Generation Techniques of Orbital Angular Momentum Beams for Wireless Communication Applications

This paper comprehensively overviews the various techniques of generating Orbital Angular Momentum (OAM) beams from microwave to optical frequencies for wireless communications applications. OAM communications has been explored extensively over the past two decades for its potential to improve spectral efficiency and increase capacity. This review paper broadly categorizes the different generation techniques into active and passive methods. Active methods include arrays of actively excited radiating antenna elements that offer dynamic control over the phase profile, enabling real-time manipulation of OAM beams. Additionally, the mutual coupling between antenna elements in uniform circular arrays is discussed, as it can significantly impact the performance and purity of OAM beams, and strategies to mitigate these effects are explored. Passive methods include spiral phase plates, reflect arrays, metasurfaces, optical rectification, difference frequency generation, laser plasma, discrete dielectric lenses, etc., which utilize static elements to manipulate the phase of an incident plane wave and create a helical phase front characteristic of an OAM wave. The paper discusses each method’s principle, advantages, and challenges, highlighting their potential applications primarily in wireless communication and additionally in optical information processing and radar. The paper also discusses methods for achieving high mode purity, which is essential for reliable OAM mode identification and efficient separation of multiplexed OAM beams. Techniques for multiplexing several OAM modes, such as concentric circular arrays considering the cross-coupling effects and mitigation methods, Butler matrices, MIMO techniques, and enhancing OAM beam propagation using lenses and superposition with non-diffracting beams, are also discussed.