## 3 Preliminaries
### CSI Data Feature Analysis  
The data of the three antennas obtained using the CSI tool are shown in Figures 2, 3, and 4, respectively. To intuitively observe the changes of the data in the time domain, the 10 groups of data were respectively smoothed by a sliding average using a window of size five (Smoothing data can enhance its observability. We find that the larger the smoothing window size, the better the smoothing effect. We chose a smooth window of size five in this paper, because when the smooth window reached that size, we could clearly see the differences between different data groups). It can be seen from the three figures that the CSI data of different groups showed different amplitude levels, respectively, and the oscillation trends of each group of data in the time domain were also somewhat different, which preliminarily reflects that there are obvious differences between IoT devices in the time domain and spatial domain. Meanwhile, some interesting phenomena can be observed: for example, the sampling data of some groups showed obvious oscillation characteristics on some antennas, but were relatively stable on other antennas, such as sampling groups 1, 3, and 10. This proves to a certain extent the incoherent characteristics of signals in space, even for very small spatial distances between several receiving antennas. It should also be noted that the CSI values received by different antennas for the same (group) signal were obviously different, thus indicating that the transmission of the signals in space will experience different attenuation processes, which also provides a prerequisite for using CSI data to further authenticate terminal device fingerprints (identity status). In order to show the statistical situation of the obtained CSI data, we calculated the average value and variance of 10 groups of data respectively. The results are shown in Figures 5 and 6, respectively. From Figure 5, it can be seen that, due to the differences in the position of the AP and the sampling time, the CSI average value of the same antenna showed obvious differences, and, for the same reason, the CSI average values corresponding to different antennas in different groups also showed differences. It is worth adding that changing the position of the AP actually corresponded to changing the position of each receiving antenna of the network card, because the wireless channel had interchangeability, which was also the main reason for the large difference in the average CSI value of each group. It can be observed from Figure 6 that the variance of different data groups showed obvious differences. The variance of groups 2, 3, and 10 was generally larger, thus indicating that the CSI data of these groups fluctuated greatly in the time domain; that is, the correlation in the time domain was poor. In comparison, the overall variance of the remaining data groups was relatively small, but, for each group of data, there was CSIvariance intensity among different antennas alternates. The fluctuation characteristics of the CSI data curve can also be observed in combination with Figures 2-4. Next, we calculated the correlation between the data of different antennas corresponding to the 10 groups of collected CSI data in order to show the signal reception coherence under the condition of tiny spatial differences between different antennas. It can be seen from Figure 7 that there was a certain correlation between the data of the different antennas. Except for the relatively small correlation between the data of groups 3 and 10, the correlation between the data of other groups of antennas was above 0.1, and the highest correlation of each group of data did not exceed 0.9. Except that the correlation between the three pairs of antennas in groups 6, 7, 8, and 9 was higher than 0.6, the correlation between the antenna data of other groups was not high. This indicates two points: (1) There were obvious differences in the correlation between antennas of each data group; (2) Although  
Figure 3: The smoothed CSI value received by antenna 2.