparative approach for different frequencies. In addition, the choice of the frequencies covers different bands of frequencies, specifically, S, C, X, Ku, and V bands. Also, the chosen frequencies include within some ISM bands, specifically 2.4, 5.8, and 60 GHz. Besides, these frequencies are selected taking into account the operational limits of the VNA and the antennas.

#### **4.3. Vector network analyzer**

VNA is one of the most used systems for microwave measurements and RF applications. It allows verifying the RF performance of microwave devices as well as their characterization in terms of network scattering parameters or *S* parameters in both magnitude and phase. The utilized vector network analyzer is an HP N5230A 4-port PNA-L. This VNA provides the combination of speed and accuracy for measuring multi-port and balanced components such as filters, duplexer, and RF modules up to 20 GHz. The N5230A VNA provides the following features and benefits:


#### **4.4. Antennas**

The same wide band antennas (Q-par Angus Ltd.) were used for transmission and reception in experiments performed between 2 and 18 GHz. The Q-par Angus Ltd. (model number WBH2-18HN/S) has a frequency range between 2 and 18 GHz, with a nominal gain between 10 and 22 dBi and a nominal beam width between 6° and 11°. The voltage standing wave ratio (VSWR) of the antenna is less than 2.5:1 (typically <2.0:1), and its cross polar is less than −17 dB. The antenna dimensions are 622 × 165 × 165 mm approximately, it weighs 2.7 kg and operates for temperatures between −40 and +70° C.

#### **4.5. System operating at 60 GHz**

In order to provide a comparative approach, an extension to the proposed system is added in order to sustain a frequency signal of 60 GHz. This is achieved using up-conversion and downconversion methods. As the operational range of the VNA is limited to 20 GHz, a 3.5 GHz signal is up-converted to 60 GHz, then transmitted. The received 60 GHz signal is downconverted to 3.5 GHz. The measurement system is shown in [49]. The up- and down-conversion processes are obtained as follows: the VNA generates a CW signal at 3.5 GHz. Mixed with the phase locked oscillator (PLO) at 56.5 GHz frequency, the 3.5 GHz frequency is up-converted to 60 GHz. The IF signal is sent to the RF block. This block is composed of a mixer, a frequency tripler, a PLO at 18.83 GHz and a Band-Pass Filter (BPF) (59–61 GHz). The local oscillator (56.5 GHz) frequency is obtained with an 18.83 GHz PLO with 70 MHz external reference and a frequency tripler. The phase noise of the 18.83 GHz PLO signal is about −110 dBc/Hz at 10 kHz off carrier. The Band Pass Filter (BPF) with a bandwidth of 59–61 GHz removes out-of-band spurious signals caused by the modulator operation. The 0-dBm obtained signal is fed into a horn antenna with a gain of 22.4 dBi and a half-power beam width (HPBW) of 10° E and 12° H. The receiving antenna, identical to the transmitting horn antenna, is connected to a BPF (59–61 GHz). The input BPF removes the out-of-band noise. The RF filtered signal is downconverted to an IF signal centered at 3.5 GHz and fed into a BPF with a bandwidth of 2 GHz. A low noise amplifier (LNA) in the band of 2–4 GHz with a gain of 45 dB (noise factor 0.5 dB) is used to achieve sufficient gain. A variable attenuator with a dynamic range of 70 dB is used to control the IF power of IF input signal.

#### **4.6. Preliminary results**

In order to provide a comparative study in terms of operational frequency, several frequencies were tested using the proposed system. The operational frequencies should cover as much radar band designations as possible, as well as industrial-scientific-medical (ISM) bands. The operational frequencies chosen in this work are 2.4 GHz (ISM S-band), 5.8 GHz (ISM C-band), 10 GHz (X-band), 16 GHz (Ku-band), and 60 GHz (ISM V-band). Choosing these frequencies allows providing a comparative approach for different frequencies. In addition, the choice of the frequencies covers different bands of frequencies, specifically, S, C, X, Ku, and V bands. Also, the chosen frequencies include within some ISM bands, specifically 2.4, 5.8, and 60 GHz. Besides, these frequencies are selected taking into account the operational limits of the VNA

VNA is one of the most used systems for microwave measurements and RF applications. It allows verifying the RF performance of microwave devices as well as their characterization in terms of network scattering parameters or *S* parameters in both magnitude and phase. The utilized vector network analyzer is an HP N5230A 4-port PNA-L. This VNA provides the combination of speed and accuracy for measuring multi-port and balanced components such as filters, duplexer, and RF modules up to 20 GHz. The N5230A VNA provides the following

