## 1 Introduction  
This paper reports the research results on the detection of human vital signs using a frequency-modulated continuous-wave (FMCW) radar with a concurrent detection approach. Human vital signs, or briefly called vital signs, show the state of the important functions of the human body. There are several known vital signs [1]. The main ones are heartbeat rate, breathing rate, body temperature, and blood pressure. If a person's vital signs are different from the normal values, then it is an early sign that this person is experiencing a health problem [2]. In this paper, only heartbeat rate and breathing rates are discussed. Henceforth in this paper, if we mention vital signs, we mean heartbeat rate and breathing rate. Breathing rate and heartbeat rate are important vital signs that are considered in the medical field and observed as a physical indicator for many diagnosis activities. Most of the existing technologies, such as respiratory meters, spirometers, and photoplethysmography (PPG) that are employed to measure patient respiration, are operated as contact devices. For long-term monitoring purposes, contact devices exhibit the issues on the patient's comfort and hygiene level. Moreover, respecting the COVID-19 virus spreading through direct contact, the use of contact devices will provide a higher risk than non-contact devices. Radar technology has been studied to develop a non-contact device for human vital signs [3]. The multi-target detection capability is an important feature that needs to be developed for serving several patients with a minimum number of devices. FMCW radar is a special type of radar that emits electromagnetic waves continuously, in the same manner as continuous-wave (CW) radar, except that FMCW radars can change their operating frequency during measurement. The basic features of the FMCW radar include: having the ability to measure tiny changes in the target distance with reference to the emitted wavelength; having the ability to measure the distance and speed of the target simultaneously; and very accurate distance measurement results [3]. In our research, the FMCW radar used is the TI IWR-1443, with an operational frequency of 77-81 GHz and a transmit power of 12 dBm [4]. The TI IWR-1443 is shown in Figure 1. In addition, as a comparison, this research also uses an oximeter. In our experiments, this FMCW radar emitted electromagnetic waves to four targets. The receiving antenna on the FMCW radar received electromagnetic waves reflected by the four targets. The output signal of the receiving antenna was further processed using signal processing, which will be explained in the following sections, so as to produce the vital signs of the four targets. In addition, an oximeter was attached to a finger belonging to each target. This oximeter also shows the vital signs of each target. Before we carried out this research, several researchers had already conducted similar experiments using the 77-81 GHz FMCW radar [5]. Still, the signal processing by these researchers was different from our signal processing. The authors of [5] used beam-forming weights to obtain the angular position of each target, whereas our method uses Two Dimensional Fast Fourier Transform (2D-FFT) to perform signal processing. Another paper [6] also discusses the detection of human vital signs using the FMCW 76-81 GHz radar. The authors of the paper [6] detected vital signs on one person lying in a bedroom environment, while our experiment concurrently detected vital signs on four people in a sitting or standing position or lying down in any room environment. The comparison of these experiments is shown in Table 1.  
Figure 1: The FMCW radar device used in this research. Thus, this method can help improve medical services [7] in measuring the vital signs of patients with sensitive skin, such as skin with burns, because the sensor is not attached to the body.