# Human Activity Signatures Captured Under Different Directions Using SISO and MIMO Radar Systems
## 4 Human Activity Signatures by Using a Distributed MIMO FMCW Radar System
### Implications on the Performance of an RF-Based Step Counter  
To illustrate the utility of the proposed distributed \(2\times 2\) MIMO radar system, we now investigate the performance of a radar-based passive step counter [49] for the MIMO configuration. As mentioned in Section 3, the radar-based passive step counter was designed only for human walking activities in accordance with Scenario 1. We have already shown the performance limitations of the SISO radar system with the radar-based passive step counter module in Section 3. In this section, we show how the integration of the radar-based passive step counter module with the proposed \(2\times 2\) MIMO radar system will mitigate the shortcomings that we encountered with the SISO radar system. Recall that illuminating the indoor environment from different perspectives, as shown in Figure 7, will enable the distributed \(2\times 2\) MIMO radar to capture the walking activity signature regardless of its direction. A basic block diagram of the \(2\times 2\) MIMO radar-based RF step counter is shown in Figure 10, where it can be seen that the step counter module has been implemented separately for Radar\({}_{1}\) and Radar\({}_{2}\). To detect the number of steps in a particular walking activity, the \(2\times 2\) MIMO radar's raw IQ data are processed by the radar signal preprocessor module (see Section 2). The radar signal preprocessor module generates the TV mean radial velocities \(\bar{v}_{11}(t)\) and \(\bar{v}_{22}(t)\) for Radar\({}_{1}\) and Radar\({}_{2}\), respectively. The TV mean radial velocity \(\bar{v}_{ii}(t)\) is smoothed by a Savitzky-Golay filter to be further processed by the RF step detection module. In Figures 5 and 11, the solid black curves show the smoothed TV mean radial velocities \(\bar{v}_{11}(t)\) and \(\bar{v}_{22}(t)\) for Radar\({}_{1}\) and Radar\({}_{2}\), respectively. By processing the smoothed mean radial velocity \(\bar{v}_{ii}(t)\), the RF step detection module detects the number of steps and their corresponding timestamps for the walking activity. The detected steps of the MIMO radar-based RF step detector are distinctly marked and labeled in Figures 5 and 11 for Radar\({}_{1}\) and Radar\({}_{2}\), respectively. For Scenario 1, the human subject started walking from the position \((x_{3},y_{2})\) and took four steps towards the position \((x_{1},y_{2})\). We can see from Figure 5a that the RF step detection algorithm has successfully det  
Figure 10: The block diagram of the RF-based step counter for a \(2\times 2\) MIMO radar system. the radar subsystem Radar\({}_{1}\). However, the RF step detection algorithm has detected five steps in Figure 10(a) due to the poor quality of the data from Radar\({}_{2}\). Thus, we can easily discard the data from Radar\({}_{2}\) and select the number of steps counted by the RF step counter associated with Radar\({}_{1}\). On the other hand, for Scenario 2, when the human subject walked with four steps from the position \((x_{2},y_{3})\) towards the position \((x_{2},y_{1})\), Radar\({}_{2}\) is able to produce an intelligible walking activity signature. For Scenario 2, we can choose the results from the Radar\({}_{2}\)-based RF step counter, which accurately counts the number of steps, as shown in Figure 10(b). Therefore, unlike the SISO-based RF step counter, the \(2\times 2\) MIMO-based RF step counter is able to detect all the steps that were taken by the human subject in Scenario 1 and 2. Moreover, for Scenario 3, when a person walked from the position \((x_{1},y_{1})\) towards the position \((x_{3},y_{3})\) taking only four steps, both radar systems Radar\({}_{1}\) and Radar\({}_{2}\) are able to produce good walking activity signatures. Thus, the RF step counter detects the four steps successfully as depicted in Figures 4(c) and 10(c) for Scenario 3. It is now clear that by observing the environment from different perspectives, the proposed \(2\times 2\) MIMO radar system is able to detect the scatterer's motion in the horizontal \(xy\)-plane.