Non-Coaxial OAM: Precoding Design, Misaligned Parameter Estimation and Capacity Compensation

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 modified 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 non-coaxial 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.