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（Holographic 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、定位与感知能力。半连续表面与密集排列的元件即便在视距（LoS）条件下也能增加空间自由度（DoFs）和频谱效率（SE）。本文基于圆盘形大智能表面（LIS），依据不同预编码设计分析了全息MIMO性能。除无线电波轨道角动量（OAM）这一广为人知的技术外，我们提出了一种基于极坐标Walsh函数的新设计。进一步，我们以信道容量为指标，将所提方案与基于奇异值分解（SVD）及完美信道状态信息（CSI）的最优方案及其他基准方案进行了性能差距分析。验证表明，所提方案性能略逊于基于OAM的方案，但为降低实现复杂度与成本提供了潜在前景。