Reconfigurable Intelligent Surfaces for 6G: Emerging Hardware Architectures, Applications, and Open Challenges

TL;DR

RISs are presented as a transformative approach for 6G wireless networks, enabling deliberate control of the propagation environment. The paper surveys hardware architectures and operation modes, including metasurface-based designs (e.g., amplification, RX-enabled, hybrid, DMA, SIM, STAR) and modes such as simultaneous reflection/sensing and EM-domain computing. It covers RIS-inspired applications across index and reflection modulation, non-coherent modulation, NGMA, ISAC, energy harvesting, and aerial/vehicular networks, with supporting prototypes and field trials. Open challenges are discussed, including power-performance tradeoffs, SIM verification, STAR-mode selection, modulation-rate limits, and deployment realism, guiding future RIS research toward practical 6G deployments.

Abstract

Reconfigurable intelligent surfaces (RISs) are rapidly gaining prominence in the realm of fifth generation (5G)-Advanced, and predominantly, sixth generation (6G) mobile networks, offering a revolutionary approach to optimizing wireless communications. This article delves into the intricate world of the RIS technology, exploring its diverse hardware architectures and the resulting versatile operating modes. These include RISs with signal reception and processing units, sensors, amplification units, transmissive capability, multiple stacked components, and dynamic metasurface antennas. Furthermore, we shed light on emerging RIS applications, such as index and reflection modulation, non-coherent modulation, next generation multiple access, integrated sensing and communications (ISAC), energy harvesting, as well as aerial and vehicular networks. These exciting applications are set to transform the way we will wirelessly connect in the upcoming era of 6G. Finally, we review recent experimental RIS setups and present various open problems of the overviewed RIS hardware architectures and their applications. From enhancing network coverage to enabling new communication paradigms, RIS-empowered connectivity is poised to play a pivotal role in shaping the future of wireless networking. This article unveils the underlying principles and potential impacts of RISs, focusing on cutting-edge developments of this physical-layer smart connectivity technology.

Figures (15)

• Figure 1: Comparison of a passive (left) with an active (right) RIS. The former's panel is implemented with simpler hardware but leads to lower signal strength at the receiving end(s). The effective tunable phase shift offered by each $n$-th meta-atom is represented by $\phi_n$. Links via a passive RIS suffer from multiplicative path loss, while those via an active RIS are subject to additive path loss. Both designs require a controller for the dynamic reflection configuration.
• Figure 2: RISs equipped with both signal reception (RX) and signal reflection units. In the left hardware architecture taha2021enabling_ALL, the RIS panel comprises conventional reflective meta-atoms, as in passive RISs, as well as active sensing devices which enable sensing of parameters of the impinging signals that becomes available to a baseband unit, via an RX RF chain, for further processing. The right hardware architecture alexandropoulos2021hybrid is realized with hybrid reflective and sensing meta-atoms alamzadeh2021reconfigurable that split their incident signal into a portion that is reflected (after tunable phase shifting) in the environment, while the remainder of the signal is fed to a reception RF chain(s) for sensing and processing at the metasurface's baseband processor. $\rho_n$ represents the latter power splitting ratio at each $n$-th hybrid meta-atom, while $\psi_n$ indicates the tunable phase shift applied in the received signal. The baseband signal processing unit of both hardware architectures can be part of the RIS controller, complementing the dynamic reflection configuration management with further processing tasks (e.g., sensing and optimization).
• Figure 3: Dynamic metasurface antennas (DMA) used as an extremely massive MIMO receiver (RX) and transmitter (TX). The metasurface comprises microstrips, each implemented as 1D or 2D waveguide, that include meta-atoms of tunable EM states. These elements are placed on the waveguides through which the received waveforms intended for information decoding (left) and the signals to be transmitted (right) are transferred. The TX and RX baseband processors, which respectively generate the outgoing signals and process the received signals, are connected to the waveguides through dedicated input and output ports via the TX and RX RF chains, respectively.
• Figure 4: A stacked intelligent metasurfaces (SIM) structure comprising a 3D slab of RISs whose EM responses are managed by a dedicated controller. By appropriately designing the tunable transmissive properties of each RIS's meta-atoms, the SIM is capable to hierarchically manipulate the energy distribution of the EM waves passing through it.
• Figure 5: (a) A holographic MIMO wireless communication system comprising SIMs placed very close to a multi-antenna TX and RX, which respectively consist of $L$ and $K$ transmissive RISs. The SIMs implement analog signal processing entirely in the EM propagation domain. (b) Strength of the end-to-end wireless channel matrix for different numbers of the metasurface layers $K$ and $L$. It is demonstrated that, as the number of layers increases, the channel matrix becomes closer to diagonal.