Cooperative Hybrid Networks with Active Relays and RISs for B5G: Applications, Challenges, and Research Directions

Among the recent advances and innovations in wireless technologies, reconfigurable intelligent surfaces (RISs) have received much attention. RISs are envisioned to be one of the enabling technologies for beyond 5G (B5G) networks. On the other hand, active (or classical) cooperative relays have played a key role in providing reliable and power- efficient communications in previous wireless generations. In this article, we focus on hybrid network architectures that amalgamate both active relays and RISs. First, we discuss each technology's operations concepts and protocols. Subsequently, we present multiple use cases of cooperative hybrid networks, where both active relays and RISs can coexist harmoniously for enhanced rate performances. In addition, we provide a case study demonstrating a communications network's achievable rate performance, assisted by either an active relay, an RIS, or both -- and with different relaying protocols. Finally, we present the reader with challenges and key research directions in this area.