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：作为最具前景的技术之一，全息多输入多输出（H-MIMO）近年来因其在扩展无线通信可能性和实现基础无线性能极限方面的巨大潜力而吸引了显著的研究兴趣。借助具有强大电磁波控制能力的近乎连续、大尺寸且节能的表面，H-MIMO开辟了在更基础的电磁域进行信号处理的机遇，为支持未来网络中极高频谱效率和能量效率的全息成像级通信铺平了道路。本文致力于实现一种通用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。我们从积分形式出发，建立了两种有效且计算高效的通道模型：一种采用复杂公式但精度更高，另一种则简洁明了但精度略有牺牲。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密的上界。研究结果表明，该容量极限随发射单元面积、接收单元面积以及所有收发天线单元间$1/{{d}_{mn}^2}$、$1/{{d}_{mn}^4}$和$1/{{d}_{mn}^6}$综合效应的乘积呈对数增长，其中$d_{mn}$表示每个发射单元与接收单元之间的距离。数值评估验证了我们信道模型的有效性，并展示了上界与实际容量之间的微小差异，这有助于预测实际系统性能。