# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 2 Technical Considerations for RIS
### Hardware Implementation of RIS
rotating and deforming the resonator, while material reconfiguration involves changing the material property parameters and conductivity of the substrate using reconfigurable materials such as liquid crystals (LC), graphene, and ferroelectric thin films [62]. Thermal control materials, such as light-induced plasma, carbon nanotubes, and vanadium dioxide (VO2), are also being investigated for reconfigurable metasurface behavior [63; 64; 65]. A comparison of methods and features for RIS reconfiguration is shown in Table 5.  
RISs can be divided into digital and analog (continuous) RISs according to the phase control scheme. Digital RISs are implemented with MEMS and PIN diodes and are widely studied due to their simple control circuitry and fast response; however, the phase quantization error introduced by digital phase control devices cannot be ignored [66]. For example, in the case of 1-bit phase control, the gain loss due to phase error is more than 3 dB, and the phase quantization error causes unacceptable gain loss [67]. Therefore, to eliminate the gain loss due to quantization error in digital RISs, analog tunable devices, namely varactor diodes, are used in analog RIS design [68]. An analog RIS using varactors can reduce the quantization loss due to phase shift because it implements continuous phase shift by changing the capacitance of the varactor diode, but it has the disadvantage of the loss characteristics of the varactor itself and the complexity of the circuit. The digital and analog unit cell structure of a transmitarray with a transmissive function during an RIS is shown in Figure 15.  
Figure 15: Unit cell structure of RIS. (**a**) Digital RIS [69]. (**b**) Analog RIS. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Type** & **Technology** & **Stability** & **Integration** & \begin{tabular}{c} **D/A** \\ **Control** \\ \end{tabular} & **Cost** & \begin{tabular}{c} **Loss** \\ **(M/T)** \\ \end{tabular} & \begin{tabular}{c} **Bias Power** \\ **Consumption** \\ \end{tabular} & **Linearity** & \begin{tabular}{c} **Switching** \\ **Time** \\ \end{tabular} \\ \hline \multirow{3}{*}{Circuit} & PIN diode & + & – & D & + & \(-/-\) & – & o & + \\ \cline{2-8}  & Varactor & + & – & D & + & \(-/-\) & + & – & + \\ \cline{2-8}  & MEMS & o & + & D & + & +/o & + & + & o \\ \hline \multirow{3}{*}{Material} & Ferroelectric & \multirow{3}{*}{\begin{tabular}{c} \end{tabular} } & \multirow{3}{*}{\begin{tabular}{c} \end{tabular} } & \multirow{3}{*}{\begin{tabular}{c} \end{tabular} } & \multirow{3}{*}{\begin{tabular}{c} \end{tabular} } & \multirow{3}{*}{\begin{tabular}{c} \end{tabular} } & \multirow{3}{*}{\begin{tabular}{c} \end{tabular} } & \multirow{3}{*}{\begin{tabular}{c} \end{tabular} } & \multirow{3}{*}{\
\begin{tabular}{c} \end{tabular} } & \multirow{3}{*}{
\begin{tabular}{c} \end{tabular} } \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1} \cline{2-8}  & & & & & & & & & \\ \cline{1-1} \cline{2-8}  & & Graphene & – & + & A & o & \(-/+\) & + & – & + \\ \cline{1-1} \cline{2-8}  & Photoconductive & o & – & A & o & \(-/-\) & – & – & + \\ \hline \multirow{3}{*}{Mechanical} & Fluidic & o & – & A & o & o/+ & + & o & – \\ \cline{1-1} \cline{2-8}  & Micromotors & – & – & A & – & + & o & + & – \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of reconfigurable techniques (+: good, o: normal, \(-:\) poor, M: microwave, T: THz). Adapted with permission from [56]. Copyright 2014 IEEE. RISs can also be categorized into passive, nearly passive, and active RISs based on their power consumption as shown in Figure 16. Passive RISs, as shown in Figure 16a, have the disadvantage of having a fixed beam steering value that is initially designed by combining unit cells with phases of 0\({}^{\circ}\) and 180\({}^{\circ}\), but they have the advantage of low fabrication cost and simplicity [70].