Emerging Technologies in Intelligent Metasurfaces: Shaping the Future of Wireless Communications

TL;DR

This survey surveys the landscape of intelligent metasurfaces for wireless networks, tracing the evolution from two-dimensional RIS to three-dimensional SIM and flexible morphable metasurfaces (FIM). It analyzes experimental demonstrations and modeling for RIS, discusses the operating principles and prototypes of SIM along with wave-domain signal processing capabilities, and surveys a breadth of applications from MIMO beamforming to ISAC and holography. The article also highlights open challenges—such as channel estimation for morphable surfaces, nonlinear activations in SIM, and practical deployment of FIMs—and sketches research directions toward energy-efficient, high-capacity, and adaptive wireless systems enabled by these technologies.

Abstract

Intelligent metasurfaces have demonstrated great promise in revolutionizing wireless communications. One notable example is the two-dimensional (2D) programmable metasurface, which is also known as reconfigurable intelligent surfaces (RIS) to manipulate the wireless propagation environment to enhance network coverage. More recently, three-dimensional (3D) stacked intelligent metasurfaces (SIM) have been developed to substantially improve signal processing efficiency by directly processing analog electromagnetic signals in the wave domain. Another exciting breakthrough is the flexible intelligent metasurface (FIM), which possesses the ability to morph its 3D surface shape in response to dynamic wireless channels and thus achieve diversity gain. In this paper, we provide a comprehensive overview of these emerging intelligent metasurface technologies. We commence by examining recent experiments of RIS and exploring its applications from four perspectives. Furthermore, we delve into the fundamental principles underlying SIM, discussing relevant prototypes as well as their applications. Numerical results are also provided to illustrate the potential of SIM for analog signal processing. Finally, we review the state-of-the-art of FIM technology, discussing its impact on wireless communications and identifying the key challenges of integrating FIMs into wireless networks.

Figures (12)

• Figure 1: The development trend of intelligent metasurfaces: From 2D RIS to 3D SIM and FIM.
• Figure 2: Four development trends of RIS technology: (a) from environmental RIS to transceiver RIS; (b) from passive RIS to active RIS; (c) from reflective RIS to transmissive RIS; (d) from channel estimation to channel training.
• Figure 3: Schematic of a SIM and its equivalent neural network structure. The forward propagation of EM waves in the SIM is equivalent to a multi-layer perceptron.
• Figure 4: Illustration of several existing SIM prototypes.
• Figure 5: A SIM-aided point-to-point MIMO system, where a pair of SIMs are deployed at the transmitter and receiver to implement MIMO precoding and combining automatically as the EM waves propagate through them.