Empowering MIMO-FSO Systems: RIS Technology for Enhanced Performance in Challenging Conditions

This paper presents a novel analytical framework to enhance the performance of reconfigurable intelligent surfaces (RIS)-integrated multiple-input-multiple-output (MIMO) free-space optical (FSO) communication systems. The study addresses critical challenges such as atmospheric turbulence, misalignment, and signal attenuation. It introduces a series-based approach to model the combined effects of Gamma-Gamma turbulence, generalized Rician pointing errors, and RIS size-related constraints. In contrast to previous studies, which often rely on oversimplified or idealized channel models, this framework provides closed-form expressions for the first time for the probability density function and cumulative distribution function of the end-to-end channel specifically designed for RIS-empowered (RIS-E) MIMO-FSO systems. These expressions capture the complex interactions between channel impairments and system parameters, enabling accurate performance evaluation in real-world deployments. The derived formulations provide key performance metrics, including outage probability, average bit error rate, ergodic capacity, data rate, and energy efficiency, for a variety of system configurations. Practical diversity combining techniques such as equal gain combining, maximal ratio combining, and selection combining are rigorously analyzed. In addition, asymptotic analyses at high signal-to-noise ratios offer simplified expressions that provide valuable insights into coding gain, diversity order, and system behavior under extreme conditions. A key contribution of this work is the investigation of the optimization of RIS placement, which improves signal alignment and reduces the outage probability, even under challenging atmospheric conditions. In addition, the study highlights the computational efficiency of the proposed framework through a detailed complexity analysis that confirms its feasibility for practical, large-scale applications. Monte Carlo simulations validate the theoretical findings, demonstrating strong agreement with the analytical results. These results confirm the transformative potential of RIS technology in mitigating turbulence-induced fading and misalignment. This research establishes RIS-E MIMO-FSO systems as a robust, energy-efficient solution for next-generation, high-bandwidth optical networks. Additionally, it provides practical deployment guidelines, ensuring the effective integration of RIS technology into real-world wireless communication infrastructures, thereby advancing the development of resilient and high-performance optical communication systems.