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Reconfigurable intelligent surface (RIS)—known also as intelligent reflecting surface (IRS), and large intelligent surface (LIS)—is a programmable structure that can be used to control the propagation of electromagnetic (EM) waves by changing the electric and magnetic properties of the surface. These elements form a low-profile 2D reflecting or transmission array, which is able
to manipulate the phase, amplitude, polarization or frequency of the incident electromagnetic waves.
With the tunable electronic circuit embedded in the element, for example, a reflection-type RIS is able to steer the incident wave of a specific frequency band to the desired reflecting direction
in real-time, generating an anomalous reflection rather than a specular reflection, and thus is capable of creating an intelligent electromagnetic environment for wireless communications
In addition to the control of EM waves, RISs can be used to sense the radio environment by integrating sensing capabilities into them. By placing intelligent surfaces in the environment where wireless systems are operating, the properties of the radio channels can be controlled at least partially.
A Large Intelligent Surface (LIS) makes new and disruptive applications that require high energy efficiency and transmission reliability, low latency and ability to interact with the environment, possible. These will enable a future where man-made structures become more and more electronically active, with integrated electronics and wireless communication making the entire environment intelligent.
The RIS is mainly considered to be a beyond 5G technology operating at frequencies from tens of gigahertzs (GHz) to terahertzs (THz). At these frequency bands, the signal propagation is heavily attenuated and can be blocked completely by the obstacles in the propagation environment. In these cases, RIS is found to be a low-cost and promising technique for combating blockage and path loss with moderate-sized surfaces and enables communication between the TX and RX.
Two approaches can be used to implement an RIS that control the impinging signals characteristics (e.g., reflection, refraction, absorption, focusing and polarization), namely, conventional antenna arrays and metasurfaces.
Metasurfaces are thin two-dimensional metamaterial layers that allow or inhibit the propagation of electromagnetic waves in desired directions. For example, metasurfaces have been demonstrated to produce unusual scattering properties of incident plane waves or to guide and modulate surface waves to obtain desired radiation properties. These properties have been employed, for example, to create innovative wireless receivers and transmitters.
In a nutshell, metasurfaces are electrically thin and dense two-dimensional arrays of structural elements possessing desired properties granted by their constitutive elements. Elements are called meta-cells, unit-cells or meta-atoms. The meta-cells size is much smaller than the signal’s wavelength (λ) typically, between λ/10 and λ/5. In addition to the control of EM propagation, metasurfaces can be used to realize complex operations such as data modulation and mathematical operations and they have been demonstrated to be capable of storing EM pulses for short time periods in the order of 15 ns.
RIS is a new type of system node leveraging smart radio surfaces with thousands of small antennas or metamaterial elements to dynamically shape and control radio signals in a goal-oriented manner. The technology will effectively turn the wireless environment into a service, inspiring a host of new use cases.
These include enhancing key-performance-indicators for various systems such as coverage and capacity, as well as enabling new applications such as localization and sensing. As an example, an RIS can reconfigure the radio environment to sense human posture and detect someone falling, a very useful application for elderly care. RIS, thanks to its associated characteristics, is expected to serve as a key technology in future wireless systems, including for 6G.
RIS can be potentially deployed for both indoor and outdoor usage, including offices, airports, shopping centres, lamp posts, and advertising billboards, and may take any shape or be integrated onto objects. Additionally, the characteristics of RIS may result in low energy consumption, making it a sustainable, environmentally friendly technology solution. RIS can be configured to operate at any part of the radio spectrum, including frequencies from sub-6 GHz to THz, and may use tools from Artificial Intelligence and Machine Learning (AI/ML) to enable systems operation and optimization.
“Transforming the wireless environment from a passive into an intelligent actor, RIS will create innovation opportunities and progressively impact the evolution of wireless system architecture, access technologies, and networking protocols. There are however many technical challenges that need to be adequately addressed before RIS can be adopted into future standards, towards commercialization of the technology, and the ETSI ISG RIS aims to identify and address some of these challenges”, says Arman Shojaeifard, Chair of ISG RIS.
