The Impact of Asymmetric Antenna Element's Radiation Pattern and Its Solution Analysis for UCA-Based OAM Communications

The uniform circular array (UCA) currently stands as the predominant method for generating orbital angular momentum (OAM) in the radio frequency band. Existing studies paid little attention to the impact of the antenna element's radiation pattern (AERP) on the OAM communication performance. In this paper, we present a UCA-based OAM communication model that considers the AERP. Through the model, it is theoretically demonstrated that perfect orthogonality of OAM modes is maintained only when the AERP is symmetric in the azimuthal angle. However, AERP is typically asymmetric due to the designing or manufacturing error, leading to a decline in OAM mode isolation. To address this issue, we propose the use of the sequentially rotated UCA (SRUCA) and prove that the mode orthogonality of SRUCA can always be maintained despite the asymmetry of the AERP. However, because the spatial orientation of each antenna element changes, the attribute of the SRUCA-based OAM communications also changes. To explore the performance difference, we further construct the electric field vector model of SRUCA-based OAM communications and theoretically prove that orthogonality among excitation modes still exists. Additionally, there is mode gain variation for the SRUCA compared with the UCA. Typically, in general communication scenarios where the communication distance is significantly larger than the UCA radius, there is 50% polarization loss (PL) for the linear polarization (LP) SRUCA when full modes multiplexing is performed. Comprehensive simulations of phase and amplitude error, mode isolation, polarization variation, and PL are performed. The results validate the theoretical analysis.