Learning-Based Beam Steering for Long-Range Orbital Angular Momentum Mode Multiplexing

The utilization of Orbital Angular Momentum (OAM) mode multiplexing offers a promising solution to alleviate the signal processing overhead in line-of-sight communication systems. However, concerns regarding the divergence of OAM beams have raised doubts about its practicality over long transmission distances. In this paper, we propose a novel beam steering strategy aimed at mitigating this issue by ensuring the successful reception of all OAM modes. To accomplish this objective, we present an unsupervised learning-based beam steering approach designed to individually steer each mode, thereby facilitating their reception at the receiver. We refer to this approach as Steered OAM (S-OAM). To mitigate inter-mode interference (IMI) from the steering process, we incorporate an equalizer into both the receiver and the training process. This ensures that our proposed S-OAM system is IMI-aware, improving its performance across steered OAM modes. Using S-OAM provides the flexibility of choosing the number of transmitted modes to accommodate low-capacity scenarios in low signal-to-noise ratio or long transmission distances, where a large number of modes lead to weak streams. Furthermore, the proposed approach leverages codebook-based phase shifters at the transmitter, eliminating the need for real-time calculation of phase shifter values and thereby maintaining a low level of signal processing burden at the transmit side, akin to traditional OAM transmission. Simulation results validate the efficacy of S-OAM, demonstrating its capability to generate multiple equally robust streams at the receiver with a total spectral efficiency reaching up to $0.5 \mathrm{bits} / \mathrm{sec} / \mathrm{Hz}$ from the theoretical upper bound.