# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
### RIS Introduction  
In conventional wireless communications, the radio channel is defined passively, as shown in Figure 6a, as defined by [PERSON] in 1948. As described earlier, an RIS is a planar surface composed of a large number of passive reflective elements, each of which can independently induce controllable amplitude and/or phase changes in the incident signal; thus, densely deploying RISs in wireless networks and smartly adjusting their reflections can flexibly reconfigure the signal propagation/radio channel between the transmitter and receiver, providing a new means to fundamentally solve the wireless channel fading \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Year** & **Ref.** & **Structure** & **Technologies** & **Function** \\ \hline
2012 & [12] & Intelligent wall & PIN diode, active FSS &
\begin{tabular}{c} Perfect transmission/ \\ reflection surface \\ \end{tabular} \\ \hline
2014 & [13] & Spatial microwave modulator & Binary phase control metasurface & Microwave field forming \\ \hline
2014 & [11] & Coding metamaterial & Binary phase control metasurface & Reconfigurable scattering pattern \\ \hline
2016 & [14] & Programmable metamaterial & PIN diode, metasurface & Reconfigurable phase, polarization, scattering \\ \hline
2016 & [15] & Reconfigurable reflectarray & Varactor, reflectarray & Adaptive reflection phase \\ \hline
2018 & [4] & Large intelligent surface &
\begin{tabular}{c} Radiation/sensing array, \\ electromagnetic active material \\ \end{tabular} & Massive MIMO substitute \\ \hline
2018 & [3] & Software controlled hypersurface & IoT gateway, metasurface &
\begin{tabular}{c} Radio absorption, polarization, \\ beam steering \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Communication applications of electromagnetic surface structures. Figure 4: Programmable metamaterials [11]. Figure 3: Reconfigurable array antenna using PIN diodes. Adapted with permission from [10]. Copyright 2011 IEEE. impairment and interference problems, and to dramatically improve wireless communication capacity and reliability. In other words, RIS-using phase-shifted metasurface structures can control the channel by controlling the reflection or transmission properties of the radio waves in the channel, thus making it an active characteristic to reconfigure the wireless channel as needed, as shown in Figure 6b. Therefore, a lot of research has been performed recently as a new paradigm to realize a smart and reconfigurable radio channel/radio propagation environment for B5G/6G wireless communication systems using RISs [17]. In particular, in the case of 5G and 6G communications, the importance of technologies utilizing RISs is increasing, as higher frequencies are used, as well as the density of the network, requiring technologies to realize high spatial resolution, i.e., an increase in channel capacity, with low hardware cost and low power consumption [18]. Currently, the frequency bands for 5G communication set by 3 GPP are divided into FR1, from 410 to 7125 MHz, and FR2, from 24.25 to 52.6 GHz [19]; FR1 is generally referred to as'sub6G' and FR2 as'mmWave'. In 5G communication, sub6G is known to cause severe shaded areas indoors due to the lack of spatial rank, but if an RIS is used, the rank can be increased by increasing the multipath component in space, and thus the throughput can be increased indoors. In the case of mmWave communication, due to the straightness of the radio waves, the radio waves are blocked by obstacles such as trees and buildings, reducing the communication coverage. However, if an RIS is used, the shaded areas are covered through the reflection of the signal, and the overall coverage can be increased [17]. As shown in Figure 7, the RIS can realize several functions in wireless channel reconfiguration,  
Figure 5: Concept of a smart wireless environment using reconfigurable metasurfaces [16]. Figure 6: Wireless channel modeling: (**a**) passive wireless channel and (**b**) active wireless channel.