The potential integration of unmanned aerial vehicles (UAVs) with free space optical (FSO) communication systems stands as a promising innovation in the realm of wireless network infrastructures. This study presents a comprehensive investigation into the application of orthogonal frequency division multiplexing (OFDM) in conjunction with UAV-based FSO technology, with a specific focus on establishing robust wireless communication links to ground sites within the evolving landscape of 5G networks. The research introduces a pioneering 4-level quadrature amplitude modulation (4-QAM)-OFDM-FSO framework tailored for UAV-to-ground communication, revolutionizing the prospects for seamless and high-throughput data transmission within dynamic network environments. Through comprehensive simulations and theoretical analyses, we unveil the system's efficacy in mitigating atmospheric turbulence, achieving heightened signal integrity, and ensuring performance adaptability over varying link distances, thus significantly addressing present limitations in traditional wireless communication models. Anchored within the context of modern wireless network infrastructures, this work serves as a crucial stepping stone for the practical application of OFDM-UAV-FSO communication systems, representing a paradigm shift in fostering resilient and agile wireless connectivity in the era of 5G networks. The inception of cutting-edge wireless networks expected to outperform the capabilities of 5G necessitates an infrastructure that can handle vast amounts of data. This infrastructure must be not only cost-effective and simple to deploy but also readily scalable to accommodate the diverse demands of front-haul and backhaul applications. Motivated by the growing interest in harnessing UAVs to extend the reach and enhance the operational efficacy of conventional cellular networks, this work introduces a novel application of UAV-ground station connections. The concept employs FSO to facilitate network traffic within both the segments. To optimize throughput, resilience, and spectral efficiency, the application of OFDM is proposed. The research considers the transmission of a 20 Gbps 4-QAM data signal across various channel conditions. It thoroughly assesses the performance implications of factors such as transmission distance and beam divergence. The study explores the correlation between pointing error, scintillation, beam divergence angle, and average spectral efficiency. A slight increase in pointing error results in a rapid rise in the scintillation index, while a larger beam divergence angle can help minimize the impact of scintillation. Adapting the beam's divergence angle based on the pointing error between the optical transceivers can reduce the effects of scintillation and improve the average spectral efficiency and channel capacity. Additionally, the relationship between pointing error, scintillation, and the determination of the optical beam divergence angle in terms of beam divergence and average spectral efficiency and channel capacity is examined, and theoretical evaluations further confirm the method's effectiveness in reducing scintillation in the presence of pointing errors. Furthermore, the simultaneous use of OFDM adaptive beam divergence control and modulation could significantly enhance the data rate. This approach aims to reduce the impact of scintillation in UAV FSO links, which often experience significant losses due to unpredictable fluctuations in the atmosphere's refractive index. The results of the simulations indicate that the integrated 4-QAM-OFDM-FSO framework can realize high data transmission rates, efficiently serving front-haul and backhaul needs, thereby signifying a significant evolutionary leap for the next generation of wireless technology. The numerical findings demonstrate the significant impact of the coherent FSO OFDM optical wireless communication (OWC) setup in UAV wireless communications to ground links, particularly in mitigating the effects of strong turbulence and pointing errors (PEs). Through the integration of spatial coherence diversity and adaptive modulation OFDM in the coherent OWC, there has been a noticeable enhancement in the average spectral efficiency (ASE). Notably, our results indicate an ASE of 53 bits/s/Hz and 37 bits/s/Hz achieved at an average transmitted optical power of 10 dBm for an aperture diameter of 10 cm, without and with PEs for the coherent OWC-FSO OFDM UAV technique, respectively. The proposed method was validated through simulations, demonstrating both improved average spectral efficiency and effective reduction of the scintillation effect. This approach holds promise for mitigating scintillation effects in UAV-FSO links.
