  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 Ndimensional 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.
