# Experimental Evaluation of a MIMO Radar Performance for ADAS Application
## 3 Results
### Number of Echoes per Unit of Time  
Finally, we have performed a count of the useful echoes available from some of the targets. It has to be said, anyway, that we have not optimized the timing of the radar, but we were only interested in the variation of the number of echoes with the PFA parameters. The most significant point is that the number of valid echoes diminishes strongly with a low value of the parameter \(\mathit{PFA}_{vel}\), as can be seen comparing the two plots in Figure 11. If the requirements of the system are about 25 echoes per second (from each target), this parameter, \(\mathit{PFA}_{vel}\), will have to be relaxed accordingly. As it has been said before, a too low value of PFAs provokes not only the decrease of false echoes but also of the good ones.  
Figure 11: Number of valid echoes per second from each target in scene 1.  
As it can be noted in Figure 11, the number of good echoes from different targets follows a different behavior. That is due to the different trajectories of the pedestrians. The distance of pedestrian \(M\) with respect to the radar has a low variation along time, so that the number of received echoes from her suffers a variation only for the lowest value of PFA. Pedestrian \(A\) is progressively approaching the radar, and most of the echoes produced have a very good signal-to-noise ratio. As a consequence, the decrease in the PFA value has little effect. On the contrary, pedestrian \(F\) is in the worst situation to be detected: at the beginning of his path, the distance to the radar is larger than for the other two targets, and, at the final state, he is nearing going out of the field of view of the radar. So, the echoes produced by him are, generally speaking, of lower amplitude. Furthermore, the echoes of the farthest pedestrian are occasionally intercepted by the other two when crossing the line of sight of the radar.  
Also worth noting is the fact that an appreciable number of frames sent by the radar have no response at all or fewer responses than the number of real targets. This is an experimental verification of the three scenarios shown in this work.  
In Figure 12, the time intervals to take into account in the configuration of the TI device are shown [25].  
As it was explained above, each chirp was sampled with a frequency \(f_{s}=12\) MHz, up to 256 samples per chirp. So, we have the effective duration of the chirp \(T_{acq}=256/12\) MHz \(=21.33\) us. However, the real period of the signal is not related to that value, as seen in the figure. The total period will be \(T_{c}=T_{idle}+T_{ramp}\). In our experiments, we took \(T_{idle}=100\) us and \(T_{ramp}=60\) us. (On the other hand, we set \(T_{ADCt}=6\) us and \(T_{TXt}=0\), which were the default values in the software pack mmWave Studio ([[https://www.ti.com/tool/MMWAVE-STUDIO](https://www.ti.com/tool/MMWAVE-STUDIO)]([https://www.ti.com/tool/MMWAVE-STUDIO](https://www.ti.com/tool/MMWAVE-STUDIO))), provided by TI). With these values, the expected number of frames per second is about 49 frames/s with 128 chirps per frame and about 24 frames/s with 256 chirps per frame. These numbers include frames without response and frames provoking false echoes. It has to be said, however, that we were not worried about the time the radar takes between frames, which is also an adjustable value. Actually, we tried to avoid the radar sending a very high number of echoes per second in the presented measurements. Adjusting the time between frames in other experiments, we observe that the radar can provide much more echoes per second than the one presented here.