# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 2 Technical Considerations for RIS
### Hardware Implementation of RIS
In [89], an optically transparent 12 GHz-operating reconfigurable metasurface based on ITO film was proposed, as shown in Figure 22a. This structure has an optical transmittance characteristic of 80%, and a photovoltaic panel was attached to the back to realize an energy source. The disadvantage of ITO is that it has relatively large electromagnetic losses due to the physical properties of the material. In 2022, [PERSON] et al. [90] designed a 2-bit photonic RIS operating at 3.5 GHz based on a metal mesh with high conductivity, which realized excellent beam steering characteristics from \(0^{\circ}\) to \(60^{\circ}\), but had a low optical transmittance of 49.3%. When lumped elements such as an RLC are used to realize an optical transparent RIS, it is difficult to obtain a high optical transmittance due to the opacity of the device; therefore, vanadium dioxide (VO2), which can be actively controlled by current and temperature, is sometimes used. In [91], a transparent RIS with 88% optical transmittance and controllable loss characteristics from \(-1.27\) dB to \(-15.38\) dB at 10 GHz was designed and fabricated by combining a mesh-implemented LC resonator and a VO2 pattern. In [92], the bottom layer was realized with a metal mesh stripe pattern and the top layer with a metal mesh lattice pattern to improve the optical transparency of the RIS device to 79% without affecting its microwave properties in the operating band of 3.53-3.7 GHz. Existing optically transparent design technologies are immature and unreliable, posing challenges  
Figure 16.1 Unit cell of an electromechanical RIS [88]. (a) Schematic diagram. (b) Driver module of the RIS supercell. (c) Photograph of the driver module. in the processing of metal vias and multilayer structures, surface mounting of components, design of direct current (DC) supply networks, and other design and fabrication issues. Therefore, designing and fabricating optically transparent reconfigurable RISs with the same electromagnetic properties as conventional metasurfaces remains a great challenge. Regarding commercial RISs, NTT Docomo, together with Metawave, conducted a field test of a metamaterial reflector prototype operating in the 28 GHz band in 2018 [93]. During the test, the BS was installed on the roof of a building, making the road below the building a coverage blind spot, as shown in Figure 23a. By placing metamaterial reflectors to cover this area and adjusting the beam to reflect in this area, SNR improvements of over 15 dB and speed improvements of over 500 Mbit/s were observed [93]. In 2020, trials were conducted with AGC Inc. with a transparent dynamic metasurface [94]. The transparent dynamic metasurface consists of two transparent substrates, one of which has a predesigned metamaterial pattern printed on it, and the distance between the two substrates can be adjusted to control the following three modes: full transmission, full reflection, and hybrid mode. In full transmission mode, the metasurface has a transmission loss of only about 1 dB, while in full reflection mode, it has a transmission loss of more than 10 dB. An improved version of the transparent dynamic metasurface was tested in 2021, which is capable of focusing millimeter waves [95]. At the focal point, the metasurface can improve the SNR by about 24 dB compared to ordinary glass. Placing a repeater at the focal point of the metasurface, as shown in Figure 23c, can be used as a solution to improve indoor coverage of millimeter waves, providing a reliable link between an outdoor mm wave BS and an indoor UE [96]. Figure 16.2 Optically transparent RIS unit cell. (**a**) ITO-based RIS. Adapted with permission from [89]. Copyright 2019 IOP Publishing Ltd. (**b**) Metal mesh-based RIS. Adapted with permission from [90]. Copyright 2022 John Wiley and Sons. (**c**) VO2-based RIS [91]. (**d**) Light transmittance-enhanced metal mesh RIS [92].