Participants

Approval was acquired from the local ethics committee before planning the experiments. Seven male and four female test subjects where measured with an average age of 34.73 ± 15.94 years and an average BMI of 23.19 ± 3.61 kg/m2. All test subjects were healthy and were briefed about the experiments that were conducted. A written consent was obtained from all participants that also allows for sharing the anonymized data. After briefing, the participants were asked to fill out a measurement protocol which includes their personal data such as age, sex, weight, and height. A summary of all participants is displayed in Table 1.

Table 1 Overview of all test subjects. Full size table

Human subjects

The study was approved by the ethics committee of the Friedrich-Alexander-Universität Erlangen-Nürnberg (No. 85_15B). All research was performed in accordance with relevant guidelines and regulations. The informed consent was obtained from all subjects in human trials.

Procedures

All measurements were recorded at the Institute for Electronic Engineering at the Friedrich-Alexander-Universität Erlangen-Nürnberg. At least two supervising persons monitored the measurements and ensured a trouble-free progress of the measurements. At the beginning of each measurement, the ECG electrodes and the respiration sensor were attached to the test subject. Next, the thorax of the test subject was auscultated to locate the positions at which a strong heart sound signal could be perceived. Since the PCG served as a reference sensor, it was usually placed on a location at which a very high signal quality was observed. The antenna of the radar system had therefore to be focused on different regions at which, however, a good signal quality was still to be expected. The distance between the antenna and the region of interest (ROI) was around 20 cm during all measurements. To maximize the signal quality, the antenna direction was chosen perpendicular to the thorax surface. The length of each measurement was around 60 s. The different measurement positions on the thorax, on the back, and on the carotid of the test subjects are depicted in Fig. 1a,b. The right and left carotid are abbreviated using the terms “CR” and “CL”, respectively. The number at the other positions describes the number of the intercostal spaces starting from the top. “R” and “L” describe the position on the right or left side. Figure 1c shows an exemplary measurement in the laboratory. A block diagram of the overall setup can be seen in Fig. 1d. In the following, all components will be described in detail.

Fig. 1 Overview of the different measurement spots, the measurement setup, and the system configuration. (a) Measurement spots at the thorax17. (b) Measurement spot at the back. (c) Experimental setup with a test subject in a sitting position. The informed consent was obtained from the persons in the image. (d) Block diagram of the overall setup17. (e) Photograph of the BB back end17. (f) Photograph of the RF front end17. Images in (a) and (b) are taken from Biodigital Inc. (https://human.biodigital.com/index.html). Full size image

Baseband back end

Figure 1e shows the baseband board that was used to digitize the sensor signals. The ECG, the radar signals, and the respiration sensor are attached to this back end to allow for simultaneous sampling of the signals. The signals are digitized using the 24 bit analog-to-digital converter ADS1298 from Texas Instruments at a sampling rate of 2000 Hz. After conversion, the raw signals are sent to the PC via Ethernet.

RF front end and Six-Port radar

A Six-Port is utilized as a quadrature interferometer for the radar application. A detailed description of the radar system is given in17. The Six-Port is a completely passive structure which basically consists of three quadrature hybrid couplers and one Wilkinson divider29. As indicated by its name, the Six-Port has two input and four output signals. The two input signals consist of a reference signal at a defined frequency and the signal that is received after reflection at the target. Inside the structure, the two input signals are superimposed under four relative and static phase shifts of 0°, 90°, 180°, and 270°. These signals are then down-converted using diode power detectors. The four resulting baseband signals \({B}_{3\ldots 6}\) form two differential and orthogonal signals I and Q which can be expressed as a complex number Z. A relative distance change of a target in front of the antenna results in a phase shift Δ\(\varphi \)29:

$$\Delta \varphi ={\rm{a}}{\rm{r}}{\rm{g}}\{\underline{Z}\}={\rm{a}}{\rm{r}}{\rm{g}}({B}_{5}-{B}_{6})+j({B}_{3}-{B}_{4}).$$ (1)

The relative distance change Δx can be easily reconstructed from Δ\(\varphi \) using29:

$$\Delta x=\frac{\Delta \varphi }{2\pi }\cdot \frac{\lambda }{2},$$ (2)

with λ being the known wavelength of the signal. However, since the unambiguousness range is limited, phase unwrapping has to be performed additionally. The whole RF front end can be seen in Fig. 1f. A reference signal at 24.17 GHz is generated using the PSG Analog Signal Generator E8257D from Keysight. The signal is split by a 10 dB coupler whereas the main part is fed to the antenna and the lesser part is directly fed into the Six-Port receiver29.

Reference sensors

In the following, the reference sensors that are used for validation are described. They consist of an ECG, a PCG, and a respiration sensor.

