Reconfigurable Intelligent Surface Aided Multi-User Communications: State-of-the-Art Techniques and Open Issues

Abstract

Reconfigurable intelligent surface (RIS)-aided communication is considered as an exciting research topic in academic and industrial communities since it provides an emerging affordable solution to achieve high quality and secure next-generation wireless systems. Especially, the deployment of RIS in multi-user wireless networks promises to reduce system hardware costs, signal processing complexity, as well as energy consumption due to small size, lightweight and ability to actively shape the wireless propagation environment. Further, by realizing a cost-effective radio environment, RIS-aided communication can be implemented to be an appealing technology for future integration with other emerging wireless applications and communication systems. Despite the positive appeal, RISs face new challenges that hinder integrating efficiently into wireless networks, such as network secrecy performance and system sum-rates, as well as achieving efficient deployment design in highly dynamic and time-varying wireless environments. To this end, we overview recent state-of-the-art techniques to address the above issues faced in the integration of RISs with various emerging multi-user communication techniques, such as Unmanned Aerial Vehicles (UAVs), Non-Orthogonal Multiple Access (NOMA), Millimeter Wave (mmWave) and Terahertz (THz) communications, Physical Layer Security (PLS), massive antennas, and Simultaneous Wireless Information and Power Transfer (SWIPT). Finally, we highlight promising future research directions of RIS-aided communication in Cell-Free Massive Multiple-Input-Multiple-Output (MIMO) systems, Rate-Splitting Multiple Access (RSMA), Light Fidelity (LiFi), and Cognitive Radio (CR) systems.

UAV-Assisted RIS for Future Wireless Communications: A Survey on Optimization and Performance Analysis

“…In [47], [48], the authors focused on analyzing the parameters such as reliability, secure transmission, channel estimation, and power optimization importantly, providing a detailed survey on the performance of the system with various optimization techniques. In [49], the authors have provided a brief summary of the UAV-assisted RIS system in a single subsection discussing its efficient results in the field. Whereas in [50], a detailed survey of the performance of a UAV with various communication and networking technologies is provided.…”

“…Reference Contribution [45] Explores the performance of RIS in PLS under different performance metrics and antenna classification [46] Implementation of deep learning techniques with RIS assisted Wireless networks [47] Discusses RIS performance in implementation with wireless networks with various optimization techniques at RIS and in the network [48] RIS-assisted THz communications and studies on implementing it with 6G networks [49] Survey on state-of-the-art technologies of RIS and its performance analysis [50] Insights and challenges on the performance of UAV assisted communication networks [51] Integration of UAV with IoT presented in various domains [52] UAV networks for enhancing the adaptability, privacy, secrecy, and scalability [53] Survey on UAV-assisted relaying networks with state-of-the-art technologies Our paper Survey on implementing the UAV-assisted RIS system in various optimization techniques and emerging technologies performance of the system. Table 3 provides a brief summary of the discussions below.…”

Intelligent metaphotonics empowered by machine learning

“…In this case, ML can be used as a feedback mechanism, redefining programming sequences of a digital metasurface to adjust to environmental changes. In recent years, this concept has developed into a new branch of intelligent metasurfaces, which rapidly gained a lot of attention for wireless communications at radio frequencies, especially in 6G and internet-of-things technologies [160][161][162][163][164][165][166][167]. Below we present only a few examples for illustrating those ideas.…”

Uplink Multiple Access for Reconfigurable Intelligent Surface-Aided Wireless Systems

“…However, as indicated in [10], despite PD-NOMA achieving user fairness via power allocation, this improvement in the performance of users with poor channel conditions may end up impacting users with good channel conditions. The integration of reconfigurable intelligent surfaces (RISs)-also known as intelligent reflecting surfaces (IRS)-and PD-NOMA [11] has been proposed as a way to address this disadvantage of PD-NOMA. RIS devices are made up of massive passive/active low-cost reflecting elements capable of smartly configuring the phase shift of incident electromagnetic signals via a RIS controller [11].…”

“…The integration of reconfigurable intelligent surfaces (RISs)-also known as intelligent reflecting surfaces (IRS)-and PD-NOMA [11] has been proposed as a way to address this disadvantage of PD-NOMA. RIS devices are made up of massive passive/active low-cost reflecting elements capable of smartly configuring the phase shift of incident electromagnetic signals via a RIS controller [11]. Hence, RISassisted NOMA systems can provide additional channel paths which are useful in providing PD-NOMA gain in unfavorable scenarios such as when the channel strengths of the far and near user are similar or mutually orthogonal as in downlink multiple-input-single-output (MISO) networks [10].…”