# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 2 Technical Considerations for RIS
### Hardware Implementation of RIS  
In this section, we present the State-of-the-Art RIS hardware architecture and some representative implementations [55]. Compared to the transceiver technology currently used in wireless networks, the main feature of the RIS concept is that it is an extension of the reconfigurable metasurface concept, which provides the possibility to make and control the EM response to the incoming radio waves, thus controlling the wireless environment. Traditionally, reconfigurable reflectarray antennas are similar in structure to an RIS, and several studies on reconfigurable metasurfaces have provided a way to dynamically control the unit cell reflection characteristics [56]. Two common ways to control the reflection response in reconfigurable reflectarray antennas are through the use of tunable delay lines and tunable resonant elements: the former technique involves coupling the radiating element to a section of transmission line whose electrical length is varied to provide different phase-shift characteristics before the coupling and re-radiation of the incident wave, and the latter technique involves changing the resonant frequency of the radiating element itself through reactive loading or switching structures [57]. Currently, the latter technique is mainly used in RISs. There are a wide variety of electromagnetic conversion and sensing functions that can be achieved with the RIS and related technologies, including absorption, reflection, polarization, focusing, and phase shifting, as shown in Figure 13. Achieving this level of functionality requires careful design of the RIS unit cell and associated processing, bias, and control circuitry. As described earlier, the RIS structure is an extension of the metasurface structure, where the metasurface structure consists of a two-sided structure of a unit cell structure with a ground (Figure 14a), the reconfigurable metasurface structure adds a bias layer to the reconfigurable element, and the RIS consists of the following three substrate layers (Figure 14c): a metasurface layer, a circuit network layer, and a control layer. Figure 12: RIS channel modeling. (**a**) Classification of approaches. (**b**) How to model channels with RIS cascade [54]. The metasurface layer of an RIS typically takes two forms. One approach is a semi-array-like structure with array elements placed in a region of \(\lambda/2\), and the other approach is a reconfigurable metasurface structure using elements with periodicity much smaller than \(\lambda/2\)[60]. Reconfigurable metasurfaces employ reconfiguration mechanisms that can be divided into the following three categories: circuit reconfiguration, geometric reconfiguration, and material reconfiguration. Circuit reconfiguration is achieved through impedance changes within unit cell elements such as switches and variable capacitances. The main advantages of RISs are low fabrication cost, low latency, and low power consumption; at microwave frequencies, most RIS designs are based on integration with other voltage-driven circuit elements such as PIN diodes, varactors, or MEMS as circuit reconfiguration, which is the most popular approach in practice [61]. For example, the ON and OFF states of a PIN diode acting as a switch in the unit cell of an RIS can provide different resonant modes, providing one, two, or more phase states. Unlike PIN diodes, which can only provide discrete phases, varactors can be used to implement continuous phase changes in RIS designs. Geometric reconfiguration involves changing the electrical shape of the device, such as  
Figure 14: Comparison of the cross-sectional structure of metasurface and RIS. (**a**) Metasurface. (**b**) Reconfigurable metasurface. (**c**) Reconfigurable intelligent surface. Adapted with permission from [59]. Copyright 2023 John Wiley and Sons. Figure 13: Hardware architecture of RIS. Adapted with permission from [58]. Copyright 2010 IEEE.