# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 4 Conclusions
The choice of which type of RIS to select depends on the requirements of the deployment scenario of the RIS. If the RIS is used outdoors, it seems that various types of configurations such as passive/active, reflective/transmissive, etc. are possible. However, in the case of a passive RIS, a review of the management entity is required because it is not an operator-oriented structure; in the case of an active RIS, the implementation of CSI on the service operator side will be essential. Therefore, a mixed RIS structure in which only some of the cells of the total unit cells comprising the RIS have sensor functions for CSI, or only some of the cells have an active structure, is expected to be the ultimate solution. In addition, since the RIS is located outdoors, it is necessary to consider securing publicity, such as an alliance between operators or government initiative to overcome the same problems of environmental pollution and duplicate investment that occurred in the existing individual base station installation by implementing a different RIS for each operator. In the case of O2I, where the base station of the RIS is located outdoors and the user is indoors, the RIS can be used as a transmissive type in the windows of buildings, etc. Therefore, an RIS structure with transmissive characteristics and a transparent RIS for the use in windows are required. Therefore, in this case, the RIS requires a reconfigurable structure with transparent unit cells. Among the current research results, the most favorable structure in terms of transparency is the reconstructed structure using VO2. In the case of the transparent RIS using metal mesh with a PIN, varactor, or RF switch, even though the overall light transmittance is high, the RIS is difficult to use due to the opacity of the lumped elements. If the RIS is located indoors, it should be used to improve performance against external electromagnetic signals from windows and other sources, so that the RIS can be used as a passive or active reflective structure. As mentioned earlier, the passive type has a fixed reflection angle and can only be used in certain scenarios, and the RIS must be constructed face-to-face for it to be effective. In the case of a passive RIS, the management entity will need to be a government, building owner, or a coalition of operators because it is operated independently of the operator. In the case of an active RIS used indoors, among the currently proposed technologies, a lumped device-based RIS with a PIN or varactor seems to be advantageous in terms of technical maturity. However, there are problems with it such as the price required for circuit configuration and the implementation of CSI technology for the RIS operation that is limited by the operator. Therefore, the recently researched liquid crystal-based RIS seems to be a promising alternative due to its price competitiveness and the use of TFT technology already perfected in the display industry; however, its disadvantage is that it is only available in millimeter-wave and higher frequency bands, and it is necessary to continue research on issues such as overcoming technical limitations on slow switching times. The commercialization of the RIS to enable B5G/6G communications requires, first, the identification of killer applications to ensure commercial viability for a complex multi-panel RIS and/or multi-user scenarios and to identify industry verticals that show the greatest benefit in terms of return on investment. Second, it requires sensing and network transmission technologies, including those related to channel modeling, channel estimation, and feedback techniques to maintain and improve the quality of communication services. Third, there is a need for concerted standardization efforts and broad consensus to determine appropriate channel models, waveforms, control signals, and performance requirements, considering possible spectrum, technology, and test solutions.