  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.
