# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 2 Technical Considerations for RIS
### Channel Modeling
\begin{table}
\begin{tabular}{c c l} \hline \hline
**Ref.** & & **Abstract** \\ \hline
[38] & \(\bullet\) & Path loss in near and far fields based on Huygens–Fresnel principle and Physical Optics (PO) method. \\  & \(\bullet\) & Modeling RIS as an EM sheet with negligible thickness. \\ \hline
[39] & \(\bullet\) & Calculate path loss with a physics-based approach. \\  & \(\bullet\) & Compute the electric field of a finite-size RIS in the near and far fields. \\ \hline
[40] & \(\bullet\) & Calculate path loss for a passive reflectarray RIS. \\  & \(\bullet\) & Calculate path loss as a function of the RIS size, link shape, and actual gain of RIS unit cells. \\ \hline
[41] & \(\bullet\) & Provide a realistic power scaling law for the far-field region of an array with the PO method. \\ \hline
[42] & \(\bullet\) & Calculate free-space path loss with physics/EM-based approach considering near-field and far-field regions of an RIS. \\  & \(\bullet\) & Validate with experiments/measurements using the fabricated RIS. \\ \hline
[43] & \(\bullet\) & Calculate path loss as a function of RIS size and transmission distance using the PO method and the Huygens–Fresnel principle. \\  & \(\bullet\) & Consider 2D homogeneous metasurfaces behaving in reflection/refraction mode in near and far fields. \\ \hline
[44] & \(\bullet\) & Analysis of the power scaling law for large RIS using a determined propagation model. \\  & \(\bullet\) & Show that an asymptotic limit can be obtained in the near field. \\ \hline
[45] & \(\bullet\) & Represents end-to-end channel model including physical parameters of RIS. \\  & \(\bullet\) & PO-based analysis using radar concept. \\  & \(\bullet\) & Each tile of RIS is modeled as an anomalous reflector. \\ \hline
[46] & \(\bullet\) & EM-compliant communication model that considers mutual coupling between RIS unit cells. \\  & \(\bullet\) & Full system representation with circuit-based models for all terminals. \\  & \(\bullet\) & Investigate inter-coupling between impedance-controlled thin dipoles and the effects of tuning elements. \\ \hline
[47,48] & \(\bullet\) & Statistical geometry-based non-stationary channel model for MIMO systems supporting RIS. \\  & \(\bullet\) & Considers various physical characteristics of the RIS, such as number and size of unit cells, Tx, RIS, and relative positions between them. \\ \hline
[49,50,51] & \(\bullet\) & Three-dimensional channel models for physical RIS characteristics and real-world deployment. \\  & \(\bullet\) & Introduction to the open-source, SimRIS channel simulator. \\  & \(\bullet\) & Consider technical specifications for sub-6 GHz and millimeter wave bands. \\ \hline
[52] & \(\bullet\) & Physics-based end-to-end model of RIS-parameterized radio channels with adjustable fading \\  & (open source PhysFad). \\ \hline
[53] & \(\bullet\) & Utilize the physics of electromagnetic (EM) fields and a modified Saleh–Valenzuela model. \\  & \(\bullet\) & Derive theoretical results for path loss, channel parameters, and received distortion power. \\ \hline \hline \end{tabular}
\end{table} 
Table 4: RIS channel modeling research trends. Figure 12: _Cont._