# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 2 Technical Considerations for RIS
### Hardware Implementation of RIS
In [81], a practical RIS design was proposed and near-field and far-field measurements for signal enhancement were performed. However, the operating bandwidth of the design [81] was 27.5 to 29.5 GHz (7% bandwidth), which cannot meet the wide bandwidth requirements of the 5G millimeter-wave band. [PERSON]. et al. [82] designed a small unit cell and a 25 \(\times\) 32 element RIS prototype operating at 28.5 GHz, but did not provide the operating bandwidth. Similarly, [PERSON] et al. [83] presented a 24 \(\times\) 24 RIS consisting of a crossed dipole-shaped unit cell operating at 29 GHz to derive a path loss model, as shown in Figure 20. In particular, they presented an RIS phase control system consisting of an Arduino microcontroller and 144 shift registers connected in 6 series and 24 parallel to control 1152 PIN diodes. In 2023, ref. [84] proposed a 20 \(\times\) 20 RIS covering the entire millimeter-wave 5G n257 (26.5 to 29.5 GHz) and n258 (24.25-27.5 GHz) bands. The fabricated RIS exhibited a broadband characteristic of 22.5-29.5 GHz (26.9%) and a beam steering characteristic of 50\({}^{\circ}\). PIN diodes or varactors typically have a large insertion loss characteristic, which further worsens overall efficiency. This can be mitigated by selecting diodes with low insertion loss, but at the cost of increased fabrication costs. Electrically controlled RISs can only realize a preset EM function, which is energy intensive in application scenarios where the EM function does not need to be reconfigured frequently. Therefore, elec  
Figure 19: Hardware implementation of RIS. (**a**) PIN diode—1 bit [76]. (**b**) PIN diode—2 bit. Adapted with permission from [78]. Copyright 2020 IEEE. (**c**) Varactor diode—1 bit. Adapted with permission from [79]. Copyright 2021 IEEE. (**d**) Varactor diode—analog. Adapted with permission from [80]. Copyright 2022 IEEE. Figure 20: Millimeter-wave RIS unit cell. 29 GHz operating unit cell. Adapted with permission from [83]. Copyright 2022 IEEE. tromechanical RISs have been proposed as RISs for signal enhancement in static areas such as indoor telecommunication shaded areas. Recently, various electromechanically reconfigurable metasurfaces based on piezoelectric actuators [85], spring devices [86], and micromotors [87] have been proposed. These reconfigurable metasurfaces, which consist of a passive metasurface and a mechanical actuation module, have the general advantages of both high efficiency and reconfigurability. However, since most of the passive metasurfaces and mechanical actuators are complex and limited in installation and only support discrete phase states, there is a need for a practical electromechanical RIS design that is easy to fabricate and assemble, low-cost, and flexible in wavefront tuning. In 2023, [PERSON] et al. [88] designed an RIS operating at 2.55 GHz consisting of a passive metasurface and a stepper motor drive module as shown in Figure 16.0 By grouping \(16\times 16\) unit cells into \(4\times 4\) supercells and controlling the phase with a 12 V voltage, they demonstrated an improvement in received signal performance of about 5.2 dB through indoor measurements. In order to realize the concept of a smart wireless environment, multiple RISs composed of reconfigurable metasurfaces are required to be installed in various environments that control the propagation channels, which requires high-quality communication services without visually affecting the RISs, if possible. Optically transparent RISs have potential applications in scenarios where high-quality communication services and good optical transparency must be provided simultaneously to a large number of mobile terminal devices. For example, optically transparent RIS can be used for glass curtain walls in urban buildings, glass walls in meeting rooms, aircraft, ships, trains, subways, shop windows and billboards in shopping malls, and showcase glass in museums, and solar panels can be installed on the back of the optically transparent RISs to achieve self-powered operation.