## 4 Results and Analysis
### Path Loss  
Figure 4 presents the path loss results for all the frequency bands. It was observed that along the LOS path, the path loss for all the frequency bands showed an obvious distance dependence, and the path loss increased with an increasing distance from the transceiver with a non-linear relationship. Moreover, it could be observed that the path loss of the visible light band exhibited more significant variation with the transceiver distance compared with the electronic wave bands, and the path loss variation for various electronic wave bands was similar. For example, when the transceiver distance increased from 3 m to 9 m, the variation in the path loss for the visible light band was 15.5 dB, while the path loss variation for the other electronic wave bands was 6.43, 5.92, 6.49, 6.98, 5.41, 5.12, and 4.55 dB, respectively. This can be attributed to the LOS transmission of VLC. That is, in the received signal, LOS components account for a majority of the received power, while NLOS components only account for a small portion. As the LOS component mainly varies depending on the transceiver distance, the path loss for the visible light channel varies significantly with the transceiver distance. In addition, at the same transceiver distance, the path loss increased with an increase in the frequency. Specifically, the path loss of the visible light channel was much higher than that of the electronic wave channel in general. At a small transceiver distance, the difference in the path loss between the electronic wave and visible light wave was relatively small. As the transceiver distance increased, the difference in the path loss between the visible light and electronic wave bands grew larger. For example, at a transceiver distance of 2.8 m, the path loss difference between the visible light signal and the 300 GHz signal, which was in the THz band, was only 8.37 dB. When the transceiver distance was 23 m, the path loss difference reached 27.10 dB. This shows that in short-range communication, there is not much difference in performance between THz communication and VLC. For long-range communication, electronic wave communication possesses great advantages over VLC. In the THz bands, a large frequency change can only bring about small changes in the path loss, and in the mmWave and sub-6 GHz bands, a smaller frequency change will lead to a large path loss difference. For instance, at a transceiver distance of 15 m, the path loss difference between 300 GHz and 220 GHz was only 3.13 dB, while the path loss difference between 60 GHz and 39 GHz was 8.58 dB, and the path loss difference between 6 GHz and 3.5 GHz was 3.07 dB. Therefore, there will be significant differences in the coverage capability when applying different frequency bands, and a strict frequency selection must be made based on the propagation characteristics during deployment. To accurately characterize the path loss, the FI model was utilized to fit the path loss for all the frequency bands, which can be expressed as  
\[PL^{FI}(d)=\alpha+10\beta\text{log}_{10}d+X_{\sigma}, \tag{7}\]  
where \(\alpha\) is the floating intercept, \(\beta\) is the slope of the model, which is the PLE, and \(X_{\sigma}\) is a zero-mean Gaussian variable with a standard derivation, \(\sigma\), representing the shadow effect. The fitting parameters for all the frequency bands are shown in Table 3.  
Figure 4: Path loss vs. transceiver distance. In 3 GPP TR 38.901, the channel parameters of the factory scenario are presented, which can support the frequency bands for 0.5-100 GHz. The path loss model parameters in the factory scenario are distinguished based on the density of scatterers and the height of the transmitter. In our research, the simulation scenario had dense metal instruments and high transmitters, which corresponds to the DH scenario with dense clutter and high base stations in 3 GPP TR 38.901, and the PLE in 3 GPP TR 38.901 is 2.19, which is much higher than that of the electronic wave bands obtained in our research.