Waveform Diversity-Based Generation of Convergent Beam Carrying Orbital Angular Momentum

Abstract

The orbital angular momentum (OAM) in radio frequency is expected to provide a new dimension to the OAM mode and benefit the degree of freedom of the electromagnetic wave. However, a central energy hollow exists in the OAM-carrying beam, and its radius varies with the OAM mode values, which is the main bottleneck in the radio frequency OAM applications. This article proposes a solution to eliminate the central hollow, by fundamentally changing the transmitted signal of the uniform circular array (UCA). To generate the unidirectional beam pattern, defined as the beaming OAM (BOAM), the orthogonal waveforms are radiated by UCA with an equidifferent initial phase. The BOAM wave can be received by one single antenna, and the separate OAM mode demodulation is then achieved based on the waveform diversity. This article studies the theoretical model of the BOAM field distribution and shows that the waveform diversity among distributed units can not only alter the doughnut-like shape into the closed beam, but also maintain the intact vortex characteristic. On this basis, the extended OAM mode multiplexing system is designed with one single UCA. A 16-element UCA is shown to be capable of generating 15 OAM modes simultaneously, with all modes superposing on the same direction. Furthermore, it is verified by the simulation that the vortex phase of each single mode can be precisely extracted from the OAM-mode-multiplexing signal.