AI 中文总结
研究启用非正方形均匀平面阵列的XL-MIMO系统，推导其有效波束聚焦距离、渐近有效自由度及相关误差界，设计三维各向异性近场码本用于低复杂度信道估计，揭示了该系统特性及性能权衡，验证码本在降复杂度下精度与三维极域码本相当。
AI 中文摘要
超大规模多输入多输出（XL-MIMO）对下一代通信系统至关重要。实际上，非正方形均匀平面阵列（UPA）的部署从根本上改变了波前特性，并由于非正方形阵列几何结构产生的孔径差异，导致沿不同轴具有各向异性的波束聚焦能力。为全面揭示这种非正方形几何结构对性能的影响，从而释放非正方形UPA的潜力，我们研究了启用非正方形UPA的XL-MIMO系统的各向异性近场特性、基本限制和信道估计。首先，我们推导了阵列长轴和短轴的有效波束聚焦距离。有趣的是，非正方形UPA的辐射空间可分为三个区域，即完全近场、各向异性近场和远场区域，随着阵列长宽比增加，各向异性区域渐近主导整个近场空间。然后，给出了启用非正方形UPA的XL-MIMO系统的渐近有效自由度，这表明在大阵列长宽比情况下，距离域复用由长轴孔径决定。此外还推导了距离估计的闭式克拉美罗界和三维位置误差界，以揭示距离、方位角和仰角估计之间因几何结构引起的性能权衡，据此确定使三维位置误差界最小的最佳阵列长宽比。最后，通过利用各向异性波前特性，我们设计了一个三维各向异性近场码本，以促进非正方形UPA的低复杂度信道估计。数值结果验证了所提出的码本在降低复杂度的情况下实现了与三维极域码本相当的精度。
英文摘要
Extremely large-scale multiple-input multiple-output (XL-MIMO) is crucial for next-generation communication systems. In practice, the deployment of non-square uniform planar arrays (UPAs) fundamentally alters wavefront characteristics and induces anisotropic beamfocusing capability along different axes due to the aperture disparity originating from the non-square array geometry. To fully uncover the performance impact of such non-square geometry and thus unleash the potential of the non-square UPAs, we investigate the anisotropic near-field characteristics, fundamental limits, and channel estimation for non-square UPA-enabled XL-MIMO systems. First, we derive the effective beamfocusing distances for the long and short axes of the array. Interestingly, the radiation space of a non-square UPA can be partitioned into three regions, i.e., the fully near-field, the anisotropic near-field, and the far-field regions, and the anisotropic region asymptotically dominates the overall near-field space as the array aspect ratio increases. Then, the asymptotic effective degree of freedom for non-square UPA-enabled XL-MIMO systems is provided, which reveals that distance-domain multiplexing is governed by the long-axis aperture in the large array aspect ratio regime. Furthermore, the closed-form Cramer-Rao bound for distance estimation and the three-dimensional (3D) position error bound (PEB) are derived to reveal the geometry-induced performance trade-offs among distance, azimuth, and elevation estimation, based on which the optimal array aspect ratio that minimizes the 3D PEB is determined. Finally, by exploiting the anisotropic wavefront properties, we design a 3D anisotropic near-field codebook to facilitate low-complexity channel estimation for non-square UPAs. Numerical results validate that the proposed codebook achieves comparable accuracy to the 3D polar-domain codebook at reduced complexity.
Comments13 pages, 9 figures, submitted to IEEE Transactions on Communications
