Abstract: Orbital Angular Momentum (OAM) is an intrinsic physical quantity in the Electro-Magnetic (EM) wave,
and the corresponding physical dimension is linearly independent with that of the electric field strength, so that the
OAM is independent with the electric field strength and constitutes the new dimension in the wireless transmission.
The vortex wave transmission system, which is the potential key technology in next-generation mobile
communications, is easily confused with the traditional Multiple-Input Multiple-Output (MIMO) transmission system.
This leads to the controversy on whether the OAM of vortex wave can provide the new dimension for the wireless
transmission. With tracing back to the history of EM wave utilization and development, the physical characteristics of
OAM are analyzed, and the fact that only the vortex wave transmission system with EM wave photon carrying
Intrinsic OAM (IOAM) can obtain the new dimension besides MIMO transmission is clarified. Compared with the
MIMO transmission, the Extrinsic OAM (EOAM) formed by the plane microwaves in the statistical OAM beam is
coupled with the space domain and cannot provide the additional new dimension. However, in the Line of Sight (LoS)
channel, the additional degrees of freedom and lower complexity can be achieved. Moreover, the typical vortex EM
wave OAM transmission system can be divided into four different regions based on the different channel capacities.
Taking the microwave band as an example, it is pointed out that only the quantum OAM transmission based on vortex
microwave photon can form a new MIMO bound with the OAM dimension beyond the traditional MIMO bound.