## 1 Introduction
### Literature Review
The correlation between power delay profiles (PDPs) was described by the temporal PDP correlation coefficient. In [14; 15; 16], the author conducted channel measurements at 142 GHz and studied three different aspects of channel characteristics in the industrial scenario. Ref. [14] focused on the influence of co-polarized and cross-polarized antennas on the path loss. The omnidirectional and directional path loss with two antenna polarization configurations produced gross cross-polarization discrimination (XPD) with a mean of 27.7 dB. Ref. [15] conducted measurements with low-RX and high-RX antenna heights of 0.5 m and 1.5 m to characterize the propagation channel for close-to-floor applications. Ref. [16] proposed a sub-THz statistical channel model for the indoor factory scenario based on an extensive dataset of radio propagation characterization in factory buildings at 142 GHz. The temporal and spatial channel parameters were modeled by distribution fitting and showed satisfactory p-scores in the goodness-of-fit test. However, the channel characteristics in the industrial scenario at frequencies other than 28 GHz and 142 GHz still need further investigation. With respect to THz bands, the research on industrial scenario channels is relatively insufficient, with the frequency points mainly concentrated at 300 GHz. In [17], the author presented channel measurement campaigns in a frequency range of 215-225 GHz in micro drilling-milling and large milling areas. The results indicated that the FI model is potentially more accurate compared to the CI model and the distribution density of scatterers in the environment has significant effects on the RMS DS and AS. In [18], channel measurements were conducted at a carrier frequency of 300 GHz with a measurement bandwidth of 2 GHz in three distinct industrial scenarios. The channel parameters of each receiver were provided, such as the path gain, path loss, RMS DS, and AS. It can be seen that there are obvious differences in the channel characteristic parameters in different scenarios. However, due to the lack of statistical analysis, the impact of scenarios on the channel parameters has not been quantified. In [19], measurements were conducted at 304.2 GHz for both time-invariant and time-variant settings. For the time-invariant setups, the evaluation results included the path gain, PDP, and RMS DS. For the time-variant setups, the evaluation results included a 3D power delay profile and the path gain versus the time for a fixed delay value. In [20], a novel ultra-wideband dual-polarized double-directional measurement was conducted at 300 GHz for the application of an access point to the inside of a machine in an industrial scenario. The results showed a sparse spatial/temporal channel with multiple paths from the different metallic objects and their influence on polarization. However, the channel characteristics in the industrial scenario at other frequencies in the THz band have not been fully investigated. For visible light bands, in two IEEE channel model standards, manufacturing cells have been defined as typical application scenarios for visible light communication (VLC). However, the channel characteristics for visible light bands in industrial scenarios are rarely researched. In the IEEE 802.15.7 [21] released in 2018 and the IEEE 802.11 bb [22], which was released in 2023, the channel frequency responses, channel direct current gain, and RMS DS for manufacturing cells were given. However, the simulation environment was too simplified, with only two machines inside, which cannot reflect the unique channel characteristics of the industrial scenario. What is more, in [23], the simulation environment was more realistic, with abundant scatterers, and the path loss, the RMS DS, and the AS were investigated at the receiving heights of 1.5 m and 2.5 m.  
Furthermore, there are works that have conducted measurements at multiple frequencies in the same industrial scenario to fairly compare the channel characteristics of different frequency bands.