I. Introduction
Electromagnetic vortex wave is a kind of nonplanar electromagnetic wave, whose phase front is a twisted structure rotating around the direction of propagation [1]–[3]. Because of carrying orbital angular momentum (OAM), it can provide an additional degree of freedom in topological charge domain, which can be used to further improve communication capacity [4]–[7]. Different from the traditional communication techniques using amplitude modulation, frequency modulation, and phase modulation, vortex waves take advantage of the OAM mode numbers for modulation. Recent studies also report that OAM waves have potential abilities for radar imaging application [8]–[10].