[论文解读] Near-Field Wideband Beamforming for Extremely Large Antenna Arrays

Mingyao Cui, Linglong Dai|arXiv (Cornell University)|Sep 21, 2021

Microwave Engineering and Waveguides被引用 27

一句话总结

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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.

研究动机与目标

• 由于波束分裂，在 ELAA-THz 宽带系统中激发近场波束成形挑战。
• 提出一个可处理的信道模型：将大阵列划分为子阵列，以分离阵间和阵内的相位效应。
• 开发一种使用相位移器和时延的 PDF 方法，以补偿远场阵内相位和近场阵间相位差异。
• 分析波束成形增益并推导出一个有效 Rayleigh 距离，作为实际近场边界。

提出的方法

• 通过将 ELAA 分成每个包含 P 天线的 K 个子阵列，引入一个分段远场信道模型。
• 将信道相位分解为阵间近场和阵内远场分量，以识别主导的波束分裂源。
• 应用具有时延的延迟相控前馈(Delay-Phase Precoding, DPP) 架构来补偿阵间相位，使用相位移器对齐阵内相位。
• 建立并求解一个优化问题，以最大化跨子载波的带宽宽带波束成形增益，在特定条件下得到闭式解 r_k' = L - r_k。
• 推导使用 Xi_P Dirichlet sinc 函数的波束成形增益表达式，并给出跨带宽的理论增益损失界限。

Figure 1: The system layout of extremely large arrays.

实验结果

研究问题

• RQ1 近场波束分裂如何在极大天线阵列的宽带范围内表现？
• RQ2 分段远场模型是否能够准确近似 ELAA 的近场宽带信道？
• RQ3 在子载波之间，如何用带有 PS 和 TD 分量的相位延迟聚焦来缓解近场波束分裂？
• RQ4 所提出的 PDF 方法对带宽上的平均波束成形增益有何影响？
• RQ5 在实际基于 ELAA 的通信中，定义近场区域的合适度量是什么？

主要发现

• 近场波束分裂导致不同频率的波束聚焦在不同位置，降低了偏离中心频率时的增益。
• 分段远场信道模型通过将阵列划分为子阵列来近似近场信道，从而实现对阵间与阵内相位差异的分开处理。
• 基于 PS 的子阵列和一个 TD 元件的 PDF 方法能够跨频补偿阵间相位并维持较高的波束成形增益。
• 在合适条件下，可将基于 TD 的最优距离参数 r_k' 设置为 r_k' = L - r_k，以最大化每个子载波的波束成形增益。
• 解析结果表明，平均波束成形增益损失因子可以用宽带损失 gamma(B,f_c,P) 与几何损失 xi(r,theta,D) 的乘积来描述，将增益与带宽、几何和阵列尺寸相关联。

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 b