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A wireless multiple-input multiple-output (MIMO) multiplexing system consists of multiple transmit and receive antennas. The channel capacity of such systems does not merely depend on the number of degrees of freedom (DoF) it can support, but also on their gains, represented by the singular values of the channel matrix. A radio frequency based orbital angular momentum (OAM) communication system can be modeled as a MIMO multiplexing system. In the ideal case, message signals transmitted on the same frequency using multiple OAM modes can be separated by using a circular array of receive antennas. In this paper, we characterize circular arrays using three parameters: angular spacing between consecutive antennas, angular span of the elements within the array and the standard deviation of the angular spacing. We show that, the effective degrees of freedom in RF based OAM communication systems depend on the characteristics of the circular array used to receive the transmitted OAM signals.Index Terms-Wireless orbital angular momentum (OAM), line of sight MIMO, effective degrees of freedom, circular antenna array

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