Investigations on OFDM UAV-based free-space optical transmission system with scintillation mitigation for optical wireless communication-to-ground links in atmospheric turbulence

Integrating a high volume of compact cellular units is widely recognized as an essential strategy for advancing future wireless communication infrastructure. This approach promises to deliver abundant bandwidth and rapid data transfer speeds (Singh et al. 2021). Although integrating compact cellular solutions within the 4G LTE framework has undergone extensive study recently, real-world adoption has been impeded by the significant expenditures and complexities associated with establishing compact cellular stations across the front-haul and backhaul segments. As the advent of fifth generation (5G) wireless systems draws nearer, the emphasis on compact cell technology is revitalized; however, ensuring efficient connectivity between front-haul and back-haul segments remains a critical challenge. This paper introduces a novel framework for compact cell integration that involves linking compact cell stations to the main network through a layered front-haul and backhaul network structure. Fundamental techniques incorporated in this new framework include line-of-sight optical data transmission using free-space optics (FSO), the application of autonomous aerial vehicles (UAVs), and the deployment of orthogonal frequency division multiplexing (OFDM). The FSO method takes advantage of the natural environment for signal transmission, utilizing optical beams to convey information (Malik and Singh 2015). FSO connections offer several benefits, such as the lack of licensing requirements, a vast spectrum availability, a minimalistic component design, energy-efficient operations, and immunity to electromagnetic or RF signal interference (Sharma and Kaler 2012). Furthermore, the rapid and flexible installation of FSO systems positions them as a promising solution for the 'last mile' connectivity challenge, with applications across terrestrial, maritime, aerospace, space, and defense communication domains (Kakati and Arya 2018; Jendeya et al. 2017). The integration of orthogonal frequency division multiplexing with FSO systems presents additional strengths, including enhanced spectral efficiency, increased range for high-speed data transmission, and improved resilience against interference (Jeyaseelan et al. 2020; Singh and Malhotra 2019; 2020; Failed 2019). For the impending deployment of 5G networks, establishing front-haul and backhaul connections can be accomplished through a blend of wired and wireless media. While wired connections typically employ copper wires and fiber optics, the latter is preferred for its superior characteristics. However, extending optical fiber to small-cell base stations may not always be feasible due to significant installation expenses and right-of-way challenges (Dahrouj et al. 2015). An economically feasible alternative is found in wireless connections for front-haul and backhaul, which transmit communications over electromagnetic waves, utilizing both the microwave spectrum and Free-Space Optics (FSO). Microwave links typically operate within the 6–60 GHz frequency band for data transmission (Siddique et al. 2015). Nonetheless, these bands are becoming congested and are associated with substantial licensing costs in many regions. In contrast, the use of FSO for point-to-point line-of-sight (LOS) connections is gaining popularity due to its rapid deployability and absence of licensing requirements. Existing wireless networks tasked with front-haul and backhaul roles often rely on non-LOS multipoint systems operating below 6 GHz, a spectrum now facing saturation, vulnerability to interference, and high operational expenses. As a result, there is a pressing need to reevaluate front-haul and backhaul approaches suited for advanced 5G networks. The deployment of Unmanned Aerial Vehicles (UAVs) is being considered for transmitting cellular and internet data in areas where traditional infrastructure is unviable, due to prohibitive costs or challenging terrains. Notable initiatives, such as Facebook's Internet.org, employ stratospheric platforms to provide internet connectivity, along with projects like SkyStation, Satellite, Sky-Tower, Skynet, HeliNet, and CAPANINA (Austin 2010). In the literature, UAVs are also referred to as unmanned aerial systems (UAS), high-altitude platforms (HAP), or networked flying platforms (NFP) (Austin 2010). These UAVs, including balloons, either crewed or uncrewed aircraft, or airships, are positioned within the stratosphere's lower layers, hovering at altitudes between 17 and 25 km above the earth's surface (Vu et al. 2018). Depending on factors such as weather, network