Phase shifting analysis and solution for UCA based OAM beam steering

Recently, uniform circular array (UCA) based orbital angular momentum (OAM) beam steering schemes have been proposed to overcome the limitations of coaxial transmission. Unlike the traditional multiple-input-multiple-output (MIMO) beam steering, OAM beam steering includes both the OAM generation and the beam steering. Generally, the true time delay (TTD) or the phase shifter (PS) are required for beam steering in the radio domain. Previous studies suggest that TTD is preferred for wideband MIMO beam steering to avoid beam squint caused by PS. However, in this paper, we theoretically prove that to generate the OAM beam ideally, PS should be used, while TTD deteriorates the mode orthogonality, which is influenced by the relative bandwidth. Once the ideal OAM beam is generated, TTD is required to prevent beam squint. Based on this analysis, we propose to use the two-stage phase-shifting (TSPS) architecture for OAM beam steering: PS for OAM generation and TTD for beam steering. Simulation results suggest that compared to the spectrum efficiency (SE) of PS based OAM communication, the SE based on the TTD significantly declines as the relative bandwidth increases. Furthermore, OAM beam steering using the TSPS architecture greatly outperforms systems that adopt a single TTD or PS network.