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Analog-Digital Beam Focusing for Line of Sight Wide-Aperture MIMO with Spherical Wavefronts

Jiyoung Yun, Hojun Rho, Wan Choi

TL;DR

This work tackles high-rate, LoS-dominant wide-aperture MIMO in the THz/mmWave regime using hybrid analog-digital architectures. It develops a near-field channel model that exposes a prolate-matrix structure, enabling an explicit, low-complexity beam focusing scheme. The authors derive the optimal planar array configuration under hardware constraints and provide a closed-form, change-of-basis precoder/combiner leveraging 2D-DFT eigenvectors, with an OMP-based realization for practical implementation. Numerical results show that the proposed arrangement and beam focusing approach achieve near-optimal spectral efficiency across SNRs and arbitrary link rotations while substantially reducing computational complexity. The findings offer a tractable path to realizing high data-rate LoS MIMO in future wireless systems with realistic hardware budgets.

Abstract

Enhancing high-speed wireless communication in the future relies significantly on harnessing high frequency bands effectively. These bands predominantly operate in line-of-sight (LoS) paths, necessitating well-configured antenna arrays and beamforming techniques for optimal spectrum utilization. Maximizing the potential of LoS multiple-input multiple-output (MIMO) systems, which are crucial for achieving high spectral efficiency, heavily depends on this. As the costs and power demands of mixed-signal devices in high frequency bands make a fully-digital architecture impractical for large-scale MIMO setups, our focus shifts to a hybrid analog-digital hardware configuration. Yet, analog processors' limitations restrict flexibility within arrays, necessitating a nuanced understanding of hardware constraints for optimal antenna configuration design. We explore array design that optimizes the spectral efficiency of hybrid systems, considering hardware constraints. We propose an optimal antenna configuration, leveraging the prolate matrix structure of the LoS channel between two planar arrays. Building on the optimal array configuration, we introduce a low-complexity explicit analog-digital beam focusing scheme that exploits the asymptotic behavior of the LoS channel matrix in the near-field region. Simulation results validate that the proposed antenna configuration and beam focusing scheme achieves near-optimal performance across a range of signal-to-noise ratios with low computational complexity, even under arbitrary rotations relative to the communication link.

Analog-Digital Beam Focusing for Line of Sight Wide-Aperture MIMO with Spherical Wavefronts

TL;DR

This work tackles high-rate, LoS-dominant wide-aperture MIMO in the THz/mmWave regime using hybrid analog-digital architectures. It develops a near-field channel model that exposes a prolate-matrix structure, enabling an explicit, low-complexity beam focusing scheme. The authors derive the optimal planar array configuration under hardware constraints and provide a closed-form, change-of-basis precoder/combiner leveraging 2D-DFT eigenvectors, with an OMP-based realization for practical implementation. Numerical results show that the proposed arrangement and beam focusing approach achieve near-optimal spectral efficiency across SNRs and arbitrary link rotations while substantially reducing computational complexity. The findings offer a tractable path to realizing high data-rate LoS MIMO in future wireless systems with realistic hardware budgets.

Abstract

Enhancing high-speed wireless communication in the future relies significantly on harnessing high frequency bands effectively. These bands predominantly operate in line-of-sight (LoS) paths, necessitating well-configured antenna arrays and beamforming techniques for optimal spectrum utilization. Maximizing the potential of LoS multiple-input multiple-output (MIMO) systems, which are crucial for achieving high spectral efficiency, heavily depends on this. As the costs and power demands of mixed-signal devices in high frequency bands make a fully-digital architecture impractical for large-scale MIMO setups, our focus shifts to a hybrid analog-digital hardware configuration. Yet, analog processors' limitations restrict flexibility within arrays, necessitating a nuanced understanding of hardware constraints for optimal antenna configuration design. We explore array design that optimizes the spectral efficiency of hybrid systems, considering hardware constraints. We propose an optimal antenna configuration, leveraging the prolate matrix structure of the LoS channel between two planar arrays. Building on the optimal array configuration, we introduce a low-complexity explicit analog-digital beam focusing scheme that exploits the asymptotic behavior of the LoS channel matrix in the near-field region. Simulation results validate that the proposed antenna configuration and beam focusing scheme achieves near-optimal performance across a range of signal-to-noise ratios with low computational complexity, even under arbitrary rotations relative to the communication link.
Paper Structure (12 sections, 5 theorems, 47 equations, 10 figures)

This paper contains 12 sections, 5 theorems, 47 equations, 10 figures.

Table of Contents

  1. Introduction
  2. System Model
  3. Near-Field Channel Model
  4. Signal Model
  5. Problem Formulation
  6. Optimal Spatial Multiplexing Gain by Analog-Digital Beam Focusing with Generic Planar Arrays
  7. Analog-Digital Beam Focusing Design with Given Antenna Configuration under Limited Antenna Aperture
  8. Numerical results
  9. Conclusion
  10. Proof of Theorem \ref{['thm:optimal_sapcing']}
  11. Array Aperture Constraint
  12. Proof of Theorem \ref{['thm:Hgain']}

Key Result

Lemma 1

For $i \in \{\mathrm{v},\mathrm{h}\}$, the normalized channel gain matrix $\frac{\Delta_i}{N_{i,\mathrm{max}}} \mathbf{G}_{\mathrm{lin}(i)}$, for any $M_{i}, N_{i}, \Delta_i$ such that $\frac{N_{i,\mathrm{max}}}{\Delta_i}\in\mathbb{N}$, suppose $M_{i}, N_{i} \le \frac{N_{i,\mathrm{max}}}{\Delta_i}$. and where and $N_{\mathrm{v, min}} \triangleq \min(N_{\mathrm{v}},M_{\mathrm{v}})$, $N_{\mathrm{h

Figures (10)

  • Figure 1: Generic UPA wide-aperture MIMO system with hybrid architecture.
  • Figure 2: Parallel UPA wide-aperture MIMO system with hybrid architecture.
  • Figure 3: Examples of optimally spaced planar array with arbitrary rotation.
  • Figure 4: Achievable rate on the array aperture for the different communication distance with the number of antenna elements $N_t = N_r = 256$, the number of data stream and RF chain $M_f=N_f=N_s=16$. The pentagram marker indicates the array aperture of the optimally spaced array.
  • Figure 5: Upper bound of the achievable rate for the different array configuration on the SNR.
  • ...and 5 more figures

Theorems & Definitions (6)

  • Lemma 1
  • Corollary 1
  • Theorem 1
  • Remark
  • Theorem 2
  • Corollary 2