Reconfigurable Intelligent Surfaces for Exploitation of the Randomness of Wireless Environments

As the wireless world moves towards the beyond fifth generation (B5G) era, innovative applications require higher reliability, data-rates and traffic demands. Technological advances, such as massive multiple-input multiple-output, full-duplexing, and high-frequency communications, have been advocated, due to their unfordable cost, power-consumption, as well as their need to operate in unfavorable electromagnetic wave propagation environment. As a remedy, the exploitation of the implicit randomness of the propagation environment through reconfigurable intelligent surfaces (RIS) to improve the quality of experience, attracts the eyes of both academia and industry. RISs transform wireless systems into smart platforms capable of sensing the environment and applying customized transformations to the radio waves. The RIS-assisted smart environments have the potential to provide B5G wireless networks with uninterrupted wireless connectivity, and enable data transmission without generating new signals but recycling existing radio waves. Motivated by the advances in this area, this chapter focuses on presenting the technology enablers and the state-of-the-art of RIS-assisted wireless systems, the need of new channel and system models as well as theoretical frameworks for their analysis and design, as well as the long-term and open research issues to be solved towards their massive deployment.