In most multiple-input multiple-output (MIMO) communication systems, antennas are spaced at least half a wavelength apart to reduce mutual coupling. In this configuration, the maximum array gain is equal to the number of antennas. However, when the antenna spacing is significantly reduced, the array gain of a compact array can become proportional to the square of the number of antennas, greatly exceeding that of traditional MIMO systems. Achieving this "superdirectivity" requires complex calculations of the excitation coefficients (beamforming vector), which is a challenging task. In this paper, we address this problem with a novel double coupling-based superdirective beamforming method. In particular, we categorize the antenna coupling effects to impedance coupling and field coupling. By characterizing these two coupling in model, we derive the beamforming vector for superdirective arrays. We prove that the field coupling matrix has the unique solution for an antenna array, and itself has the ability to fully characterize the distorted coupling field. Based on this proven theorem, we propose a method that accurately calculates the coupling matrix using only a number of angle sampling points on the order of the number of antennas. Moreover, a prototype of an independently-controlled superdirective antenna array is developed. Full-wave electromagnetic simulations and real-world experiments validate the effectiveness of our proposed approaches, and superdirectivity is achieved in reality by a compact array with 4 and 8 dipole antennas.

翻译：在多数多输入多输出（MIMO）通信系统中，天线通常间隔至少半个波长以减少互耦。在此配置下，最大阵列增益等于天线数量。然而，当天线间距显著减小时，紧凑阵列的阵列增益可变得与天线数量的平方成正比，远超传统MIMO系统。实现这种“超指向性”需要复杂的激励系数（波束赋形向量）计算，这是一项具有挑战性的任务。本文提出一种新颖的基于双重耦合的超指向波束赋形方法以解决该问题。具体而言，我们将天线耦合效应分类为阻抗耦合与场耦合。通过对这两种耦合进行建模表征，我们推导出超指向阵列的波束赋形向量。我们证明了场耦合矩阵对天线阵列存在唯一解，且其本身具备完整表征畸变耦合场的能力。基于此已证明的定理，我们提出一种仅需数量级与天线数相当的角向采样点即可精确计算耦合矩阵的方法。此外，我们研制了一款独立可控的超指向天线阵列原型。全波电磁仿真与实际实验验证了所提方法的有效性，并通过采用4单元和8单元偶极天线的紧凑阵列在现实中实现了超指向性。