We present a unified model for connected antenna arrays with a large number of tightly integrated (i.e., coupled) antennas in a compact space within the context of massive multiple-input multiple-output (MIMO) communication. We refer to this system as tightly-coupled massive MIMO. From an information-theoretic perspective, scaling the design of tightly-coupled massive MIMO systems in terms of the number of antennas, the operational bandwidth, and form factor was not addressed in prior art. We investigate this open research problem using a physically consistent modeling approach for far-field (FF) MIMO communication based on multi-port circuit theory. In doing so, we turn mutual coupling (MC) from a foe to a friend of MIMO systems design, thereby challenging a basic percept in antenna systems engineering that promotes MC mitigation/compensation. We show that tight MC widens the operational bandwidth of antenna arrays thereby unleashing a missing MIMO gain that we coin "bandwidth gain". Furthermore, we derive analytically the asymptotically optimum spacing-to-antenna-size ratio by establishing a condition for tight coupling in the limit of large-size antenna arrays with quasi-continuous apertures. We also optimize the antenna array size while maximizing the achievable rate under fixed transmit power and inter-element spacing. Then, we study the impact of MC on the achievable rate of MIMO systems under line-of-sight (LoS) and Rayleigh fading channels. These results reveal new insights into the design of tightly-coupled massive antenna arrays as opposed to the widely-adopted "disconnected" designs that disregard MC by putting faith in the half-wavelength spacing rule.

翻译：我们提出了一个统一模型，用于描述在紧凑空间内集成大量紧密耦合天线的连接天线阵列，背景为大规模多输入多输出（MIMO）通信。我们将此系统称为紧耦合大规模MIMO。从信息论角度来看，先前研究未涉及在紧耦合大规模MIMO系统设计中，如何按天线数量、工作带宽和外形尺寸进行规模化设计。我们通过基于多端口电路理论的远场（FF）MIMO通信物理一致性建模方法，探讨了这一开放研究问题。在此过程中，我们将互耦（MC）从MIMO系统设计中的敌人转变为朋友，从而挑战了天线系统工程中促进MC缓解/补偿的基本观念。我们表明，紧耦合MC拓宽了天线阵列的工作带宽，释放了一种我们称之为“带宽增益”的缺失MIMO增益。此外，我们通过建立准连续孔径大尺寸天线阵列极限下紧耦合的条件，解析推导了渐近最优的天线间距与尺寸比。同时，我们在固定发射功率和阵元间距下优化天线阵列尺寸，以最大化可实现速率。随后，我们研究了MC在视距（LoS）和瑞利衰落信道下对MIMO系统可实现速率的影响。这些结果为紧耦合大规模天线阵列的设计提供了新见解，与广泛采用的依赖半波长间距规则而忽视MC的“非连接”设计形成对比。