# 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
### Implications on the Performance of an RF-Based Step Counter  
In this subsection, we investigate how the SISO radar system affects the performance of an RF-based step counter under three different experimental scenarios. The radar-based passive step counter [49] was developed in order to count the number of steps in an unobtrusive manner for an SISO FMCW radar system. The authors of [49] compared the performance of their radar-based passive step counter with the wearable Garmin Forerunner 935 step counter. The reported accuracy of the radar-based passive step counter was more than 98%, which was similar to the accuracy of the Garmin Forerunner 935 step counter system. However, the walking activity of a person was restricted to merely Scenario 1, where a person would walk either towards or away from the SISO radar system. This restriction was naturally enforced by the limitations of the SISO radar system in the context of activity direction. In this section, the performance of the radar-based passive step counter is shown for Scenarios 1, 2, and 3. To detect the number of steps in a particular walk activity, the SISO radar's raw data are processed according to the block diagram shown in Figure 4. The Ancortek radar in a SISO configuration produces raw in-phase and quadrature (IQ) data for the wireless channel link \(A_{i}^{T_{x}}\)-\(A_{j}^{R_{x}}\). The raw IQ data are processed by the radar signal preprocessing block (see Section 2) to generate the radial velocity distribution \(p_{11}(v,t)\) and mean radial velocity \(\vartheta_{11}(t)\). The Savitzky-Golay smoothing filter [51] has been adopted to smooth the mean radial velocity \(\vartheta_{11}(t)\) using the MATLAB command \"smooth(y,span,'sgolay',degree)\", where \"y\" is the input vector or mean radial velocity vector \(\tilde{v}_{11}(t)\), \"span\" is the number of data points used for smoothing, \"sgolay\" is the Savitzky-Golay smoothing filter, and  
Figure 3: For a SISO radar system, the measured radial velocity distribution \(p_{11}(v,t)\) and mean radial velocity \(\vartheta_{11}(t)\) of a human walking activity for (**a**) Scenario 1, (**b**) Scenario 2, and (**c**) Scenario 3. \"degree\" is the polynomial degree of the Savitzky-Golay filter. In our experiments, the \"span\" and \"degree\" are chosen to be 60 and 4, respectively. The solid black curves in Figure 5a-c show the smoothed mean radial velocity \(\bar{v}_{11}(t)\) for Scenarios 1, 2, and 3, respectively. The smoothed mean radial velocity \(\bar{\phi}_{11}(t)\) is processed by the RF step detector to detect the number of steps and their corresponding timestamps. The detected steps of the RF step detector are distinctly marked and labeled in Figure 5. In 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 detected the four steps. However, in Scenario 2, when the human subject walked with four steps from the position \((x_{2},y_{3})\) towards the position \((x_{2},y_{1})\), the SISO radar system is unable to produce an intelligible walking activity signature. Consequently, the RF step detection algorithm is unable to detect all the steps that were taken by the human subject. Apparently, the RF step detector missed one of the four steps in Scenario 2 as shown in Figure 5b. Moreover, in Scenario 3, where a person walked from the position \((x_{1},y_{1})\) towards the position \((x_{3},y_{3})\) taking only four steps, the SISO radar system is able to produce a fair walking activity signature. Thus, the RF step detector is able to detect the four steps successfully as depicted in Figure 5c. Thus, we can conclude that the monostatic SISO radar system is not sufficient to capture the human micro-Doppler signatures with high precision in all directions. It is now evident that the aforementioned limitations of the SISO radar system restrict the detection of the scatterer's motion in the horizontal \(xy\)-plane.