Superdirective array may achieve an array gain proportional to the square of the number of antennas $M^2$. In the early studies of superdirectivity, little research has been done from wireless communication point of view. To leverage superdirectivity for enhancing the spectral efficiency, this paper investigates multi-user communication systems with superdirective arrays. We first propose a field-coupling-aware (FCA) multi-user channel estimation method, which takes into account the antenna coupling effects. Aiming to maximize the power gain of the target user, we propose multi-user multipath superdirective precoding (SP) as an extension of our prior work on coupling-based superdirective beamforming. Furthermore, to reduce the inter-user interference, we propose interference-nulling superdirective precoding (INSP) as the optimal solution to maximize user power gains while eliminating interference. Then, by taking the ohmic loss into consideration, we further propose a regularized interference-nulling superdirective precoding (RINSP) method. Finally, we discuss the well-known narrow directivity bandwidth issue, and find that it is not a fundamental problem of superdirective arrays in multi-carrier communication systems. Simulation results show our proposed methods outperform the state-of-the-art methods significantly. Interestingly, in the multi-user scenario, an 18-antenna superdirective array can achieve up to a 9-fold increase of spectral efficiency compared to traditional multiple-input multiple-output (MIMO), while simultaneously reducing the array aperture by half.

翻译：超方向性阵列可能实现与天线数目 $M^2$ 成正比的阵列增益。在超方向性的早期研究中，鲜有从无线通信角度开展的探索。为利用超方向性提升频谱效率，本文研究了采用超方向性阵列的多用户通信系统。我们首先提出一种场耦合感知（FCA）多用户信道估计方法，该方法考虑了天线耦合效应。以最大化目标用户功率增益为目标，我们提出多用户多径超方向性预编码（SP），作为我们先前基于耦合的超方向性波束成形工作的扩展。此外，为降低用户间干扰，我们提出干扰零陷超方向性预编码（INSP），作为在消除干扰的同时最大化用户功率增益的最优解。随后，考虑欧姆损耗，我们进一步提出正则化干扰零陷超方向性预编码（RINSP）方法。最后，我们讨论了众所周知的窄方向性带宽问题，并发现该问题在多载波通信系统中并非超方向性阵列的根本性缺陷。仿真结果表明，所提方法显著优于现有最优方法。有趣的是，在多用户场景下，与传统多输入多输出（MIMO）相比，18天线超方向性阵列可实现高达9倍的频谱效率提升，同时阵列孔径减半。