# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 2 Technical Considerations for RIS
### Passive, Active, and Hybrid RIS  
The RIS is an array structure consisting of a very large number of passive elements that reflect electromagnetic signals in a desired manner to reconstruct the propagation characteristics of the wireless environment. Thanks to its high array gain, low cost, low power, and negligible noise, the RIS is expected to improve channel capacity, expand coverage, and reduce power in future 6G networks. The \"RIS is passive\" because it passively shifts the phase of a reflected signal without any active RF components. However, while the signal processing of the RIS is passive, the RIS itself is not completely passive as it uses active components to drive and reconfigure. For example, if a PIN diode is used to control the phase, current is required to keep the PIN diode ON, which is connected to some active components, and is called a nearly passive RIS. An important advantage of the RIS is that the noise introduced by a passive RIS is negligible, resulting in a \"square-law\" array gain [126]. This means that the receiver SNR achieved by passive beamforming of the RIS is asymptotically proportional to \(N^{2}\) when the number \(N\) of the RIS unit cells is large. Thus, it can be seen that the desired signal power at the receiver is proportional to \(N^{2}\) due to the coherent superposition of the received signals, while the unwanted noise power at the receiver is invariant due to the negligible noise assumption [71]. Therefore, the RIS is expected to result in significant capacity gains in wireless systems by the square law, but in practice, these capacity gains are usually only observed in communication scenarios where the direct link between the transmitter and receiver is completely blocked or very weak [127]. On the other hand, in scenarios with strong direct links, the RIS can achieve very little visible gain,  
Figure 31: Application of STAR-RIS in wireless environment. Adapted with permission from [124]. Copyright 2021 IEEE. and many RIS elements are needed to compensate for this loss [45]. This is due to the \"multiplicative fading\" or double path loss introduced by the RIS, i.e., the equivalent path loss of the transmitter-RIS-receiver link is the product (not the sum) of the path losses of the transmitter-RIS and RIS-receiver links, which is typically thousands of times larger than the direct link [45]. As a result, due to the \"multiplicative fading\" effect, a passive RIS is almost impossible to achieve a noticeable capacity gain in many wireless environments. Many existing studies on the RIS often ignore this effect by only considering scenarios where the direct link is severely impaired [128]. Therefore, addressing the fundamental performance bottleneck caused by the \"multiplicative fading\" effect is an important challenge for the RIS to improve its practicality in future 6G wireless networks. To solve these problems, the concept of an active RIS was proposed in [71; 129]. The characteristic of an active RIS is that it cannot only adjust the phase shift, but can also amplify the attenuated received signal to the normal intensity level at the first hop. Therefore, an active RIS can effectively avoid the double path loss attenuation. The hardware architecture of an active RIS is fundamentally different from a passive RIS. To amplify the signal, active RISs are equipped with phase transition circuits and reflective amplifiers (e.g., current inverting converters) [71]. Unlike the low-power passive RIS, the amplifiers in active RISs and BSs can consume similar power, so the power consumption can no longer be ignored. The SNR values of passive RISs and active RISs are plotted against the RIS unit cell number \(N\) in Figure 32. Here, \(N\) ranges from 10 to 1000 in Figure 32a and from \(10^{4}\) to \(3\times 10^{6}\) in Figure 32b. From these figures, we can see that when \(N\) ranges from 10 to 1000, the SNR achievable by the user is about 40 dB higher for the active RIS-enabled system compared to the passive RIS-enabled system.