# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 2 Technical Considerations for RIS
### Channel Modeling  
As with other communication technologies, channel modeling (combined with interference analysis and RIS modeling) provides a communication model for performance evaluation of wireless networks and is an essential part of RIS research to understand RIS use cases in future wireless networks. Furthermore, one of the most important aspects of implementing next-generation wireless communication technologies is to develop accurate and consistent channel models that can be effectively validated by real-world measurements. From this perspective, it is important to consider not only the physical propagation effects in different environments, such as the varying path loss and scattering behavior in a large indoor office or street environment, but also the effects of RIS on the end-to-end channel model, such as the near-field effects and potential spatial correlation of an adjacent RIS, and the mutual coupling between RIS elements. In addition, the different frequency ranges, bandwidths, and RIS architectures, such as reflective array-based or metasurface-based RISs, are also considered as a promising alternative to design the RISs. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Operation Mode** & **Characteristics and Functions** \\ \hline Reflection & \(\bullet\) & Can act as a reflector in a wireless environment; used to improve coverage, mitigate interference, and increase capacity. \\ \hline \multirow{4}{*}{Refraction} & \(\bullet\) & Refracting incident electromagnetic waves in different directions by adjusting their phase as they pass through the RIS. \\  & \(\bullet\) & Electromagnetic waves pass through the panel because there is no shielding layer inside the RIS panel. \\  & \(\bullet\) & Outdoor to indoor scenario: To improve coverage of some specific areas inside a building, the RIS is used as a glass window to focus the incident electromagnetic waves on different target areas. \\ \hline \multirow{4}{*}{Absorption} & \(\bullet\) & Colliding radio waves with a specific center frequency and a specific bandwidth are ideally absorbed completely, eliminating reflected waves. \\  & \(\bullet\) & Interference mitigation, privacy, and information security—implementing RIS on a building’s exterior to shield electromagnetic waves, isolating electromagnetic waves from each other indoors and outdoors or from different indoor spaces. \\ \hline \multirow{4}{*}{Backscattering} & \(\bullet\) & Reflected waves cover large areas rather than precise locations. \\  & \(\bullet\) & A balance between gain and effective area is required to achieve wide-angle blind spot coverage. \\  & \(\bullet\) & Backscatter mode is used for passive RISs manufactured to reflect incident EM signals in a specific direction. \\ \hline \multirow{2}{*}{Transmitting} & \(\bullet\) & Integrated into wireless transmitters to help form and steer transmitted radio waves. \\  & \(\bullet\) & Dynamic Metasurface Antenna (DMA). \\ \hline \multirow{3}{*}{Receiving} & \(\bullet\) & RIS receives and processes radio signals. \\  & \(\bullet\) & Embeds waveguides in each RIS element or group of elements to pass incident radio signals to the receiving hardware. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: RIS Operation Modes. Figure 11: RIS aided wireless communication network example. based, play an important role in the underlying channel model. Furthermore, modified LOS probabilities and path loss indices can be derived by considering the potential locations of RISs in a real wireless environment [35]. Channel modeling typically requires a tradeoff between complexity and accuracy. For example, in most ray tracing-based channel modeling approaches, considering only a single reflection makes the model simpler but not perfectly accurate, while higher-order reflections, which can characterize more accurate channel models, are time-consuming to compute and simulate.