## 4 Discussion  
The experiment succeeded in conducting the concurrent detection of the vital signs of four people. The results of measuring the vital sign of each target do not affect each other. Factors that should be considered are as follows: the distance between targets and radar should not exceed 2.55 m; any objects should not block the space between the targets and the radar. The power transmitted by the radar is safe for humans because it is 12 dBm or around 15.8 mW. Its power density is lower than the Maximum Permissible Exposure (MPE) recommended by the Institute of Electrical and Electronics Engineers (IEEE) [26]. The factor of the radar's antenna that can be considered is its parameters including its directivity. Directivity is a measurement of the capability of an antenna to focus radiated power in a certain direction. It is corresponded to the radiation intensity. The directivity of an antenna is the ratio of the attained radiation intensity in a certain direction to that of an isotropic antenna. In reality, someone is usually more interested in the peak directivity of the main lobe. Gain of an antenna equals to directivity of the antenna multiplied by radiation efficiency [27]. The antenna configuration of TI-IWR1443 consists of 3 Tx (transmitter) antennas and 4 Rx (Receiver) antennas. In our design, only 2 Tx antennas and 4 Rx antennas are used. Because the transmitter antennas operate in alternate mode, there are 8 signals received, namely from Rx1 to Rx8. According to the TI-IWR1443 User's Guide, the antenna peak gain of TI-IWR1443 is \(>\)10.5 dBi across the frequency band of 76 to 81 GHz; the horizontal 3 dB-beamwidth is approximately \(\pm\)28 degrees, and elevation 3 dB-beamwidth is approximately \(\pm\)14 degrees; the horizontal 6 dB-beamwidth is approximately \(\pm\)50 degrees, and the elevation 6 dB-beamwidth is approximately \(\pm\)20 degrees [28]. Thus, these antenna parameters are sufficient to support measurements of four objects at once. There are three methods used to display the data of measurement results, namely: mapping the distance and angle of each target from the radar; the waveform appearance \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Target 1** & **Target 2** & **Target 3** & **Target 4** \\ \hline Heartbeat rate & 69 & 80 & 69 & 75 \\ \hline Breathing rate & 10 & 21 & 16 & 21 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Oximeter measurement results. Figure 13: (a) Target 1; (b) Target 2; (c) Target 3; (d) Target 4.  
of the chest displacement, heartbeat and breathing of each target; and the numerical appearance of all targets. From the range angle map, we can see the data as follows: the distance of each target from the radar; the heartbeat rate of each target; the breathing rate of each target; and the identification of the target sequence number according to the data shown by the numeric display. Likewise, the angle of the position of each target in the room can be seen. The distance of each target or objects can be identified because their reflected signals produce different beat signals. The distance or range of each target can be derived from the frequency and phase of these beat signals. Figure 14 clarifies the concept [29]. It can also be said that the range angle map shows several strong signatures that indicate the target positions. The target positions are determined by the combination of range and angle data. The range data are obtained from the beat frequency, and the angle data are obtained by exploiting the feature of the IWR 1443 FMCW module. The result in range angle map indicates that four targets are located as (r\({}_{1}\), \(\theta_{1}\)), (r\({}_{2}\), \(\theta_{2}\)), (r\({}_{3}\), \(\theta_{3}\)), and (r\({}_{4}\), \(\theta_{4}\)). We can see in these maps that the locations of the four objects are the same.