# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 2 Technical Considerations for RIS
### Hardware Implementation of RIS
Also, in 2019, Lumotive (a U.S. startup) and TowerJazz (a global foundry) announced beam steering ICs based on liquid crystal metasurface technology for LiDAR systems in autonomous vehicles. In 2019, Pivotal Commware proposed a new technology called Holographic Beamforming (HBF) using Software Defined Antenna (SDA). The proposed holographic beamformer has the advantage of lower cost and power consumption compared to other transmission technologies such as massive MIMO and phased array [20]. In 2023, Greenerwave announced a commercialized active RIS packaged as an AP extender with a 500 MHz bandwidth in the frequency range of 27.5 to 29.5 GHz [96]. In addition, Japan Display of Japan announced an RIS that combines liquid crystal metasurface technology with TFT technology, operating at 28 GHz and capable of beam steering of \(\pm 60\) degrees, as shown in Figure 24[97]. Implementing RIS hardware and applying it to real-world communications requires the following considerations. First, how to set up the experimental setup to measure the performance of an RIS. As it is already known, when the size of the array antenna is \(D\), the near-field and far-field regions are determined by \(2D^{2}/\lambda\). As the operating frequency increases and the size of the RIS increases, the RIS is more likely to be located in the near field. In the past, unit cell design has primarily focused on far-field scenarios. However, in indoor environments, near-field applications such as smart homes are becoming more prevalent. Studying the theory of electromagnetic field distribution in the near and far fields and analyzing the factors affecting the design of these components can effectively improve the efficiency of RIS systems in indoor scenarios. The second is whether the phase control should be 1 bit or 2 bit, or whether finer resolution control is required. As discussed earlier, the phase resolution and phase shift range of each unit cell have an important impact on the RIS radiation performance. In addition, the single functional cell of the RIS prototype often induces discrete phase changes due to the limitations of the hardware structure. Since a 1-bit RIS can only provide two phase states, the antenna efficiency is low and the sidelobes are high, reducing the antenna  
Figure 16.3 NTT Docompo RIS trials in (**a**) 2018. Adapted with permission from [93]. Copyright 2019 IEEE (**b**) 2020, and (**c**) 2021. Figure 16.4 RIS using liquid crystal metasurfaces [97]. gain by more than 3 dB [66]. To mitigate the performance degradation caused by 1-bit phase quantization, some researchers have designed multibit RIS boards; however, multibit designs increase system complexity and hardware cost. Two-bit phase quantization is a compromise between system design complexity and component performance, with antenna gain loss of less than 1 dB and improved sidelobes [98]. Therefore, when designing RIS, researchers should consider the high precision gain brought by multiple bits and the complexity of system design. However, through block matching of different cells and controllable coupling design, \"supercells\" of continuous phase change can be realized in RIS prototypes; therefore, it is necessary to develop RIS prototypes that can arbitrarily adjust the phase resolution to meet high-precision requirements. The third is how to realize hardware with broadband characteristics. According to [PERSON]'s theory, communication performance is limited by bandwidth; therefore, the design of an RIS with a wide operating bandwidth is essential for high-speed wireless communication systems. To increase the bandwidth, research is being conducted on unit cell design with double resonant elements and unit cell design using an air gap. Fourth, it is necessary to determine the size of the RIS and the optimal number of unit cells for sufficient communication performance.