# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 2 Technical Considerations for RIS
### Noise and Power in RIS  
Due to the passive operation mechanism of the RIS, the noise power introduced by the RIS is very low, and many studies on RISs have assumed that the noise power of the RIS is negligible. However, since the radiating elements of the RIS unit cell are mostly patch antennas, the RIS actually generates noise [142]. Regardless of whether the RIS is passive or not, as long as the temperature is not absolute zero, the Brownian motion of the electrons is naturally present and thus noise is generated. The Johnson-Nyquist noise states that the noise power introduced into each device \(p_{n}\) is not lower than the theoretical limit of \(p_{n}=kT_{p}B\), which is the product of the Boltzmann constant \(k=1.3807\times 10^{23}\) JK\({}^{-1}\), the operating bandwidth \(B\), and the thermal noise temperature \(T_{p}\), with a standard value of 290 K [142]. The \(SNR_{0}\) input to the receiver through the antenna is given by Equation (1), where \(\eta_{r}\) is the radiation efficiency, \(\tau\) is the mismatch loss, \(S_{i}\) is the input signal, \(T_{r}\) is the noise temperature of the receiver, and \(T_{a}\) is the noise temperature of the antenna [143]. \[SNR_{0}=\frac{\tau\eta_{r}S_{i}}{kB\big{(}\tau\eta_{r}T_{a}+\tau(1-\eta_{r})T _{p}+T_{r}} \tag{1}\]  
It can be seen from Equation (1) that if the RIS has sufficient directivity and gain, the signal power is sufficiently greater than the internal noise plus \(kT_{p}B\). However, if the noise power due to low radiation efficiency, mismatch losses, etc. is greater than that, the output signal-to-noise ratio, \(SNR_{0}\), through the RIS will decrease. In fact, the noise power introduced into each unit cell due to system noise is higher. For example, a passive reflector operating at an operating frequency of 915 MHz and a 250 kHz subcarrier has a noise power of about \(-107\) dBm, which is well above the natural lower bound [143]. If an RIS with a 100 MHz operating bandwidth has 10,000 unit cells, the noise power of the RIS can be estimated to be at least \(-54\) dBm. If the path loss of the RIS-receiver link is \(-30\) dB, the noise power at the receiver will be \(-84\) dBm, which is not small compared to the original noise power of the receiver [126]. In this case, when the number of RIS unit cells becomes large enough, the total noise at the receiver is gradually dominated by the noise introduced by the RIS, which becomes non-negligible and adversely affects the channel capacity [144]. As the RIS passively reflects the incident signal without high-power components such as power amplifiers and ADC/DACs, it consumes nearly zero power to realize phase shifting compared to conventional techniques such as massive MIMO and relays [144]. Therefore, the RIS is expected to significantly reduce the power consumption required for future 6G communications [105]. However, because hardware power is required to drive and reconfigure the RIS [105], the energy consumption may be higher than expected when the RIS is used in a real-world environment. For example, the power consumption  
Figure 35: Hybrid RIS. **(a)** Hybrid relay–RIS structure [136]. **(b)** Hybrid RIS structure. Adapted with permission from [140]. Copyright 2022 IEEE. to drive an RIS with 1600 1-bit unit cells has been measured to be about 11.2 W [145]. In addition, reconfiguring the RIS requires additional power to support the RIS controller and transmit control signals, and realizing beamforming of the RIS requires additional power for channel estimation, channel feedback, and beamforming design. Therefore, the total power consumption generated by the RIS installation may not be low, and it is necessary to study the RIS with low-power structure and corresponding transmission methods to achieve energy efficiency in future 6G networks.