On the restriction of utilizing orbital angular momentum in radio communications

The orbital angular momentum (OAM) is considered as one of the most fundamental physical quantities in optics. It was recently discovered that the optical OAM can be also applied to radio communications. Nevertheless, OAM-based radio communications are not applicable for long-distance scenarios. In the existing studies, both the transmitting and receiving antenna arrays are facing each other perfectly on the same axis in free space, which however is not the general case in practical wireless communication systems. In this paper, we analyze the performance of OAM-based radio communications with disalignment antenna arrays, indicating the existence of an oblique angle. Both numerical and simulation results demonstrate that the capacity of OAM-based radio communication decrease significantly with disalignment antenna arrays, and the rotation phase is seriously distorted. Moreover, the performance is poorer with larger oblique angles.