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

Krishan Kumar Tiwari, Giuseppe Caire

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.

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

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, independently controlled beams can be achieved by stacking 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.
Paper Structure (8 sections, 12 equations, 8 figures, 1 table)

This paper contains 8 sections, 12 equations, 8 figures, 1 table.

Table of Contents

  1. Introduction
  2. The AMAF-RIS module: design principles
  3. RIS Array and Far-Field Array Response
  4. AMAF-RIS Illumination and Beam-Steering
  5. Multi-beam multiuser MIMO
  6. MU-MIMO Example
  7. Power Efficiency Analysis
  8. Conclusions

Figures (8)

  • Figure 1: 3D geometry, conceptual (not to scale).
  • Figure 2: Spherical coordinates definition for the RIS coordinate system S2.
  • Figure 3: Ground footprint (coordinate system S1) of the RIS PEM spotbeam pointing at the RIS boresight (coordinate system S2), for a $16\times 16$ RIS with $2\times 2$ AMAF at distance $F = 6$.
  • Figure 4: RIS PEM amplitude profile for a $16\times 16$ RIS with $2\times 2$ AMAF at distance $F = 6$.
  • Figure 5: Ground footprints: an example set of 4 PEM spot beams, with perfect beam pointing onto the 4 UEs.
  • ...and 3 more figures

Theorems & Definitions (1)

  • Remark 1