# Human Activity Signatures Captured Under Different Directions Using SISO and MIMO Radar Systems
## 3 Human Activity Signatures Measured by Using a SISO FMCW Radar System
### Results for the Monostatic SISO Configuration  
To show the limitations of the aforementioned monostatic SISO radar system, we recorded the human motion in three different directions, which are depicted in Figure 1. We process the recorded raw radar data by means of radar signal preprocessing, as described in Section 2. The radial velocity distribution \(p_{11}(v,t)\) has been computed by using (14), and the mean radial velocity \(\partial_{11}(t)\) has been obtained from (15) for the human walking and falling activities. Figures 2 and 3 show the measured radial velocity distribution \(p_{11}(v,t)\) over time \(t\) corresponding to a falling and walking activity, respectively. The black dashed lines in Figures 2 and 3 represent the measured mean radial velocity \(\partial_{11}(t)\). We can see from Figures 2 and 3 that the measured mean radial velocity \(\partial_{11}(t)\) provides a descriptive statistic, which quantitatively summarizes all the main features of the TV radial velocity distribution \(p_{11}(v,t)\).  
Recall from Figure 1 that in Scenario 1, a person moves parallel to the SISO radar boresight--i.e., from position \((x_{3},y_{2})\) to \((x_{1},y_{2})\)--whereas in Scenario 2, the person moves perpendicular to the SISO radar boresight--i.e., from position \((x_{2},y_{3})\) to \((x_{2},y_{1})\). Figure 2a,b shows a person falling parallel (Scenario 1) and perpendicular (Scenario 2) to the radar boresight direction, respectively. Note that in Scenario 1, the person suddenly moves closer to the SISO radar system upon falling. Thus, the falling activity produces high and abrupt positive changes of the radial velocity distribution \(p_{11}(v,t)\), which leads us to suppose that the fall of a person parallel to the radar boresight will be captured perfectly by the SISO radar system, as illustrated in Figure 2a. On the other hand, a fall perpendicular  
Figure 1: A SISO radar system in the presence of a moving person in a cluttered indoor environment.  
Figure 2: For a SISO radar system, the measured radial velocity distribution \(p_{11}(v,t)\) and mean radial velocity \(\partial_{11}(t)\) of a human falling activity for (**a**) Scenario 1, (**b**) Scenario 2, and (**c**) Scenario 3.  
to the radar boresight does not cause a high and abrupt change in the radial velocity distribution \(p_{11}(v,t)\), as shown in Figure 2b.  
The same observations can be made for a person walking in the direction parallel and perpendicular to the radar boresight presented in Figure 3a,b, respectively. Thus, the activity fingerprints captured by the SISO radar system as shown in Figures 2 and 3 are not sufficient to effectively classify different human activities, especially when the direction of the activity is perpendicular to the radar's boresight. Furthermore, in Scenario 3, the person moves diagonally in the \(3\times 3\) grid of Figure 1; i.e., from \((x_{3},y_{3})\) to \((x_{1},y_{1})\). The SISO radar system is able to generate adequate human activity signatures in Scenario 3 as shown in Figures 2c and 3c for the falling and walking activities, respectively. It should be mentioned that unlike the falling activity related to Scenario 3, the initial and final positions of the walking activity are \((x_{1},y_{1})\) and \((x_{3},y_{3})\), respectively. As the person is walking away from the SISO radar system in Scenario 3, the radial velocity components of the TV radial velocity distribution \(p_{11}(v,t)\) are negative, as can be seen in Figure 3c.