The vortex electromagnetic wave carried by multiple orthogonal orbital angular momentum (OAM) modes in the same frequency band can be applied to the field of wireless communications, which greatly increases the spectrum efficiency. The uniform circular array (UCA) is widely used to generate and receive vortex electromagnetic waves with multiple OAM-modes. However, the maximum number of orthogonal OAM-modes based on UCA is usually limited to the number of array-elements of the UCA antenna, leaving how to utilize more OAM-modes to achieve higher channel capacity with a fixed number of arrayelements as an intriguing question. In this paper, we propose an N-dimensional quasi-fractal UCA (ND QF-UCA) antenna structure in different fractal geometry layouts to break through the limits of array-elements number on OAM-modes number. We develop the N-dimensional OAM modulation (NOM) and demodulation (NOD) schemes for OAM multiplexing transmission with the OAM-modes number exceeding the array-elements number, which is beyond the traditional concept of multiple antenna based wireless communications. Then, we investigate different dimensional multiplexing transmission schemes based on the corresponding QF-UCA antenna structure with various array-element layouts and evaluate the optimal layout type and dimension to obtain the highest channel capacity with a fixed number of array-elements. Simulation results show that our proposed schemes can obtain a higher spectrum efficiency, surpassing those of alternative array-element layouts of QF-UCA and the traditional multiple antenna systems.

翻译：在同一频段内由多个正交轨道角动量（OAM）模式承载的涡旋电磁波可应用于无线通信领域，从而显著提升频谱效率。均匀圆阵（UCA）被广泛用于产生和接收多OAM模式的涡旋电磁波。然而，基于UCA的正交OAM模式最大数量通常受限于UCA天线的阵元数目，如何利用更多OAM模式在固定阵元数条件下实现更高信道容量成为一个值得探究的问题。本文提出了一种在不同分形几何构型下的N维准分形UCA（ND QF-UCA）天线结构，以突破阵元数量对OAM模式数量的限制。我们发展了适用于OAM模式数超过阵元数的OAM复用传输的N维OAM调制（NOM）与解调（NOD）方案，这超越了传统基于多天线无线通信的概念。随后，我们研究了基于相应QF-UCA天线结构（具有不同阵元布局）的多维复用传输方案，并评估了在固定阵元数下为获得最高信道容量的最优布局类型与维度。仿真结果表明，所提方案能获得更高的频谱效率，优于QF-UCA的其他阵元布局方案及传统多天线系统。