Inter-Mode Interference Supression for SPP-Based OAM Systems: Expectation Propagation over 3D Factor-Graph

Orbital angular momentum (OAM) has gained significant interest in the field of radio vortex wireless communications due to the orthogonality it offers among different OAM modes. However, this orthogonality is contingent upon a challenging prerequisite: the precise alignment of transmit and receive antennas. Misaligned transceivers introduce phase turbulence, causing severe inter-mode interference (IMI) among OAM modes and complicating demultiplexing at the receiver end. To address this issue, our paper presents an innovative approach using a spiral phase plate (SPP) to generate OAM signals. We construct a three-dimensional (3D) factor-graph by incorporating time delay as an additional dimension and develop corresponding message passing mechanisms. Consequently, we propose a novel 3D- message passing algorithm (3D-MPA) for low-complexity multi-mode detection. Our simulation results demonstrate that the 3D-EPA method outperforms the traditional MMSE method.