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 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 spectrum efficiency given a fixed number of array-elements 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 multiplex transmission schemes based on the corresponding QF-UCA antenna structure with various array-elements layouts. Simulation results show that our proposed schemes can obtain a higher spectrum efficiency.

The vortex electromagnetic wave carrying 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 array-elements as an intriguing question. In this paper, we propose anN-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 theN-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.

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.