Degrees of freedom of OAM-based communication systems

This paper studies the degrees of freedom (DOF) of a line-of-sight communication system employing OAM-based orthogonal multiplexing. Analytical expressions of the DOF number are obtained with or without the requirement of OAM-carrying waves. It is found that the DOF number of an OAM-based system strongly depends on the relative sizes between the transmitter and the receiver. It is also found that the DOF number of an OAM-based system is less than that of a comparable system not restricted to using OAM-carrying waves, suggesting that OAM-based multiplexing does not offer any additional performance benefits over traditional spatial multiplexing techniques.