A Revisit of Orbital Angular Momentum Multiplexing in Multipath Environment

It is commonly believed that orbital angular momentum (OAM) multiplexing is only suitable for short-range communications in line-of-sight (LoS) scenario and multipath propagation would be detrimental for OAM communications. It has been demonstrated very recently that OAM multiplexing could work in rich isotropic multipath environment when the conventional spatial equalization is used for data detection. Moreover, the resulting channel capacity is comparable to that of a conventional multiple-input multiple-output system. Nevertheless, the rich isotropic multipath environment is an ideal multipath scenario. In this paper, we investigate the performance of OAM multiplexing in arbitrary multipath environment. Contrary to the common belief, it is shown that multipath can be beneficial for OAM multiplexing in terms of channel capacity. Particularly, the OAM capacity increases with enlarged angular spread of the channel and reaches its maximum when the angular spread is comparable to the divergence angle of the OAM beam. Based on the study, the OAM multiplexing is further investigated for base station (BS) applications. It is shown that OAM based BS antennas are comparable to (or even outperform) the conventional BS antennas in terms of channel capacity.