## 4 Results and Analysis
### RMS Delay Spread  
Figure 5 illustrates the RMS DS results in the industrial scenario. Firstly, as the transceiver distance increased, the RMS DS for the electronic wave bands showed an overall trend of first increasing and then decreasing, with the boundary point at a transceiver distance of 13 m. At the receiver closest to the transmitter, only a few NLOS components could be received, and the LOS component power accounted for a large proportion of the received power; thus, the RMS DS was at its minimum. As the transceiver distance increased, the number of NLOS components received from different directions increased, resulting in an increased RMS DS. When the transceiver distance continued to increase and exceeded 13 m, the RMD DS did not increase continuously as in most cases but instead decreased with an increasing distance. This can be attributed to the height of the transceivers, which was lower than the height of most metal instruments on both sides. The high-order reflection MPCs from both sides were obstructed by the metal instruments and could not reach the receivers, and only some low-order reflection rays from the transmitter direction could be received, leading to a decrease in the RMS DS. Therefore, in industrial scenarios, the height of the transmitter and receiver relative to that of the metal equipment can have a great impact on the RMS DS for electronic wave channels. Especially when the scenario size is large, a lower height of the transceiver will lead to a decreasing trend in the RMS DS with an increasing transceiver distance. For the visible light band, the RMS DS increased with an increase in the transceiver distance. Even when the distance was greater than 13 m, there was no decreasing trend as in the electronic wave bands. This is because visible light signals exhibit Lambertian radiation on the surface of metal objects, and the reflected optical signals will have a wide range of angles. Although the metal instruments on both sides of the receivers may block some of the optical signals from propagating into the surrounding space, multiple reflections will still occur between the two rows of metal instruments. As the distance increases, the received NLOS path increases, resulting in an increase in the RMS DS. Figure 5: RMS DS vs. transceiver distance. Thirdly, from a frequency perspective, the RMS DS of the visible light band was significantly lower than that of the electronic wave band, while in the electronic wave band, the impact of the frequency on the RMS DS was not significant. On the one hand, the low RMS DS of the visible light band is attributed to the LOS transmission of VLC. The LOS components that have a low time delay account for a large proportion of the received signal power; thus, the RMS DS is relatively small. On the other hand, for the electronic wave band, the distribution of the MPCs that reaches the receiver is similar, which leads to a similar RMS DS. In specular reflection on the surface of metal instruments, the attenuation of signals is relatively small in various frequency bands. There is a low probability that the MPCs experience excessive power attenuation after multiple reflections due to a frequency increase which prevents it from reaching the receivers. Therefore, for various electronic wave bands, there is an obvious frequency dependence of the RMS DS. Finally, it was found that the RMS DS of both the electronic wave and visible light bands could be fitted well by a lognormal distribution, as shown in Figure 6. The fitting parameters are given in Table 4. For the mean of the RMS DS fitting model, that of the visible light channel was obviously lower than that of the electronic wave channel, which was \(-7.93\)\(\log_{10}(\mathrm{s})\). In the electronic wave bands, the value of \(\mu\) was not significantly changed with the frequency, ranging from \(-7.72\) to \(-7.79\)\(\log_{10}(\mathrm{s})\), with an average value of \(-7.74\)\(\log_{10}(\mathrm{s})\).