# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 2 Technical Considerations for RIS
### Differences between Relay and RIS [101]
It has been shown that an RIS with large metasurfaces and massive devices can outperform active relays in terms of spectral efficiency and energy efficiency [21; 101; 106]. The differences between an RIS and relay are shown in Figure 27 and Table 6, respectively. \begin{table}
\begin{tabular}{c c c} \hline
**Parameters** & **RIS** & **Relay** \\ \hline \multirow{4}{*}{Definition} & The reflecting surface which scatters & The device which decodes incoming \\  & incoming EM waves with controllable & EM signal, amplifies and retransmit it \\  & delay/phase and polarization is known as an & RIS. \\ \hline Operating mechanism & Passive/active reflection & Active reception and transmission \\ \hline Duplex & Full & Half/full \\ \hline Number of transmit RF & Zero & N \\ Chains required & & \\ \hline ADC/DAC and amplifier & Not required & Required \\ \hline Energy consumption & Low & High \\ \hline \multirow{4}{*}{Spectral efficiency formula} & Spectral efficiency (IRS): & Spectral efficiency of Relay : \\  & Where, & \\ \cline{1-1}  & \(\rho\) - Signal Transmit Power & Where, \\ \cline{1-1}  & \(N\) - IRS elements & \(\rho\) - Signal Transmit Power \\ \cline{1-1}  & \(\beta_{SR}\) -Path Loss between Source and Reflector & \(\beta_{SR}\) -Path Loss between Source and Relay \\ \cline{1-1}  & \(\beta_{RD}\) -Path Loss between Reflector and Destination & \(\beta_{RD}\) -Path Loss between Relay and Destination \\ \hline \end{tabular}
\end{table} 
Table 6: Comparison of RIS and relay [101]. Figure 27: Comparison of RIS and relay. Adapted with permission from [101]. Copyright 2020 IEEE. (a) RIS. (b) Relay. RISs consist of a composite material multilayer structure with metal or dielectric patches printed on a grounded dielectric substrate, with reconfigurability implemented through low-power and low-complexity electronic circuits (switches or varactors). RISs typically do not require dedicated power amplifiers, mixers, and DACs/ADCs, making them less complex than relays if implemented using mass-produced and inexpensive large-area electronics. In addition, passive or nearly passive RISs are not affected by noise because they do not amplify or regenerate signals, but their performance can be degraded with phase noise [107; 108]. RISs configured to act as reflectors are not affected by HD constraints and loopback self-interference, unlike relays. RISs are well suited for near-passive implementations because even metasurfaces without reconfigurability can be implemented with completely passive components, and low-power active components (switches or varactors) are only needed to ensure reconfigurability. The low-power nature of switches and varactors also lends itself to near-passive implementations using energy harvesting. In relay-assisted systems, it is typically assumed that the total RF power is allocated between the transmitter and relay to ensure total power constraint. In RISs, the total RF power is used by the transmitter; in addition, the power reflected and scattered by the RIS depends on the transmittance, which can be optimized through proper design of the metasurface [14]. In the ideal case, the total power reflected by the RIS is equal to the total power of the incident radio waves. From a system model perspective, the RIS can be considered a full-duplex MIMO relay with no self-interference. In addition to the differences in system models, there are also differences in how performance is viewed in different cases. Finally, from an operator perspective, there are differences between the RIS and relay in how they are actually deployed and applied. RISs can be deployed in a more flexible way than relays because they can operate in a passive or nearly passive manner. Power and performance losses can be compensated for by increasing the number of reflective elements. The most important characteristic of RISs is that they can be independent of the power supply, which enables flexible, on-demand deployment of 6G networks. It not only complements coverage, but also provides on-demand services for users and traffic.