We live in a world of waves, most often electromagnetic waves. Over the past few decades, we have learned how to use many types of waves to transfer data between two or more users, without the need of utilizing any electrical conductors as the means of transfer.

What if we could finally control and optimize the propagation of electromagnetic waves as we wish?

Fifth generation networks (5G) and future networks (6G) are key enablers for the industry, society, French and European digital sovereignty. Indeed, in the dynamic of the national recovery plan called “France Relance”, 5G and future telecommunication networks (6G) have been identified as a target market with high potential of growth, in which France has renowned scientific and technological expertise.

• A world of electromagnetic waves. The history of wireless communications started with understanding fundamental electric and magnetic phenomena, as well as with related experiments and inventions that were carried out during the second half of the 18th century and the first decades of the 19th century. Wireless communications are defined and are characterized by the transfer of information between two or more users without the need of utilizing any electrical conductors as the medium to perform the transfer. The most common and standardized wireless technologies utilize electromagnetic waves for the transfer of information. Thanks to the development and wide adoption of five wireless telecommunication standards and the recently started activities on 6G wireless networks, we do live in a world of electromagnetic waves.

• Design of 5G telecommunication networks. Current wireless systems utilize a variety of transmission technologies, communication protocols, and network deployment strategies. These include millimeter-wave communications, massive multi-input–multi-output (MIMO) systems, and ultra-dense heterogeneous networks. Currently available solutions are often based on the deployment, design, and optimization of transmitters, receivers, and network infrastructure elements with power amplification and digital signal processing capabilities, as well as the availability of backhaul and power grid. Communication engineers usually design the transmitters, receivers, network elements, and transmission protocols by assuming not to be able to control how the electromagnetic waves propagate through a wireless environment and how they interact with the material objects available in the considered environment. When an electromagnetic wave impinges, for example, upon a metallic wall or upon a glass window, the reflected and refracted waves are not directly controlled by the network operator but are determined by the properties of the electromagnetic waves and the constitutive elements of the material objects that interact with the electromagnetic waves.
• Towards smart radio environments. Let us imagine now that the material objects that are available in the wireless environment (such as a city or a building) are covered or even made of metamaterials, i.e., artificial (engineered) materials that, when illuminated by electromagnetic waves, are able to modify their physical properties, including the phase, amplitude, direction of reflection, direction of refraction, polarization of the incident waves. These engineered materials can therefore appropriately control and shape the incident waves as desired, and can ultimately allow network designers to jointly optimize the propagation of the electromagnetic waves emitted by the transmitters, the way that the waves interact with the objects available in the considered environment, and the way that the waves are decoded by the receivers, as illustrated in Fig. 1. A wireless network such as the one shown in Fig. 1 is called programmable wireless environment or smart radio environment (SRE).

• The enabling technology: Reconfigurable intelligent surfaces. The key technology to realize SREs, i.e., to program a wireless environment at will, is the reconfigurable intelligent surface (RIS). An RIS is a two-dimensional metamaterial (a metasurface) that can be viewed as a very thin sheet of adaptive and inexpensive metamaterial, which, like a wallpaper, covers parts of walls, buildings, ceilings and can shape the radio waves that illuminate it, in a way that they can be programmed and controlled using an appropriate control system. An important property of an RIS is its ability to be reconfigurable after its deployment in a wireless network. A conceptual illustration of an RIS is shown in Fig. 2. Unlike typical 5G transceivers, an RIS does not emit new electromagnetic waves (like base stations), but it is used for communication purposes, like a mirror. An RIS is an almost passive electronic device that is able to focus the radiated energy, hence significantly reducing the power consumption and the exposure to electromagnetic waves. In simple terms, radio waves bounce off an RIS and can be intelligently redirected to 6G terminals. The RIS technology is currently being specified by the European Telecommunications Standards Institute (ETSI). A video produced by ETSI is available here.

• Use cases and applications. Exploiting the unique properties of RISs opens the way to different use cases. This includes boosting the coverage and throughput in a specific area, or ensuring more secure communications, by directing the electromagnetic only towards the intended users, minimizing the risk of interception from unauthorized users. An RIS can notably reduce the level of exposure to the electromagnetic waves: To achieve the same throughput, 6G base stations will be able to transmit at a lower power level. Consequently, in addition to its ability of manipulation of the electromagnetic waves, an RIS is considered a sustainable physical layer technology, because it is an almost passive surface that does not require, a priori, any, or very little, energy supply. Research activities on RIS and SRE are therefore part of a national and European vision of 6G that prioritizes, or takes into account by design, new telecommunication networks with a reduced environmental impact and an increased level of security. An RIS facilitates, therefore, the design of eco-responsible and sustainable telecommunication networks.

• PEPR-NF. In the context of the PEPR-NF project (https://pepr-futurenetworks.fr/), several research and development activities are underway on RIS and SRE, notably within the projects PERSEUS, YACARI, SYSTERA and FOUND.

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