ECG

A three channel ECG serves as main reference sensor for all cardiovascular signals. Three leads are attached to the body according to clinical standard30: one electrode at the right arm (RA), one at the left arm (LA), and one at the left leg (LL). The positions are indicated by the numbers 1–3 in Fig. 1c. Standard snap electrodes were used for the measurements wherein for hygiene reasons a new set of electrodes was used for each test subject. The ECG leads 3 (LA-LL) and 2 (RA-LL) according to Einthoven30 are recorded. Lead 1 can be simply calculated by subtracting lead 3 from lead 2.

PCG

A digital PCG was used as reference sensor for the heart sound signals. For this purpose, the Electronic Stethoscope Model 3200 from Littmann was utilized. The PCG is connected to the PC via Bluetooth. The raw measurements are exported as .wav files and imported into MATLAB. After re-sampling and synchronisation, the PCG signals are also stored as an array in the .mat files. According to the documentation of the PCG, the signals are amplified between 20 … 1000 Hz and lower frequency sounds are emphasized between 20 … 200 Hz.

Respiration sensor

As a reference sensor for respiration, a passive temperature-based airflow sensor was employed. The air heats up quickly in the lungs when breathing due to the large surface area of the capillaries, so an increase in temperature can be registered at the nose when exhaling. Analogously, the sensor cools down when inhaling. Using this mechanism, a qualitative respiration curve can be created.

Measurement protocol

During the measurements, various scenarios were carried out. These scenarios are described in the following. Please note that not all scenarios could be carried out with all test subjects, e.g., some could not perform the task that was needed for the post-exercise measurement. During all measurements, the participants were asked to stay calm and avoid any artifacts that might interfere with the measurements.

Default scenario

The default scenario makes up the majority of the measurements in the database. In this standard setting the test subject is sitting or standing comfortably and breathing freely. Both radar and PCG are placed on ROIs at which the heart sounds have a high signal quality. Different ROIs are possible for this scenario, however, they are all located on the thorax as seen in Fig. 1a. An exemplary illustration of a default measurement can be seen in Fig. 2a. Depicted is a segment of synchronised signals from all sensors. “Respiration”, “Radar pulse”, and “Radar HS” (heart sounds) result from filtering the “Radar raw” signal in the corresponding frequency bands.

Fig. 2 Exemplary signals of different scenarios. (a) Default, (b) Distance variation, and (c) Apnea scenario. Full size image

Carotid

In this scenario the sensor is placed or focused on the carotid artery, either at the left or right side as seen in Fig. 1a. Heart sounds are expected to be detected at these ROIs as they travel as transverse vibrations along the ventricular walls and along the large vessels31,32,33.

Distance variation

During this scenario, the test subject is sitting in an office chair which is gradually moved away from the antenna so that the distance increases. It is attempted to keep the focus of the antenna on the same ROI during the distance variation. The test subject is moved in steps: the chair is moved away for around 5 … 10 cm and than kept still for around 15 … 20 s. This is repeated until the measurement time of 60 s is over. Using this scenario, it is attempted to determine the influence of the distance on the signal quality.

Speech

The speech scenario is aimed to observe the measurability of vital signs, in particular heart sounds, when the test subject is speaking during the measurement. A defined random text is given to the participants and is read aloud at an arbitrary speed until the measurement time is over.

Back

During this scenario, the ROI is located on the back. An ROI on the left side is chosen which is approximately at heart level. This way, the signal quality shall be maximized.

Angle variation

During angle variation, the angle between the thorax surface and the direction of the antenna is changed from a perpendicular 90° to both 60° and 120°.

Subscenarios

In addition to the above mentioned scenarios, subscenarios are performed. These are performed in addition to the main scenarios, e.g., the post-exercise scenario in addition to a back measurement. Not all possible combinations are performed since this would result in an excessively large number of recordings.

Apnea

During apnea, the test subject is asked to breath freely during the first 30 s of the measurement and to hold the breath as long as possible during the second 30 s. It is furthermore distinguished if this maneuver is performed after inhalation or exhalation.

Post-exercise

These measurements are recorded after the test subject has done 20 squats. Through this stimulation of the cardiovascular system, heart rate, and cardiac output are increased.

Exemplary signals of three different scenarios can be seen in Fig. 2. Figure 2a shows a default measurement. Displayed are the raw radar distance signal, the filtered respiration sensor signal (0.05 Hz … 1.7 Hz), the three ECG leads (L1–L3), the radar signal filtered in the pulse frequency range (0.7 Hz … 15 Hz), the PCG signal, and the radar signal filtered in the heart sound frequency range (16 Hz … 80 Hz). For visibility reasons, only a 30 s segment of the whole measurement is shown. Figure 2b shows a distance variation measurement. As can be seen in the radar raw signal, the distance between antenna and body surface has been successively increased. After each increment, the person sat still again for a defined period of time. Please note that the radar distance signal is inverted for a better comparability with the respiration sensor signal. A movement towards the antenna is now reflected by a positive or rising radar signal while a negative signal indicates a target which is moving away. Figure 2c shows an apnea measurement. While the person was breathing normally in the beginning, the breath was hold from second eight and onwards.