Degrees of Freedom of OAM-Based Line-of-Sight Radio Systems

This paper studies the degrees of freedom (DOFs) of a line-of-sight (LOS) communication system, employing orbital angular momentum (OAM)-based orthogonal multiplexing to enhance the spectral efficiency. Aperture and ring receive domains are considered, and analytical expressions of the DOF numbers are obtained. For both cases, the DOF numbers are found to be strongly dependent on the relative sizes between the transmitter and the receiver because of the divergence of the OAM-carrying beams. It is also found that the DOF number of an OAM-based system is less than that of a comparable system allowed to use any supported waves, suggesting that OAM-based multiplexing does not offer any additional performance benefits over traditional spatial multiplexing techniques. Some practical implications of the OAM-based radio are discussed. A numerical study is also presented to evaluate the analytical findings and to address the impacts of the assumptions adopted to reach these findings. The analytical DOF expressions are evidenced to have very good performance in both high-DOF and low-DOF scenarios.