# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
### RIS Introduction
such as creating a virtual line-of-sight (LOS) link to bypass obstacles between base stations and terminals through smart reflection and thus improving channel ranking conditions by adding signal paths in the desired direction; improving channel statistics/distribution by converting Rayleigh/fast fading to Rician/slow fading for ultra-high reliability; and suppressing/nullifying co-channel/inter-cell interference. In addition to the functional advantages of improving communication performance, the RIS has various practical advantages. First of all, a passive RIS consisting of a reflective element, such as a low-cost printed dipole, can be implemented and operated at a much lower hardware/energy cost than a conventional active array antenna because it can only passively reflect the incident radio waves without RF circuits at the transmitting radio frequency. RISs also operate in full-duplex (FD) mode and are free of antenna noise amplification and self-interference, which is an advantage over traditional active relays such as half-duplex (HD) relays, which have lower spectral efficiency, and FD relays, which require sophisticated techniques to eliminate self-interference. RISs are typically low-profile, lightweight, and conformal in shape, making them easy to mount/remove and deploy/replace on environmental objects. Finally, RISs act as auxiliary devices in wireless networks and can be transparently integrated, offering great flexibility and compatibility with existing wireless systems such as cellular or WiFi [20]. Since 2018-2019, when the concept of the RIS was first introduced, several considerations and critical perspectives have been presented on the advantages of the RIS as well as its practical implementation in wireless communication systems [21]. The first is the accurate electromagnetic modeling of the RIS, and most RIS-related studies assume that the metasurface comprising the RIS is an ideal reflector, but the hardware design for phase control, i.e., phase control resolution, RIS substrate characteristics, transmission/reflection/polarization characteristics of the incident radio waves, and the size of the RIS, should be considered. For example, modeling the size of the RIS is a very important issue because if the RIS does not reach a certain size, it is not possible to obtain excellent  
Figure 7: Application of wireless communication system using RIS. Adapted with permission from [20]. Copyright 2021 IEEE. SNR characteristics even with currently used relays. The second is channel modeling, which includes channel sensing and estimation, and most RIS-related studies assume that Channel State Information (CSI) can be perfectly obtained from base stations, RISs, and user terminals. However, currently, neither the base station nor the user terminal can recognize the RIS, and in the case of a wireless communication channel containing an RIS, the path loss between the base station-RIS and the RIS-user terminal must be considered; therefore, a real-time channel sensing and estimation method that considers multiplicative fading is required, unlike conventional channels. The third is the design of an RIS with excellent efficiency, which requires a large size and many array elements to operate over long distances, and a design that considers energy efficiency to solve this problem, and a design that considers the reflection efficiency depending on the direction and location of the RIS. Fourth, as for the performance parameters of the RIS, in the case of communication modules in general, radio frequency compatibility, signal compatibility, and resource management are tested for commercialization; however, in the case of the RIS, it is necessary to study which performance parameters should be tested because various combinations of the direction of incident radio waves and the direction of transmission/reflection are possible, as well as reflection performance.