  Orbital angular momentum (OAM) carried electromagnetic waves have the
potential to improve spectrum efficiency in optical and radio-frequency
communications due to the orthogonal wavefronts of different OAM modes.
However, OAM beams are vortically hollow and divergent, which significantly
decreases the capacity of OAM transmissions. In addition, unaligned
transceivers in OAM transmissions can result in a high bit error rate (BER).
The Talbot effect is a self-imaging phenomenon that can be used to generate
optical or radio-frequency OAM beams with periodic repeating structures at
multiples of a certain distance along the propagation direction. These periodic
structures make it unnecessary for the transceiver antennas to be perfectly
aligned and can also alleviate the hollow divergence of OAM beams. In this
paper, we propose Talbot-effect-based fractal OAM generation and detection
schemes using a uniform circular array (UCA) to significantly improve capacity
and BER performance in unaligned OAM transmissions. We first provide a brief
overview of fractal OAM. Then, we propose the fractal OAM beam generation and
detection schemes. Numerical analysis and simulations verify the effectiveness
of our proposed fractal OAM generation scheme and also demonstrate improved
capacity and BER performance compared to normal OAM transmissions. We also
analyze how the receive UCA radius and the distance between the UCAs impact the
capacity and BER performances.
