# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 2 Technical Considerations for RIS
### Hardware Implementation of RIS
In this case, it is possible to realize the required unit cell periodicity at 60 GHz with about 1 mm \(\times\) 1 mm [75]. The general architecture of the RIS consists of three layers and one RIS controller, as described earlier. The top layer acts as an antenna that directly controls the incoming electromagnetic waves. This layer consists of numerous reconfigurable conductive patches on a dielectric substrate. For practical implementation, this layer can also be equipped with a dedicated RF sensor to detect radio signals, which can directly sense the surrounding radio signals to support intelligent RIS configuration mechanisms. Then, an RF choke structure is placed in the middle layer to minimize RF signal leakage. Finally, control elements that reconfigure the reflective elements in real time are placed on the bottom layer. These control elements are typically implemented using commercial microcontrollers, SDR devices such as USRPs, or FPGAs, and interface with the entire network to receive and apply RIS control commands, as shown in Figure 18. A gateway interface is used to facilitate the transfer of information between the RIS and network equipment such as  
Figure 17: Bias circuits in the RIS. (**a**) Optical control bias circuit. Adapted with permission from [72]. Copyright 2023 IEEE. (**b**) Chip bias circuit. Adapted with permission from [73]. Copyright 2023 IEEE. base stations [76], and a wireless transceiver is used to provide the unit cell configuration information of the RIS through the gateway interface, which is usually coupled with a microcontroller. The microcontroller provides configuration signals to the bias network based on commands received from the network equipment and may also process data received from onboard error detection or sensing hardware before passing this information to the network controller. Most of the early RIS research was designed around the WiFi band. In 2020, [PERSON] et al. [77] designed RFocus, a two-dimensional surface with 3200 rectangular arrays of passive antenna elements operating at 2.4 [GHz], as shown in Figure 19a. Each passive unit cell has a size of \(\lambda/4\times\lambda/10\), and electromagnetic waves are reflected or refracted. They showed that RFocus surfaces can be fabricated at low cost and can improve the average signal strength by a factor of 9.5. [PERSON]. et al. [78] first designed a 2-bit 16 \(\times\) 16 RIS with a phase difference of 90\({}^{\circ}\) based on PIN diodes operating at 2.3 GHz and 28.5 GHz, as shown in Figure 19b. By using modular hardware and flexible software based on USRP, high antenna gains of 21.7 dBi at 2.3 GHz and 19.1 dBi at 28.5 GHz were achieved with low power characteristics. In 2021, [PERSON]. et al. [79] designed a size 55 \(\times\) 20 1-bit RIS operating at 5.8 GHz based on a varactor diode, as shown in Figure 19c, and measured a power gain of 26 dB compared to the reference value of replacing the RIS with a copper plate in an indoor test where the transmitter and receiver were separated by a 30 cm thick concrete wall, and 27 dB in a short-range outdoor measurement. Long-range measurements also confirmed successful transmission of a 32 Mbps data stream over 500 m and low-power operation of 1 W. With the RIS utilizing the continuous control characteristics of varactor diodes, [PERSON]. et al. [80] proposed the concept of using a 14 \(\times\) 14 RIS as ambient backscatter communications (AmBC) for IoT devices operating at 5.15-5.75 GHz (Figure 19d) and confirmed this with experimental measurements. Figure 19: _Cont._  
Figure 18: Example of control structure in RIS. Adapted with permission from [59]. Copyright 2023 John Wiley and Sons. Most of the published work on RISs focuses on idealized RISs and is limited to theoretical analysis and most of the experimental studies have been conducted in sub-6 GHz bands; there are not many studies related to practical RIS design and experimental characterization for millimeter-wave bands.