# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 2 Technical Considerations for RIS
### Channel Modeling
To consider the RIS in channel modeling, the 3 GPP's channel model is shown in Table 3. In urban scenarios, the base station (BS) to an RIS channel model can generally be modeled using the Uma/Umi model, i.e., depending on the height of the BS-RIS. The RIS-UE (user equipment) channel can be modeled as InH/InF if the link is indoors; otherwise, it can be modeled as Uma/Umi. If the distance between the RIS and UE or between the BS and RIS is very small, a pure LOS channel model can be applied. RIS-RIS channels can be cascaded by multiple RISs on the same link. In Outdoor-to-Indoor (O2I) use cases, the RIS-RIS channel can be outdoors or indoors, and for coverage extension, the RIS-RIS channel should be modeled as an Uma/Umi channel model [34; 36]. Recently, research has been performed on propagation channel modeling to modify the radio propagation environment by including an RIS [37]. While many studies in the literature have modeled the path loss of RIS-enabled communication systems, end-to-end channel modeling has recently received significant attention. When modeling the total path loss of an RIS-enabled system, the near-field and far-field effects of the RIS should also be considered. Another important point is the methodology followed in modeling the end-to-end channel, while many researchers follow a physics-based channel modeling strategy, is that there are also statistical channel modeling strategies that consider the physical characteristics of reflection and scattering effects, statistical channel modeling strategies, scattering in the environment, and statistical characteristics of clusters. An electromagnetic (EM) compatible channel model has also been proposed. Channel models that consider EM characteristics such as mutual coupling between subwavelength unit cells have also been proposed. Considering the existing RIS-enabled channel modeling research, channel and propagation modeling approaches can be broadly classified (Figure 12a). Since there is no clear distinction between specific approaches in this classification, channel modeling studies where more than one approach is used can provide a broader perspective. One method of RIS channel modeling is based on the 3 GPP standardized channel model, which has a modeling framework consisting of three parts (Figure 12b): general parameters, small-scale parameters (SSP), and channel impulse response (CIR). The model generates large scale parameters (LSPs) and SSPs for both Tx-RIS and RIS-Rx subchannels. Table 4 summarizes the research related to RIS channel modeling. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Scenarios**} & **BS–UE** & **BS–RIS** & **RIS–UE** & **RIS–RIS** \\  & **Channel Model** & **Channel Model** & **Channel Model** & **Channel Model** \\ \hline Urban deployment: & & & & \\ avoiding indoor UE’s penetration loss & Uma/Umi & Uma/Umi & InH/InF & Uma/Umi/InH/InF \\ \hline Urban deployment: & & & & \\ UE-controlled RIS & Uma/Umi & Uma/Umi & LoS & N/A \\ \hline Urban deployment: RIS close to BS & Uma/Umi & LoS & Uma/Umi & Umi \\ \hline Urban deployment: & & & & \\ coverage extension & Uma/Umi & Uma/Umi & Uma/Umi & Uma/Umi \\ \hline Indoor scenario & InH/InF & InH/InF/LoS & InH/InF/LoS & InH/InF \\ \hline \multicolumn{5}{c}{Uma: Urban Macro, Umi: Urban Micro, InF: Indoor Factory, InH: Indoor Hotspot.} \\ \end{tabular}
\end{table} 
Table 3: 3 GPP channel models that are applicable to RIS use cases.