## 5 Characterization Using the Time Domain Measurement Data
### Measurement Results
#### 5.1.4 PSD Results  
The PSD value of the noise is shown in Figure 9. PSD means the probability that the interval of adjacent pulses exceeds a fixed value \(\tau_{s}\). From the measurement results, it can be seen that at the 315 MHz frequency point, the PSD value measured near the manual spot welder was higher than that measured near the welder robot, while the PSD value measured in the office area was the smallest. This shows that the noise power spectrum density value of the office area was smaller than the values of the other two measurement positions, while the uncertainty caused by manual operation caused electromagnetic noise to appear more frequently. In the 779 MHz and 916 MHz bands, the PSD values measured at the three locations were similar, which may be due to the frequency increase, and the noise power spectrum density values at the three measurement locations tended to be consistent.  
Combining the values of the four statistical parameters shown in Figures 6-9, it can be concluded that the electromagnetic noise generated by the manual spot welder and the welder robot operation was similar, but it was not completely consistent. This was because the two kinds of mechanical equipment used the same welding head, so the noise situation was similar. However, when working, the height of the welding head and the position of the welding car were different, resulting in slightly different noises, but the two were generally similar.  
Figure 7: Noise amplitude distribution (NAD) results. (**a**) NAD of vertical polarization at 315 MHz; (**b**) NAD of horizontal polarization at 315 MHz; (**c**) NAD of vertical polarization at 779 MHz; (**d**) NAD of horizontal polarization at 779 MHz; (**e**) NAD of vertical polarization at 916 MHz; (**f**) NAD of horizontal polarization at 916 MHz.  
Since the tail of the APD curve represents the maximum measured noise power, it can be seen from the APD results that the maximum measured noise power in the office area was significantly lower than the maximum measured noise power in the other two measurement positions under the same frequency point and the same antenna polarization mode. PDD results indicate that the office area was less affected by electromagnetic noise with a longer duration than the other two measurement positions. PSD results indicate that the pulse interval of electromagnetic noise in the office area was larger than the other two measurement positions. From these three data, it can be seen that the office area was less affected by electromagnetic noise than the other two measurement positions, which was due to the fact that both manual spot welding machines and welding machines were located inside the welding workshop, and the workshop environment was more noisy and electromagnetic interference was stronger than that of the office area. However, the office area was not far from the welding workshop and also had interference by electromagnetic noise radiated from the workshop, so the noise power measured in the office area was only slightly lower than that measured in the welding workshop.  
In addition, we can conclude from the NAD results that an electromagnetic noise higher than -90 dBm was generated at every 0.1 ms interval at 315 MHz near the welding robot. However, the received power of the user equipment in the IIOT scenario was about -90 dB. Therefore, these electromagnetic noises will greatly affect the reliability of the communication system. Therefore, it is necessary to further study the anti-noise problem in industrial environments.  
Figure 8: Pulse duration distribution (PDD) results. **(a)** PDD of vertical polarization at 315 MHz; **(b)** PDD of horizontal polarization at 315 MHz; **(c)** PDD of vertical polarization at 779 MHz; **(d)** PDD of horizontal polarization at 779 MHz; **(e)** PDD of vertical polarization at 916 MHz; **(f)** PDD of horizontal polarization at 916 MHz.