Orbital angular momentum (OAM) is a fundamental property of electromagnetic waves and is inherently carried by all frequencies of waves. In particular, OAM applications at radio frequencies (RFs) have attracted much attention due to their unique functionalities in communications. Enabled by the present wireless communication system, employing a circular antenna array (CAA) is a highly compatible and cost-effective approach for generating RF-OAM beams. In this paper, recent progress on RF-OAM communication systems with CAAs from the transceiver to the controller is summarized. A new scheme for RF-OAM generation and beam steering systems is proposed based on dual-wavelength control using CAA with the energy-conserved feature, where an integrated chip incorporating an arrayed waveguide grating (AWG) structure is introduced. Either microwave photonic phase shift or optical true time delay (OTTD) can be operated by this chip, benefiting from the phase amplification concept and the design of delay channels. Utilizing their combination, RF-OAM beams with arbitrary OAM states can be achieved for multiplexing and steered simultaneously. Furthermore, RF-OAM beam steering can be achieved without distortion in the desired transmission direction. This photonics-assisted method with an integrated chip is a potential strategy for improving the signal quality and reducing the system complexity of RF-OAM transmission systems.

By introducing the arrayed waveguide router (AWGR) optical true time-delay (OTTD) architecture in bent-pipe satellite optical inter-link to optically assist the RF phased array in Low-earth orbit satellites will extend the multi-hop bent-pipe satellite beam-hopping protocol proposed in DVB-S2X. It solves the challenge of beam steering with the support of precise, broadband, and wide-range scanning. This architecture utilizes a subarray to combine the advantages of AWGR and a high-precision RF phase shifter to realize the beam pointing without an oblique view. Unlike the traditional digital and analog phased array architecture, the introduction of OTTD can solve the problem of beam squint and also ensure the high-precision scanning of the beam.

A novel microwave beam steering system with spectral filtering using optical true time delay is proposed. 90 degrees beam steering with four elements and spectral filtering with 26-dB passband-to-stopband contrast ratio are simultaneously obtained.