The efficacy of holographic multiple-input multiple-output (MIMO) communications, employing two-dimensional (2-D) planar antenna arrays, is typically compromised by finite degrees-of-freedom (DOF) stemming from limited array size. The DOF constraint becomes significant when the element spacing approaches approximately half a wavelength, thereby restricting the overall performance of MIMO systems. To break this inherent limitation, we propose a novel three-dimensional (3-D) array topology that strategically explores the untapped vertical dimension. We investigate the performance of MIMO systems utilizing 3-D arrays across different multi-path scenarios, encompassing Rayleigh channels with varying angular spreads and the 3rd generation partnership project (3GPP) channels. We subsequently showcase the advantages of these 3-D arrays over their 2-D counterparts with the same aperture sizes. As a proof of concept, a practical dipole-based 3-D array, facilitated by an electromagnetic band-gap (EBG) reflecting surface, is conceived, constructed, and evaluated. The experimental results align closely with full-wave simulations, and channel simulations substantiate that the DOF and capacity constraints of traditional holographic MIMO systems can be surpassed by adopting such a 3-D array configuration.

翻译：全息多输入多输出（MIMO）通信通常采用二维（2-D）平面天线阵列，但其有效性受限于由有限阵列尺寸所导致的有限自由度（DOF）。当阵元间距接近半波长时，DOF约束变得显著，从而限制了MIMO系统的整体性能。为突破这一固有局限，我们提出一种新颖的三维（3-D）阵列拓扑，该拓扑战略性探索了未充分利用的垂直维度。我们研究了采用3-D阵列的MIMO系统在不同多径场景下的性能，涵盖具有不同角度扩展的瑞利信道及第三代合作伙伴计划（3GPP）信道。随后，我们展示了这些3-D阵列相比具有相同孔径尺寸的二维阵列所具备的优势。作为概念验证，我们设计、构建并评估了一种兼容电磁带隙（EBG）反射面的实用偶极子3-D阵列。实验结果与全波仿真高度吻合，信道仿真也证实：通过采用此类3-D阵列配置，可超越传统全息MIMO系统的自由度与容量限制。