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Codebook Design and Baseband Precoding for Pragmatic Array-Fed RIS Hybrid Multiuser MIMO

Krishan Kumar Tiwari, Giuseppe Caire

TL;DR

A low-complexity HDA MU-MIMO framework with: user-beam association via standard beam acquisition; dynamic user grouping (one user per beam); effective baseband MIMO channel estimation using 3GPP-compliant pilots; and downlink transmission with zero-forcing precoding under per-antenna power constraints.

Abstract

In our previous work [2], we introduced a hardware- and power-efficient architecture for hybrid digital-analog (HDA) multiuser MIMO (MU-MIMO) based on stacking identical basic modules. Each module consists of a small active multi-antenna feeder (AMAF) placed in the near field of a larger reflective intelligent surface (RIS). Each AMAF is driven by one RF chain and conveys one spatial stream, achieving a multiplexing gain of $K$ with $K$ stacked modules. While [2] focused on module design and efficiency compared to active arrays, performance was evaluated only under pure line-of-sight (LOS) conditions. This work extends our approach in several ways. First, we propose a simple, pragmatic method for designing phase-only flat-top beams for the AMAF-RIS module, enabling wide angular coverage with low ripple and sidelobes. This design supports hierarchical beamforming codebooks for efficient beam acquisition. Second, we evaluate MU-MIMO performance under realistic mmWave multipath channels including both LOS and non-LOS (NLOS) components modeled using a 3D von Mises-Fisher distribution. We propose a low-complexity HDA MU-MIMO framework with: user-beam association via standard beam acquisition; dynamic user grouping (one user per beam); effective baseband MIMO channel estimation using 3GPP-compliant pilots; and downlink transmission with zero-forcing precoding under per-antenna power constraints. Results show high spectral efficiency and multiplexing gain while preserving hardware simplicity and power efficiency. Crucially, the approach is fully compliant with 3GPP 5GNR beam acquisition and sounding reference signaling mechanisms.

Codebook Design and Baseband Precoding for Pragmatic Array-Fed RIS Hybrid Multiuser MIMO

TL;DR

A low-complexity HDA MU-MIMO framework with: user-beam association via standard beam acquisition; dynamic user grouping (one user per beam); effective baseband MIMO channel estimation using 3GPP-compliant pilots; and downlink transmission with zero-forcing precoding under per-antenna power constraints.

Abstract

In our previous work [2], we introduced a hardware- and power-efficient architecture for hybrid digital-analog (HDA) multiuser MIMO (MU-MIMO) based on stacking identical basic modules. Each module consists of a small active multi-antenna feeder (AMAF) placed in the near field of a larger reflective intelligent surface (RIS). Each AMAF is driven by one RF chain and conveys one spatial stream, achieving a multiplexing gain of with stacked modules. While [2] focused on module design and efficiency compared to active arrays, performance was evaluated only under pure line-of-sight (LOS) conditions. This work extends our approach in several ways. First, we propose a simple, pragmatic method for designing phase-only flat-top beams for the AMAF-RIS module, enabling wide angular coverage with low ripple and sidelobes. This design supports hierarchical beamforming codebooks for efficient beam acquisition. Second, we evaluate MU-MIMO performance under realistic mmWave multipath channels including both LOS and non-LOS (NLOS) components modeled using a 3D von Mises-Fisher distribution. We propose a low-complexity HDA MU-MIMO framework with: user-beam association via standard beam acquisition; dynamic user grouping (one user per beam); effective baseband MIMO channel estimation using 3GPP-compliant pilots; and downlink transmission with zero-forcing precoding under per-antenna power constraints. Results show high spectral efficiency and multiplexing gain while preserving hardware simplicity and power efficiency. Crucially, the approach is fully compliant with 3GPP 5GNR beam acquisition and sounding reference signaling mechanisms.
Paper Structure (21 sections, 22 equations, 14 figures, 1 table)

This paper contains 21 sections, 22 equations, 14 figures, 1 table.

Table of Contents

  1. Introduction
  2. System Model
  3. AMAF-RIS Module and Pragmatic PEM Design
  4. Multi-Module MU-MIMO Architecture
  5. Multipath Channel Model with vMF Scatterers
  6. Users-to-beam assignment
  7. Hierarchical Codebook with Flat-Top Beams
  8. Pragmatic Flat-Top Beam Design
  9. Best separable approximation
  10. Beam widening by phase perturbation
  11. Local Optimization
  12. Extension to Planar Array
  13. Hierarchical Codebook and Effective Channel
  14. System Set-up and Performance Evaluation
  15. 3D Multipath Scenarios and Channel Characterization
  16. ...and 6 more sections

Figures (14)

  • Figure 1: Array-fed RIS NewOldIdea.
  • Figure 2: Array-fed RIS hardware hardware.
  • Figure 3: Marginal distributions of 2D $|{\bf U}_1|$.
  • Figure 4: Binary phase profile.
  • Figure 5: $F/D$-wise far-field radiation patterns of an ULA with $N_p = 40$ antennas and amplitude profile ${\bf q}$, multiplied by the binary phase profile of Fig. \ref{['fig:BinaryPattern']}.
  • ...and 9 more figures

Theorems & Definitions (1)

  • Remark 1