# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
###### Abstract  
Reconfigurable intelligent surfaces (RISs) are programmable metasurface structures that can control the propagation of electromagnetic waves by changing the electrical and magnetic properties of the surface. They can be used to intelligently reconfigure the wireless environment to improve the capacity and coverage of wireless networks. In recent years, numerous theoretical innovations and prototype tests have demonstrated that the RIS has the advantages of low cost, low power consumption, and easy deployment, and creates many potential opportunities and broad application prospects in 5G and future 6G networks. In this paper, starting from the technological development of RISs, we discussed the technical issues of RISs. The standardization of RISs, types of RISs according to operation modes, channel modeling, considerations for hardware implementation, differences from existing communication modules and the need for active RIS implementation, noise and power characteristics to ensure the efficiency of RISs, and performance parameters of RISs and field test results of RISs in indoor and outdoor environments were reviewed. By resolving the current technical issues of RISs, it is expected that RISs will be successfully used for B5G/6G communication through commercialization.  
reconfigurable intelligent surfaces; metasurface; B5G/6G communication 2023212489 12  
One of the largest applications of phase-gradient metasurface structures is beam steering, where the phase shift of each unit cell in a metasurface is used to control the direction of transmitted or reflected radio waves. This phase change can be accomplished in the following two ways: passively, by designing each unit cell in the metasurface to have a different phase, and actively, by controlling the same unit cell electrically or mechanically to control its phase, amplitude, polarization, etc. [9]. In 2011, [PERSON] et al. [10] designed and fabricated a 160 \(\times\) 160 reflectarray antenna with beam steering characteristics for millimeter-wave imaging at 60 GHz using PIN diode switching, as shown in Figure 3, suggesting that beam steering is possible due to the phase change caused by PIN diode switching. In 2014, [PERSON] et al. [11] proposed the concepts of coding metamaterials, digital metamaterials, and programmable metamaterials, and showed that it is possible to control the incoming electromagnetic waves by turning on/off (phase change) each unit structure comprising the metamaterial, as shown in Figure 4. Attempts have been made to apply electromagnetic surface structures such as these metamaterials to communications, as shown in Table 1.  
Figure 1: Expected 6G technologies.  
Figure 2: Metasurface structure—a generalized application of [PERSON]’s law.  
In 2019, [PERSON] et al. [16] presented the concept of a reconfigurable intelligent metasurface to realize a smart wireless environment by solving the following two major problems of wireless communication: meeting user requirements for radio waves by controlling the reconfigurable metastructure that can control the scattering, reflection, and refraction characteristics of radio waves, and by solving the problem of increasing power consumption due to the transmission of new signals, as shown in Figure 5.