Degrees of Freedom of UCA-Based Mode Division Multiplexing MIMO Systems

The degrees of freedom (DOF) of UCA-based single-ring orbit angular momentum (OAM) communication system has been derived in this paper. It is found that the DOF of an mode division multiplexing (MDM) multiple-input multiple-output (MIMO) system depends on the sizes of both the transmit aperture and the receive aperture, the frequency of carrying wave, and the communication distance. For a given communication configuration, the simulation results show that the beam waist selection is crucial to maximize the number of DOF. Moreover, according to the expression of DOF, the OAM technique can perfectly combines with the millimeter wave communications due to low complexity in hardware design.