The holographic multiple-input-multiple-output (MIMO) communications refer to the MIMO systems built with ultra-dense antenna arrays, whose channel models and potential applications have attracted increasing attentions recently. When the spacing between adjacent array elements is larger than half wavelength, the effect of mutual coupling can generally be neglected in current antenna designs. However, in holographic MIMO communications, the influence of strong mutual coupling on antenna characteristics is inevitable, resulting in distorted radiation patterns and low radiation efficiencies. In this paper, starting from the analytical correlation and efficiency models, we investigate how the mutual coupling affects the capacity of a space-constrained MIMO system from the aspects of degree of freedom (DOF) and antenna efficiency. The involved fundamental concepts of correlation, DOF, efficiency and mutual coupling are crucial for both antenna and wireless-communication engineers when designing emerging MIMO communication systems.

翻译：全息多输入多输出（MIMO）通信是指采用超密集天线阵列构建的MIMO系统，其信道模型和潜在应用近年来受到日益广泛的关注。当相邻阵列单元间距大于半波长时，当前天线设计中通常可忽略互耦效应。然而，在全息MIMO通信中，强互耦对天线特性的影响不可避免，会导致辐射方向图畸变及低辐射效率。本文从解析相关性模型与效率模型出发，从自由度和天线效率两个维度研究互耦如何影响空间受限MIMO系统的容量。所涉及的相关性、自由度、效率与互耦等基本概念，对于天线工程师和无线通信工程师在设计新兴MIMO通信系统时具有关键指导意义。