Misalignments in Orbital Angular Momentum-based Optical Wireless Communication Systems

Abstract

Orbital angular momentum (OAM) has demonstrated significant potential under ideal conditions, but its performance is highly sensitive to misalignments. The impact of misalignments on the system capacity in the OAM multiplexed optical wireless communication (OWC) system remains underexplored. This paper studies the effects of receiver aperture and misalignment on the received power and total capacity of OAM multiplexed OWC systems. It also investigates the effect of different mode selection strategies, namely Continuous Mode Selection (CMS), Fixed Mode Selection (FMS), and Exhaustive Search (ES), on the system capacity. We found that smaller receiver apertures experience an exponential increase in received power as aperture size increases, up to a certain threshold. Additionally, misalignment has different effects on different modes. For lower-order modes, power consistently decreases with increasing misalignment, whereas higher-order modes exhibit an initial increase in power under small misalignments. Furthermore, simulations indicate that total channel capacity improves as the number of users increases under ideal or near-ideal alignment. However, capacity significantly drops with misalignment due to reduced received power and increased cross-talk from modal coupling. Appropriate mode selection can reduce this power loss and cross-talk, improving the system’s capacity, particularly for moderate misalignments. Among the mode selection strategies, ES provides the highest capacity across all misalignment ranges but is computationally intensive. FMS, on the other hand, offers a practical balance by significantly outperforming CMS in terms of capacity under moderate misalignment conditions while maintaining reduced complexity. Overall, this study highlights the importance of an optimal mode selection strategy to enhance system capacity and improve misalignment resilience in OAM multiplexed OWC systems.