Stacked Intelligent Metasurfaces: Modeling and Optimization

The integration of stacked intelligent metasurfaces (SIM) into wireless networks offers a promising approach to enhance the performance of communications and sensing. SIMs are fabricated by stacking several layers of programmable metasurfaces, each made up of a large number of response-tunable unit elements. These spatially stacked metasurface layers enable advanced signal processing directly in the domain of the propagation of electromagnetic waves, supporting functions such as beamforming, interference suppression, and sensing, while reducing the amount of radio-frequency components, thus energy consumption, and minimizing processing delays. This chapter provides an overview of the emerging SIM technology, discussing its hardware architectures, advantages, and potential applications for wireless communications. The utilization of SIMs in enabling wave-domain beamforming and integrated sensing and communications is also discussed. Additionally, channel estimation approaches for SIM-assisted communication systems are presented. The chapter also provides insights into the architectural benefits and potential application scenarios of SIM, as well as the challenges for SIM configuration in dynamic wireless application scenarios.