New dimension in vortex electro-magnetic wave transmission with orbital angular momentum

Orbital angular momentum (OAM) is a physical quantity in the electro-magnetic (EM) wave which has the physical dimension linearly independent with that of the electric field strength, so that the OAM can constitute the new dimension in the wireless transmission. With tracing back to the history of EM wave utilization and development, the physical characteristics of OAM were analyzed, and the fact that only the vortex wave transmission system with EM wave photon carrying intrinsic OAM could obtain the new dimension besides MIMO transmission was clarified. Compared with the MIMO transmission, the extrinsic OAM formed by the plane microwaves in the statistical OAM beam was coupled with the space domain and could not provide the additional new dimension. However, in the line of sight (LoS) channel, the additional degrees of freedom and lower complexity could be achieved compared with MIMO transmission. Moreover, the typical vortex EM wave OAM transmission systems could be divided into four different regions based on the different channel capacities. Taking the microwave band as an example, it is pointed out that the quantum OAM transmission based on vortex microwave photon can form the new MIMO bound with the OAM dimension, which is beyond the traditional MIMO bound. © 2022 Editorial Board of Journal on Communications. All rights reserved.