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：全息多投入-多输出通信是指以超常天线阵列建造的MIMO系统,这些天线阵列的频道模型和潜在应用最近引起越来越多的注意。当相邻阵列元素之间的间距大于半波长时,在目前的天线设计中通常会忽略相互连接的影响。然而,在全息多投入-多输出通信中,对天线特性的强烈相互连接的影响是不可避免的,导致辐射模式扭曲和低辐射效率。在本文中,从分析相关性和效率模型开始,我们从自由度和天线效率的方面来调查相互连接如何影响空间限制的MIMO系统的能力。在设计新兴的MIMO通信系统时,涉及相关性、DF、效率和相互连接的基本概念对天线和无线通信工程师都至关重要。</s>