## 1 Introduction
### Literature Review
There is sufficient research on channel characteristics such as the path loss, RMS DS, and angle spread (AS), and many influencing factors, such as the antenna height, antenna polarization configurations, and time-varying effects, have been taken into account. In [6], the path loss, DS, and AS were analyzed and modeled in a factory-like scenario at 28 GHz. The path loss was modeled by both the CI model and the floating intercept (FI) model, and the FI model showed high accuracy. Moreover, the RMS DS and AS were modeled by a linear function, and the cumulative distribution function (CDF) fitting line as well as the fitting parameters were given. Furthermore, it was found that the antenna heights would have an impact on these channel characteristics. In [10], a ray-tracing-based \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Frequency Band** & **Frequency Point** & **Modeling Approach** & **Channel Characteristics** & **Influence Factors** & **Ref.** \\ \hline \multirow{4}{*}{THz} & 220 GHz & Channel measurement & \begin{tabular}{c} Path loss, RMS DS, AS, \\ power delay angle profile \\ \end{tabular} & Environmental density & [17] \\ \cline{2-6}  & 300 GHz & Channel measurement & \begin{tabular}{c} Path gain, path loss, \\ RMS DS, AS \\ \end{tabular} & Scenario size & [18] \\ \cline{2-6}  & 304.2 GHz & Channel measurement & \begin{tabular}{c} Path gain, PDP, RMS DS \\ \end{tabular} & \begin{tabular}{c} Time-invariant and time-variant \\ settings, antenna height \\ \end{tabular} & [19] \\ \cline{2-6}  & 300.7 GHz & Channel measurement & \begin{tabular}{c} Received power, PDP \\ \end{tabular} & \begin{tabular}{c} Penetration through glass, \\ blockage by frame losses \\ \end{tabular} & [20] \\ \hline \multirow{4}{*}{VLC} & \multirow{4}{*}{VLC} & Ray-tracing-based simulation & \begin{tabular}{c} The channel frequency responses, \\ channel direct current gain, RMS DS \\ \end{tabular} & \begin{tabular}{c} Comparison of visible light \\ and infrared frequency bands \\ \end{tabular} & [21] \\ \cline{2-6}  & & Ray-tracing-based simulation & \begin{tabular}{c} The channel frequency responses, \\ channel direct current gain, RMS DS \\ \end{tabular} & \begin{tabular}{c} Various receiving positions \\ \end{tabular} & [22] \\ \cline{2-6}  & & Ray-tracing-based simulation & \begin{tabular}{c} Path loss, RMS DS, AS \\ \end{tabular} & Antenna height & [23] \\ \hline \multirow{4}{*}{Multiple bands} & 3–4 GHz and 38–40 GHz & Channel measurement & \
\begin{tabular}{c} Channel gain coefficient, RMS DS, \\ and Ricean K factor \\ \end{tabular} & LOS and NLOS cases & [24] \\ \cline{2-6}  & 3.7 GHz and 28 GHz &
\begin{tabular}{c} The Volcano ray-tracing \\ channel simulation and \\ channel measurement \\ \end{tabular} & Channel gain, RMS DS, AS & LOS and NLOS cases & [25] \\ \cline{2-6}  & 6.75 GHz, and 305.27 GHz & Channel measurement & Path loss, normalized power, DS & LOS and NLOS cases & [26] \\ \cline{2-6}  & 28 GHz, 38 GHz, and 220 GHz & Channel measurement & Path loss, RMS DS, AS, K factor & Scenario size & [27] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._simulation was conducted at 28 GHz in a light factory and a heavy factory. The path loss was analyzed and modeled by the CI model, and the PLEs were 1.9 and 2.1 for the light and heavy factories, respectively. In [11], the author presented an extensive measurement campaign at 28 GHz inside a factory environment. It was found that the path gain in LOS cases could be well represented by the Friis free space formula, with an RMS error of 3.6 dB. In [12], the path loss, RMS DS, and AS in the factory environment at 28 GHz were analyzed and compared to the indoor hotspot (InH) model. The author proposed a path loss model similar to the FI model and used a logarithmic function to model the RMS DS. In [13], time-varying THz channel measurements at 105 GHz were conducted in an indoor factory. The number of MPCs and the RMS DS in LOS and NLOS environments were analyzed and compared.