Uniform Circular Antenna (UCA) is a convenient way to generate multiple Orbital Angular Momentum (OAM) beams simultaneously, and OAM-MIMO is promised to be an augmentation to enhance capacity. In this paper, concentric UCA which is an elaborate arrangement of MIMO antennas is adopted. OAM-MIMO transceiver mode selection and precoding architectures are proposed to improve the coaxial capacity performance. Non-coaxial parameters and their effect on mathematical channel matrix are analyzed, and

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... ified non-coaxial parameters are further introduced to simplify the channel expression. Using the central antenna to separate transmitting (TX) misalignment and receiving (RX) misalignment, modified noncoaxial parameters and deviation matrices can be obtained with reduced complexity, dealing with the lack of non-coaxial parameters in practice. Moreover, to make for the orthogonality between different OAM modes, pre-transmitting and pre-receiving matrices are designed employing estimated modified non-coaxial parameters or deviation matrices to transfer the non-coaxial channel into a block circulant one. The provided simulation results verify the superiority of the proposed mode-selection and precoding design as well as the effectiveness of the proposed non-coaxial pre-processing scheme. INDEX TERMS Orbital angular momentum, precoding, misalignment, non-coaxial parameter estimation, compensation. 37726 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ VOLUME 9, 2021 GUANGHUI YU received the master's and Ph.D. degrees in automatic control from the Beijing Institute of Technology, Beijing, China, in 1998 and 2003, respectively. Since 2003, he has been acted as a Radio Expert with the Wireless Advanced Research Department, ZTE Corporation. His research interests include Wimax, 2G, 3G, 4G, 5G, and B5G/6G design in RAN especially involved in multiplexing access, MIMO, interference management, channel modeling, and network architecture. He is also involved as one of the main researchers in the link and system simulation platform takes part in all kinds of 3GPP RAN1 activities.