On the Spectral Efficiency of Orbital Angular Momentum with Mode Offset

Thanks to its low transceiver complexity, mode division multiplexing (MDM) using orbital angular momentum (OAM) has been recently investigated as a new physical layer wireless transmission technique. This is due to the fact that different OAM modes are spatially orthogonal to each other, thus, perfectly suitable for spatial multiplexing and/or diversity. The orthogonality amongst different OAM modes is achieved in the absence of any mode offset between the transceivers’ phase-shifting feeding networks. However, the inherent presence of OAM mode offset destroys the orthogonality between different modes, which gives rise to inter-mode interference (IMI). Therefore, in this paper, in order to theoretically analyze the negative impact of OAM mode offset of MDM-OAM systems on the spectral efficiency of line-of-sight free-space wireless communication systems, an explicit signal-to-interference-plus-noise ratio expression for each OAM mode is derived, through which the spectral efficiency degradation is evaluated. Furthermore, in order to support our theoretical analysis, a $9\times 9$ MDM-OAM practical communication experiment for different OAM mode settings is carried out at a microwave frequency of 5.8 GHz. In addition, by leveraging the fact that all circulant matrices always have the same eigendirections, regardless of the entries of the matrix, a low-complexity optimal OAM mode selection scheme is proposed which significantly improves the spectral efficiency of the system. Finally, in order to completely null-out the impact of OAM mode offset, a baseband zero-forcing stage is embedded at the receiver.