Conventional Reconfigurable intelligent surfaces (RIS) for wireless communications have a local position-dependent (phase-gradient) scattering response on the surface. We consider more general RIS structures, called nonlocal (or redirective) RIS, that are capable of selectively manipulate the impinging waves depending on the incident angle. Redirective RIS have nonlocal wavefront-selective scattering behavior and can be implemented using multilayer arrays such as metalenses. We demonstrate that this more sophisticated type of surfaces has several advantages such as: lower overhead through coodebook-based reconfigurability, decoupled wave manipulations, and higher efficiency in multiuser scenarios via multifunctional operation. Additionally, redirective RIS architectures greatly benefit form the directional nature of wave propagation at high frequencies and can support integrated fronthaul and access (IFA) networks most efficiently. We also discuss the scalability and compactness issues and propose efficient nonlocal RIS architectures such as fractionated lens-based RIS and mirror-backed phase-masks structures that do not require additional control complexity and overhead while still offering better performance than conventional local RIS.

翻译：传统用于无线通信的可重构智能表面（RIS）具有局部的、随位置变化的（相位梯度）散射响应。本文考虑更一般的RIS结构——称为非局部（或重定向型）RIS，其能根据入射角选择性调控入射波。重定向型RIS具有非局部的波前选择性散射行为，可通过多层阵列（如超透镜）实现。研究表明，这类更复杂的表面具有多项优势：通过基于码本的可重构性降低开销、实现解耦的波操控、以及通过多功能操作在多用户场景中提升效率。此外，重定向型RIS架构能充分利用高频段波传播的方向性特性，可最高效地支持集成前传与接入（IFA）网络。本文还探讨了可扩展性与紧凑性问题，并提出高效的非局部RIS架构，例如分段式透镜RIS与镜面背衬相位掩膜结构，这些方案在无需额外控制复杂度与开销的同时，性能优于传统局部RIS。