On the Spectral Efficiency Limits of an OAM-Based Multiplexing Scheme

As reported in several recent publications, a spatial multiplexing involving the transmission of orthogonal waves carrying orbital angular momentum (OAM) is unable to provide spectral efficiency improvements with respect to the conventional techniques. In this letter, we emphasize how the limits of an OAM multiple transmission between antenna arrays can be derived from the Shannon capacity formula, taking as a reference the performance of a multiplexing method based on the higher order channel modes. Our approach clearly indicates that the two techniques offer the same on-axis performance. Conversely, small misalignments in the arrays positions affect the OAM scheme, highlighting the greater robustness of a traditional multiplexing method in the context of radio communications.