Near-Field Wideband Beamforming for Extremely Large Antenna Arrays

TL;DR

This work addresses near-field beam split in extremely large antenna arrays (ELAAs) operating over THz bandwidths by introducing a piecewise-far-field channel model and a phase-delay focusing (PDF) method based on the delay-phase precoding (DPP) architecture. By partitioning the array into small sub-arrays, the method decouples inter-array near-field phase discrepancies from intra-array far-field ones, enabling joint control of time delays and phase shifters to align beams across the bandwidth. The paper provides analytic insights into beamforming gain, introduces the effective Rayleigh distance to better characterize the near-field region for communications, and demonstrates substantial gains in beamforming performance, spectral efficiency, and energy efficiency through simulations. This approach offers a scalable, hardware-aware solution for robust wideband ELAA beamforming in 6G THz networks and establishes a practical metric for near-field range in practical deployments.

Abstract

The natural integration of extremely large antenna arrays (ELAAs) and terahertz (THz) communications can potentially achieve Tbps data rates in 6G networks. However, due to the extremely large array aperture and wide bandwidth, a new phenomenon called "near-field beam split" emerges. This phenomenon causes beams at different frequencies to focus on distinct physical locations, leading to a significant gain loss of beamforming. To address this challenging problem, we first harness a piecewise-far-field channel model to approximate the complicated near-field wideband channel. In this model, the entire large array is partitioned into several small sub-arrays. While the wireless channel's phase discrepancy across the entire array is modeled as near-field spherical, the phase discrepancy within each sub-array is approximated as far-field planar. Built on this approximation, a phase-delay focusing (PDF) method employing delay phase precoding (DPP) architecture is proposed. Our PDF method could compensate for the intra-array far-field phase discrepancy and the inter-array near-field phase discrepancy via the joint control of phase shifters and time delayers, respectively. Theoretical and numerical results are provided to demonstrate the efficiency of the proposed PDF method in mitigating the near-field beam split effect.Finally, we define and derive a novel metric termed the "effective Rayleigh distance" by the evaluation of beamforming gain loss. Compared to classical Rayleigh distance, the effective Rayleigh distance is more accurate in determining the near-field range for practical communications.

Figures (13)

• Figure 1: The system layout of ELAA.
• Figure 2: This figure illustrates the normalized beamforming gain in the physical space. We consider four scenarios: (a) the far-field narrowband scenario, (b) the near-field narrowband scenario, (c) the far-field wideband scenario, and (d) the near-field wideband scenario. In each sub-figure, the beam energy of the lowest, the center, and the highest frequencies are plotted (e.g., the three lines in the sub-figures (c) and (d)).
• Figure 3: Schematic diagrams of (a) far-field channel model, (b) near-field channel model, (c) piecewise-far-field channel model, (d) layout of the piecewise-far-field model, and (e) channel phase against the antenna index. The number of antennas is 256, the carrier frequency is 100 GHz. The user is located at $(x,y) = (10 \: \text{m}, 0\: \text{m})$. With $K = 4$ sub-arrays, the piecewise-far-field channel model can well approximate the near-field channel model.
• Figure 4: A single-user example of the delay-phase precoding architecture DPP_Tan2019 for performing the PDF method.
• Figure 5: Quadratic fitting $1 - \left(1 - \Xi_P(\frac{B}{2f_c})\right)x^2$ of the Dirichlet sinc function $\Xi_P(\frac{B}{2f_c}x)$, where $x\in[-1,1]$, $P = 32$, $B = 5$ GHz, and $f_c = 100$ GHz.