Performance Enhancement of OAM With OFDM-IM for FSO Communications Using LLR Detection

Orbital angular momentum (OAM) communication has emerged as a promising technology for significantly enhancing data transmission efficiency and overall system performance in free-space optical (FSO) communication systems. However, its performance is severely affected by atmospheric turbulence (AT), which leads to signal attenuation, crosstalk, and degradation in system capacity and bit error rate (BER). To address these challenges, this paper proposes a novel approach that integrates OAM multiplexing with orthogonal frequency division multiplexing using index modulation (OFDM-IM) under varying AT conditions. Additionally, a low-complexity log-likelihood ratio detector is employed for efficient signal detection. Simulation results demonstrate that the proposed system outperforms conventional OAM-MIMO in terms of BER and capacity across various turbulence regimes and OAM modes. The proposed system is evaluated using different parameters, including propagation distance, active indices, and the number of users. The results suggest that our proposed system effectively balances resilience to turbulence and spatial multiplexing, ensuring sustained capacity in challenging FSO environments.