With the current deployment of the fifth generation (5G) of communication systems, it is now a critical time to identify enabling technologies for the sixth generation (6G) of communication systems. 6G systems are expected to fulfill more stringent requirements than 5G systems, on transmission capacity, reliability, latency, coverage, energy consumption, and connection density. Existing 5G technologies, such as millimeter-wave communications, massive multi-input multi-output systems, ultra-dense heterogeneous networks, are mainly focused on the system design at the transmitter and receiver sides, and on the deployment of additional network infrastructure with power amplification and digital signal processing capabilities and backhaul availability. The purpose of currently available 5G technologies is mainly to cope with or to capitalize on often-unfavorable wireless propagation environments. In fact, the wireless environment has been conventionally modeled as an exogenous entity that cannot be controlled but can only be adapted to. According to this design paradigm, communication engineers usually design transmitters, receivers, and transmission protocols based on the specific properties of the wireless channels and for achieving desired and target performance.
Recently, reconfigurable intelligent surfaces (RISs) have emerged as a promising technology for their capability of customizing the wireless propagation environment through nearly passive signal transformations. An RIS is a planar structure that is engineered to have properties that enable the dynamic control of the electromagnetic waves, though, e.g., signal reflections, refractions, focusing, collimation, and their combination. In wireless communications, RISs are intended to realize so-called programmable and reconfigurable wireless propagation environments, i.e., wireless environments that are not viewed and treated as random uncontrollable entities but become part of the network design parameters that are subject to optimization for supporting diverse performance metrics and quality of service needs to fulfill the stringent requirements of 6G networks. Recent applications of RISs in wireless communications include their user as nearly passive relay-type surfaces, signal-RF multi-stream multi-antenna transmitters, and reconfigurable ambient backscatters.