Misalignment-Robust Codebook-Based Beamforming for OAM Mode Multiplexing Systems

In this paper, we present an algorithm to construct a misalignment-robust beamforming (BF) codebook for orbital angular momentum mode multiplexing (OAM-MM) systems. The BF codebook is constructed for a given set of antenna misalignment (AM) parameters, i.e., the AM type, rotation angles, and displacement values. We use the sorted QR-decomposition of the augmented misaligned channel matrix to design a BF codebook that is applied to received signals using phase shifters only. Subsequently, by employing the successive interference cancellation (SIC), the transmitted symbols corresponding to each OAM mode are estimated. The combination of codebook-based BF in the analog domain and the SIC in the digital domain offers an effective strategy to solve the AM problem without sacrificing the computational savings associated with the application of OAM-MM. Simulation results confirm that by adopting the proposed approach, the sum-rate of the misaligned OAM-MM system approaches the theoretical upper bound.