Envisioned as one of the most promising technologies, holographic multiple-input multiple-output (H-MIMO) recently attracts notable research interests for its great potential in expanding wireless possibilities and achieving fundamental wireless limits. Empowered by the nearly continuous, large and energy-efficient surfaces with powerful electromagnetic (EM) wave control capabilities, H-MIMO opens up the opportunity for signal processing in a more fundamental EM-domain, paving the way for realizing holographic imaging level communications in supporting the extremely high spectral efficiency and energy efficiency in future networks. In this article, we try to implement a generalized EM-domain near-field channel modeling and study its capacity limit of point-to-point H-MIMO systems that equips arbitrarily placed surfaces in a line-of-sight (LoS) environment. Two effective and computational-efficient channel models are established from their integral counterpart, where one is with a sophisticated formula but showcases more accurate, and another is concise with a slight precision sacrifice. Furthermore, we unveil the capacity limit using our channel model, and derive a tight upper bound based upon an elaborately built analytical framework. Our result reveals that the capacity limit grows logarithmically with the product of transmit element area, receive element area, and the combined effects of $1/{{d}_{mn}^2}$, $1/{{d}_{mn}^4}$, and $1/{{d}_{mn}^6}$ over all transmit and receive antenna elements, where $d_{mn}$ indicates the distance between each transmit and receive elements. Numerical evaluations validate the effectiveness of our channel models, and showcase the slight disparity between the upper bound and the exact capacity, which is beneficial for predicting practical system performance.

翻译：全息多输入多输出（H-MIMO）作为最具前景的技术之一，因其在拓展无线可能性与实现基本无线极限方面的巨大潜力，近期引发了显著的研究兴趣。凭借近乎连续、大尺寸且节能的表面以及强大的电磁波控制能力，H-MIMO开启了在更基础的电磁域进行信号处理的机会，为实现未来网络中支持极高频谱效率和能量效率的全息成像级通信铺平了道路。本文试图实现一种广义的电磁域近场信道建模，并研究在视距（LoS）环境下配备任意放置表面的点对点H-MIMO系统的容量极限。从积分表达式出发，我们建立了两种有效且计算高效的信道模型：一种公式复杂但精度更高，另一种简洁但精度略有牺牲。此外，我们利用所提出的信道模型揭示了容量极限，并基于精心构建的分析框架推导出一个紧致上界。结果表明，容量极限随发射单元面积、接收单元面积以及所有发射与接收天线单元上$1/{{d}_{mn}^2}$、$1/{{d}_{mn}^4}$和$1/{{d}_{mn}^6}$的联合效应之积呈对数增长，其中$d_{mn}$表示每个发射单元与接收单元之间的距离。数值评估验证了所提出信道模型的有效性，并展示了上界与实际容量之间的轻微差异，这有助于预测实际系统性能。