Review Paper on Hardware of Reconfigurable Intelligent Surfaces

Recently reconfigurable intelligent surface (RIS) has attracted great attention because it can create a smart wireless environment. Hence it can enhance the capacity and coverage of the wireless network significantly. A thorough review of RISs has been presented in this paper focusing on the hardware aspect of the RIS. Beyond-5G/6G communication will have a smart propagation environment, where RIS can be used for such communications. RIS consists of various small unit cells. The unit cells should have some tunning mechanism to reflect or transmit the incoming waves in the desired direction. It is possible to tune the impedance of the unit cells using PIN diodes, varactor didoes, microelectromechanical (MEMS), thermal, and other ways. In this paper, the background of RIS has been discussed where RIS will play a significant role in beyond-5G/6G communications. We have also added the theoretical background of RIS and motivations to writing this paper. After that several published papers in the literature have been presented so that the readers can get an overall idea about the RIS and its hardware. Hence, this paper will be very useful for practitioner engineers and researchers. RISs have been presented in various tables and various parameters have been presented. We have discussed challenges and solutions for the hardware of the RIS design. We have also discussed potential research and research gap that can be explored in the future. Lastly, we have added a conclusion for this review paper. In our manuscript, we have added 154 references. There are various kinds of RIS available in the literature. We have added different types of RISs in this manuscript. The magnitude and phase of the reflection and transmission coefficients are the main parameters of any kind of RIS.