A Modular Pragmatic Architecture for Multiuser MIMO with Array-Fed RIS

TL;DR

This paper introduces a modular, energy-efficient multiuser MIMO architecture using array-fed RIS, optimized for high-frequency bands, with a focus on practical implementation and high spectral efficiency.

Contribution

It presents a novel modular RIS architecture with active multi-antenna feeders for high-frequency MIMO, emphasizing practicality and energy efficiency.

Findings

High spectral efficiency demonstrated

Low sidelobe and high gain beamforming achieved

Energy efficiency surpasses conventional active arrays

Abstract

We propose a power- and hardware-efficient, pragmatic, modular, multiuser/multibeam array-fed RIS architecture particularly suited to operate in very high frequency bands (high mmWave and sub-THz), where channels are typically sparse in the beamspace and line-of-sight (LOS) is required to achieve an acceptable received signal level. The key module is an active multi-antenna feeder (AMAF) with a small number of active antennas placed in the near field of a RIS with a much larger number of passive controllable reflecting elements. We propose a pragmatic approach to obtain a steerable beam with high gain and very low sidelobes. Then, $K$ independently controlled beams can be achieved by stacking $K$ of such AMAF-RIS modules. Our analysis takes in full account: 1) the near-end crosstalk (NEXT) between the modules, 2) the far-end crosstalk (FEXT) due to the sidelobes; 3) a thorough energy efficiency comparison with respect to conventional active arrays with the same beamforming performance. Overall, we show that the proposed architecture is very attractive in terms of spectral efficiency, ease of implementation (hardware complexity), and energy efficiency.