Quasi-Fractal UCA Based Two-Dimensional OAM Orthogonal Transmission

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) structure is widely used to generate or receive vortex electromagnetic wave with multiple OAM-modes. However, the maximum number of orthogonal OAM-modes based UCA is usually limited to the number of array-elements of the UCA antenna, leaving how to utilize more OAM-modes given a fixed number of array-elements as an intriguing question. In this paper, we propose a quasi-fractal UCA (QF-UCA) antenna structure in different fractal geometry layouts to break through the limits of array-element number on OAM-modes number. We develop the two-dimensional OAM modulation (TOM) and demodulation (TOD) schemes for OAM multiplexing transmission with the OAM-modes number exceeding the array-element number, which is beyond the traditional concept of multiple antennas based wireless communications. Then, we study four different types of array-element layouts for QF-UCA and evaluate the optimal layout to obtain the maximum number of OAM-modes with fixed number of array-elements. Simulation results show that our proposed scheme can obtain the maximum number of orthogonal streams, which is larger than those corresponding to other array-element layouts of QF -UCA and the traditional multiple antenna systems.