Holographic multiple-input multiple-output (MIMO) is deemed as a promising technique beyond massive MIMO, unleashing near-field communications, localization, and sensing in the next-generation wireless networks. Semi-continuous surface with densely packed elements brings new opportunities for increased spatial degrees of freedom (DoFs) and spectrum efficiency (SE) even in the line-of-sight (LoS) condition. In this paper, we analyze holographic MIMO performance with disk-shaped large intelligent surfaces (LISs) according to different precoding designs. Beyond the well-known technique of orbital angular momentum (OAM) of radio waves, we propose a new design based on polar Walsh functions. Furthermore, we characterize the performance gap between the proposed scheme and the optimal case with singular value decomposition (SVD) alongside perfect channel state information (CSI) as well as other benchmark schemes in terms of channel capacity. It is verified that the proposed scheme marginally underperforms the OAM-based approach, while offering potential perspectives for reducing implementation complexity and expenditure.

翻译：全息多输入多输出（MIMO）被视为超越大规模MIMO的潜在技术，可在下一代无线网络中释放近场通信、定位与感知的潜力。半连续表面结合密集排列的单元为即使在视距（LoS）条件下增加空间自由度（DoFs）和频谱效率（SE）带来了新机遇。本文基于不同预编码设计，分析了配备盘形大智能表面（LISs）的全息MIMO性能。超越经典的无线电波轨道角动量（OAM）技术，我们提出了一种基于极坐标Walsh函数的新设计方案。此外，我们表征了所提方案与基于奇异值分解（SVD）及完美信道状态信息（CSI）的最优情况，以及其它基准方案在信道容量方面的性能差距。验证结果表明，所提方案性能略逊于基于OAM的方法，但在降低实现复杂度和成本方面具有潜在优势。