Spherical Stacked Intelligent Metasurfaces: A Paradigm for Full-Space Wave-Domain Processing

This article introduces the concept of spherical stacked intelligent metasurface (S-SIM), establishing a new paradigm for full-space electromagnetic field manipulation and wave-domain signal processing. By extending conventional planar stacked metasurfaces into a concentric spherical architecture, the S-SIM enables isotropic field interaction and full-space coverage. This article first establishes a unified modeling framework for S-SIM that integrates the architectural design, full-space wave-domain signal formulation, and channel representation to characterize inter-layer wave interactions and propagation characteristics. Building on this foundation, an iterative layer- by-layer beamforming algorithm is developed to optimize the phase responses of meta-atoms across multiple layers. Furthermore, a user-centric node division multiple access (NDMA) scheme is developed, allowing adaptive multiuser access and interference management in distributed three-dimensional environments. Case studies demonstrate that the proposed S-SIM system achieves more uniform full-space coverage and higher average spectral efficiency compared with the state-of-the-art (SoA) planar SIM. Finally, several open research directions are discussed, including hardware implementation, wave-domain computing, cooperative networking, and intelligent optimization, which lay the foundation on the practical deployment of S-SIM systems.