The performance 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) antenna array 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：采用二维平面天线阵列的全息多输入多输出（MIMO）通信系统的性能通常受限于由有限阵列尺寸导致的自由度限制。当阵元间距接近半波长量级时，自由度约束变得显著，从而限制了MIMO系统的整体性能。为突破这一固有局限，我们提出了一种新颖的三维天线阵列，通过战略性开发未利用的垂直维度来提升系统性能。我们研究了采用三维阵列的MIMO系统在不同多径场景下的性能表现，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，我们展示了具有相同孔径尺寸的三维阵列相比二维阵列的优势。作为概念验证，设计、构建并评估了一种借助电磁带隙（EBG）反射面实现的实用偶极子三维阵列。实验结果与全波仿真高度吻合，信道仿真进一步证实，采用此类三维阵列配置可超越传统全息MIMO系统的自由度和容量限制。