[Paper Review] Beam Focusing for Near-Field Multi-User MIMO Communications

Haiyang Zhang, Nir Shlezinger+4 more|arXiv (Cornell University)|May 27, 2021

Antenna Design and Analysis21 citations

TL;DR

This paper proposes beam focusing in near-field multi-user MIMO systems using large-scale antenna arrays at mmWave/THz frequencies, enabling high-rate communication by focusing energy at specific spatial locations rather than just directions. It develops optimized precoding schemes for fully-digital, hybrid phase-shifter, and dynamic metasurface antenna architectures, achieving reliable multi-user transmission even at similar angles—unlike conventional far-field beamforming.

ABSTRACT

Large antenna arrays and high-frequency bands are two key features of future wireless communication systems. The combination of large-scale antennas with high transmission frequencies often results in the communicating devices operating in the near-field (Fresnel) region. In this paper, we study the potential of beam focusing, feasible in near-field operation, in facilitating high-rate multi-user downlink multiple-input multiple-output (MIMO) systems. As the ability to achieve beam focusing is dictated by the transmit antenna, we study near-field signaling considering different antenna structures, including fully-digital architectures, hybrid phase shifter-based precoders, and the emerging dynamic metasurface antenna (DMA) architecture for massive MIMO arrays. We first provide a mathematical model to characterize near-field wireless channels as well as the transmission pattern for the considered antenna architectures. Then, we formulate the beam focusing problem for the goal of maximizing the achievable sum-rate in multi-user networks. We propose efficient solutions based on the sum-rate maximization task for fully-digital, (phase shifters based-) hybrid and DMA architectures. Simulation results show the feasibility of the proposed beam focusing scheme for both single- and multi-user scenarios. In particular, the designed focused beams are such that users residing at the same angular direction can communicate reliably without interfering with each other, which is not achievable using conventional far-field beam steering.

Motivation & Objective

• To address the challenge of supporting high-rate, multi-user MIMO communications in the near-field (Fresnel) region, where conventional far-field beamforming fails due to spherical wavefronts.
• To exploit beam focusing—focusing energy at specific spatial locations rather than angular directions—to enable orthogonal communication links even when users are at similar angles.
• To design efficient precoding strategies tailored to practical antenna architectures: fully-digital, hybrid phase-shifter-based, and dynamic metasurface antennas (DMAs).
• To maximize the sum-rate in multi-user downlink MIMO systems under hardware constraints of different antenna architectures.
• To demonstrate the feasibility and performance gain of beam focusing in both single- and multi-user scenarios using realistic channel models and hardware constraints.

Proposed method

• Develops a mathematical model for near-field MIMO channels that accounts for spherical wavefronts and user-specific pathloss, incorporating array geometry and propagation distances.
• Proposes a sum-rate maximization framework for beam focusing, formulating the precoding design as an optimization problem under power and hardware constraints.
• For fully-digital architectures, derives a closed-form solution for the optimal digital precoder given a fixed analog beamforming matrix, using maximal ratio transmission with power normalization.
• For hybrid and DMA architectures, formulates the optimization as a non-convex problem over phase shifts and amplitude weights, which is solved via suboptimal but efficient iterative algorithms.
• Reformulates the sum-rate maximization problem using vectorization and Kronecker products to enable efficient computation, particularly for hybrid and DMA systems.
• Applies the triangle inequality to derive the optimal phase shifts for beam focusing, showing that the optimal phase for each element aligns with the path delay and phase shift of the desired focal point.

Experimental results

Research questions

• RQ1 Can beam focusing in the near-field region enable reliable multi-user MIMO communication when users are located at similar angular directions?
• RQ2 How does beam focusing improve sum-rate performance compared to conventional far-field beamforming in massive MIMO systems with large arrays?
• RQ3 What are the optimal precoding strategies for different practical antenna architectures (fully-digital, hybrid, DMA) in near-field multi-user MIMO?
• RQ4 How do hardware constraints (e.g., phase shifters, amplitude control) affect the performance of beam focusing in near-field MIMO?
• RQ5 To what extent can beam focusing support orthogonal communication links between users at the same angle, overcoming interference limitations of far-field systems?

Key findings

• Beam focusing enables reliable communication between users at the same angular direction by spatially concentrating energy at distinct locations, eliminating inter-user interference.
• The proposed beam focusing scheme achieves significant sum-rate gains over conventional far-field beamforming, particularly in dense multi-user scenarios.
• For fully-digital architectures, the optimal digital precoder is derived in closed form as maximal ratio transmission with full power, maximizing the signal-to-interference-plus-noise ratio.
• The optimal phase shifts for beam focusing are shown to be $ \psi_{i,l}^{*} = k|\mathbf{p}_{m}-\mathbf{p}_{i,l}| + \beta_{i}\rho_{i,l} $, aligning with path delay and phase shift for maximum gain.
• Simulation results confirm that the beam focusing scheme is effective in both single- and multi-user scenarios, with stable performance across different user distributions and array configurations.
• The hybrid and DMA architectures achieve near-optimal performance with reduced hardware complexity, demonstrating the practical feasibility of beam focusing in real-world systems.