Scalable RIS-Aided Beamforming Strategies for Near-Field MU-MISO via Multi-Antenna Feeder

TL;DR

This paper proposes a modular RIS-aided beamforming framework with a novel antenna architecture that enables flexible, scalable, and fair multi-user communications in near-field regimes, balancing complexity and performance.

Contribution

It introduces the AT-RIS architecture combining active multi-antenna feeders with transmissive RIS, enabling decoupled, configurable beamforming strategies for near-field MU-MISO systems.

Findings

Non-diagonal schemes maximize sum rate with high angular separability.

Diagonal T-RIS with focusing scheme offers robust, fair, and scalable performance.

Trade-offs exist between spectral efficiency, complexity, and fairness.

Abstract

This paper investigates a modular beamforming framework for reconfigurable intelligent surface (RIS)-aided multi-user (MU) communications in the near-field regime, built upon a novel antenna architecture integrating an active multi-antenna feeder (AMAF) array with a transmissive RIS (T-RIS), referred to as AT-RIS. This decoupling enables coordinated yet independently configurable designs in the AMAF and T-RIS domains, supporting flexible strategies with diverse complexity-performance trade-offs. Several implementations are analyzed, including diagonal and non-diagonal T-RIS architectures, paired with precoding schemes based on focusing, minimum mean square error, and eigenmode decomposition. Simulation results demonstrate that while non-diagonal schemes maximize sum rate in scenarios with a limited number of User Equipments (UEs) and high angular separability, they exhibit fairness and scalability limitations as UE density increases. Conversely, diagonal T-RIS configurations, particularly the proposed focusing-based scheme with uniform feeder-side power allocation, offer robust, fair, and scalable performance with minimal channel state information. The findings emphasize the critical impact of UEs' angular separability and reveal inherent trade-offs among spectral efficiency, complexity, and fairness, positioning diagonal AT-RIS architectures as practical solutions for scalable near-field MU multiple-input single-output systems.