Misalignment-Robust Modulation Scheme for UCA-Based OAM Multiplexing Systems

Orbital Angular Momentum (OAM) has attracted significant attention in the field of wireless communications due to its potential for high-capacity data transmission through mode multiplexing. However, OAM experiences severe performance degradation due to inter-mode interference (IMI) caused by antenna misalignment. In this paper, we propose a novel method to suppress IMI caused by misalignment for uniform circular array (UCA)-based OAM multiplexing. We discovered that IMI can be minimized when the IQ data of each OAM mode are close to each other in the constellation, enhancing robustness to misalignment. Therefore, we propose a method, where the IQ data of one OAM mode (reference OAM mode) is selected from the entire constellation range, and the IQ data of the other OAM modes are selected from a range close to this reference OAM mode. The proposed method is shown to prevent system capacity degradation compared to traditional method when the antenna is tilted. Moreover, in the proposed method, the reference OAM mode stream shows minimal performance degradation when the antenna is tilted. Furthermore, our proposed method is well-suited for adaptive modulation, effectively adjusting to varying communication conditions to maintain optimal performance.