Beam Axis Misalignment Suppression Method Employing Efficient Beamforming and IMI Cancellation for OAM Multiplexing

In orbital angular momentum (OAM), it is important to retain the alignment of the beam axis in the entire transmitter and receiver because beam axis misalignment reduces the received signal power and causes inter-mode interference (IMI). In this study, we propose an efficient method of transmit beamforming (BF), in addition to a low-complexity IMI canceller that targets the IMI from neighboring OAM modes, to simultaneously enhance the received signal power and combat the IMI for OAM multiplexing during beam axis misalignment. In the proposed approach, the precoding weight of the transmit BF is efficiently calculated by approximating the phase fluctuation owing to the beam axis misalignment using the Taylor series with the misaligned angle of the transmitting uniform circular array (UCA) and link distance. With the combination of the BF and low-complexity IMI canceller, compensation of the beam axis misalignment and suppression of the remaining IMI are realized. The effectiveness of our proposed method is verified via computer simulations in comparison with the case where only the low-complexity IMI canceller is employed at the receiver in terms of system capacity.