## 1 Introduction  
Fifth-generation (5G) wireless networks have already been installed in several countries as of mid-2019, and the first 5G mobile devices are already on the market. As the world is watching the 5G network start to take off, a new evolution of mobile communications has emerged, one that includes three use cases with distinct requirements: increased mobile Internet access, ultrareliable and low-latency communications, and vast machine-to-machine interactions. From this standpoint, the wireless industry is looking ahead to Beyond-5G (B5G), or sixth-generation (6G), technological breakthroughs in anticipation of new applications that includes the three use cases while fulfilling the increased demand in wireless communication and sensing networks with greater and greater agility, coverage, and throughput [1]. Previously, due to mobile data and cellular growth, it has become essential to install more small cells to counter the rapid increase in the number of mobile subscribers. This has opened the door for distributed antenna systems (DASs), a network of spatially separated antennas that are connected to common sources via transport medium (coaxial line or fiber optic line) to provide wireless service within a geographic area or structure. Because DAS systems are substantially smaller than typical macro cell or cell tower counterparts, they are far superior solutions for congested, high-volume areas. DAS networks tend to give better network coverage than typical cell towers, in addition to taking up less space. The quality of a cellular signal degrades as the distance between a receiver and transmitter increases; however, to ensure that mobile users are always within close proximity to a strong and fast signal, DAS is used by strategically placing receivers in targeted locations. However, there are some major drawbacks of utilizing the DAS system in the cellular communications network. When expanding the network coverage and bandwidth, one of the main factors for design planning is the cost. DAS systems, unfortunately, are not cheap. In fact, DAS networks are substantially more expensive than small cell networks [1]. The necessity for fiber-optic cables to connect each radio head to a central hub, as well as the creation of the hub and the location of the cellular base stations, all contribute to the price difference. Backhaul routing is also a concern for the DAS network. The installation process can be complicated because each of the radio heads used to broadcast the cellular signal requires routed fiber-optic connections. In order for the system to work, each of the radio heads must be connected to the central hub via fiber-optic cable. With all of these considerations in mind, researchers have started to uncover another breakthrough or alternative in enhancing the cellular coverage. In much the same way that DAS began as a promising research subject a few years ago and became a critical enabler for the evolution of commercial deployment from fourth-generation (4G) to 5G, reconfigurable intelligent surfaces (RISs) have attracted significant interest from academic and the telecommunication industry at this early stage of research [2]. RISs could be a promising option for creating a software-configurable smart radio environment. An RIS is a large, thin meta-surface made of metallic or dielectric material that is made up of a collection of passive subwavelength scattering elements with a particularly defined physical structure. Its elements can be manipulated in software to change the electromagnetic (EM) propagation properties such as phase shifting of the incident of reflected radio frequency (RF) signals. Other distinguished characteristics of RIS are as follows: RISs are made of cost-effective materials, are practically passive, and do not require a specific energy source [3; 4; 5]. They may also be simply installed on building facades, factory ceilings, and internal spaces because they have a physical appearance similar to that of a mirror.