**•** Full 4-port *S* parameter and balanced measurements up to 20 GHz

**•** <0.006 dB of trace noise at 100 kHz intermediate frequency band width (IFBW)

**•** Automatic port extension automatically corrects for in-fixture measurements

**•** Advanced connectivity with LAN, universal serial bus (USB), and general purpose interface

The same wide band antennas (Q-par Angus Ltd.) were used for transmission and reception in experiments performed between 2 and 18 GHz. The Q-par Angus Ltd. (model number WBH2-18HN/S) has a frequency range between 2 and 18 GHz, with a nominal gain between 10 and 22 dBi and a nominal beam width between 6° and 11°. The voltage standing wave ratio (VSWR) of the antenna is less than 2.5:1 (typically <2.0:1), and its cross polar is less than −17 dB. The antenna dimensions are 622 × 165 × 165 mm approximately, it weighs 2.7 kg and

C.

In order to provide a comparative approach, an extension to the proposed system is added in order to sustain a frequency signal of 60 GHz. This is achieved using up-conversion and down-

and the antennas.

features and benefits:

**•** 120 dB dynamic range at 20 GHz

**•** <4 μs/point measurement speed

operates for temperatures between −40 and +70°

**4.5. System operating at 60 GHz**

bus (GPIB) interfaces

**4.4. Antennas**

**4.3. Vector network analyzer**

40 Advanced Biosignal Processing and Diagnostic Methods

The chest displacement varies between 4 and 12 mm due to respiration, while it ranges between 0.2 and 0.5 mm due to heart beating [19]. The measurement of this small displacement is the objective of this work. The variation of the phase of *S*21 is directly proportional to the chest displacement and indirectly proportional to the wavelength of the signal according to the following relationship:

$$
\Delta\theta(t) = \frac{4\pi\Delta x(t)}{\lambda} \tag{3}
$$

where *λ* is the wavelength of the transmitted signal and *Δ* is the chest displacement.

In order to validate the proposed system, measurements were performed at different frequen‐ cies. This section describes the measurements setup. As the frequency range of the antenna is between 2 and 18 GHz, and the maximum frequency of the VNA is 20 GHz, a set of experiments were performed between 2 and 18 GHz. Specifically, experiments were performed at 2.4, 5.8, 10, and 16 GHz. Another operational frequency, 60 GHz, was used via up- and downconversion methods between 3.5 and 60 GHz. These two versions of the system were tested with a total output power of −10 dBm and for a distance of 1 m between the antennas and the person. The total output power, in other words radiated power, is the transmitted power added to the gain of the antenna. Measurements were performed on a 27-year-old healthy person, while holding the breath for a duration of 10 s [50]. The number of points taken for this window of time is 20,000 points. Hence, a sampling frequency of 2 kHz is obtained.

Performing a measurement begins by generating a continuous wave signal at the desired operational frequency. This CW signal, generated by the VNA, is driven to the transmitting antenna that is directed to the subject's chest. Reflected off the chest of the person under test, the signal is received by the receiving antenna and is driven back to the VNA. The phase variation *S*21, which corresponds to the difference in terms of phase between the transmitted and the received signal, is computed. The difference in phase is due to the chest displacement. Hence, it contains information about the cardiopulmonary signals when breathing normally and about the heartbeat signal when holding the breath.

The theoretical values of the phase variation due to the chest displacement ranges and the average phase variation obtained by measurements when operating at 2.4, 5.8, 10, 16, and 60 GHz are shown in **Table 1**.


**Table 1.** Theoretical and measured phase variations due to chest displacement.

For each of the utilized frequencies, the phase variation ranges within the theoretical limits. **Figure 2** shows the phase variations due to heartbeat signals detected at different frequencies and plotted within the same scale. It can be noticed that the phase variation increases when

**Figure 2.** Phase variation of *S*21 due to heartbeat signal measured when holding the breath at different operational fre‐ quencies: (a) 2.4 GHz, (b) 5.8 GHz, (c) 10 GHz, (d) 16 GHz, (e) 60 GHz, and (f) all frequencies over the same scaling.

the frequency increases. Thus, higher sensitivity to small displacements is obtained at higher frequencies. The use of higher frequencies will reduce the noise effect and increase the accuracy in detecting the peaks of the signal.