load, and service coverage, the movements of UAVs can be either manually controlled by a ground operator or completely autonomous (Alzenad et al. Jan. 2018). The integration of UAVs with cutting-edge communication technologies has emerged as a pivotal area of innovation, particularly in the context of wireless network infrastructures. Leveraging the capabilities of UAVs in tandem with FSO communication systems represents a transformative avenue for enhancing wireless connectivity, especially in the evolving landscape of 5G networks. In this study, we conduct an extensive exploration of the application of OFDM in conjunction with UAV-based FSO technology, with a specific focus on establishing robust wireless communication links to ground sites (Alzenad et al. Jan. 2018; Alkholidi and Altowij 2014; Andrews et al. 1999). The research introduces a pioneering 4-QAM-OFDM-FSO framework tailored for UAV-to-ground communication, aiming to mitigate existing limitations and address critical challenges in traditional wireless communication models. The current state of wireless communication technologies, particularly in the context of 5G networks, presents notable gaps in addressing the demand for high-throughput, agile, and resilient communication links, especially in dynamic and challenging environmental conditions. The utilization of UAVs as a platform for Free Space Optical communication introduces unprecedented potential but also warrants thorough exploration to understand and optimize the performance of such systems. At the intersection of OFDM, UAVs, and FSO technology, there exists a clear research gap that necessitates in-depth investigations to effectively harness the potential of this novel integration and contribute to the enhancement of next-generation wireless network infrastructures (Andrews et al. 1999; Petkovic and Dordevic 2014; Ma et al. 2015; Epple 2010; Kumar 2013; Elsayed et al. 2022a; Hayal et al. 2023). The literature in the domain of UAV-based FSO communication and the application of OFDM within this context highlights a fundamental shift toward enhancing wireless communication capacities. Past studies have elucidated the potential of UAVs as aerial nodes for establishing communication links, particularly in scenarios where traditional ground-based infrastructure encounters limitations. Furthermore, the efficacy of OFDM in mitigating intersymbol interference and enhancing spectral efficiency has been widely documented within terrestrial wireless communication systems, prompting exploration of its potential application within the unique constraints and dynamics of UAV-assisted FSO setups. Moreover, within the realm of 5G technologies, the imperative for resilient and agile wireless connectivity further amplifies the relevance of the investigation. The endeavour to establish robust and high-throughput wireless communication links within challenging environments characterizes a pressing need, aligning closely with the aspirations and requirements of modern network infrastructures (Hayal et al. 2023; Elsayed et al. 2022b, 2018; Yousif and Elsayed 2019; Yousif et al. 2019). We elucidate on the innovative simulation framework established, which evaluates performance across several metrics, including bit error rate (BER) and Q-factor, and critically examines real-world deployment considerations such as beam divergence and atmospheric challenges. This rigorous approach solidifies the practical implications of the theoretical model for the future deployment of 5G and beyond. Furthermore, we delineate the advanced simulation framework developed for this research, which includes not only performance metrics such as BER and Q-factor analysis but also addresses practical deployment factors like beam divergence angle, scintillation, and atmospheric conditions (Elsayed et al. 2018; Elsayed and Yousif 2020a, b; El-Mottaleb et al. 2021; Abd et al. 2020). This comprehensive approach provides a better understanding of how UAV-based FSO systems could be implemented in a real-world, 5G context. Additionally, we highlight the methodological advancement of utilizing a 4-level quadrature amplitude modulation (4-QAM) scheme integrated into the OFDM-FSO system (Abd et al. 2020; Elsayed et al. 2022c; Singh et al. 2022; Elfikky and Rezki 2024; Elsayed and Yousif 2020c). This illustrates a significant improvement in data rate capabilities, which is essential for front-haul and backhaul applications in 5G networks. Scintillation introduces signal errors, undermining the reliability of communication networks. This paper investigates the relationship between pointing errors (PEs) and scintillation by examining beam divergence angles. The findings show that even slight increases in pointing errors significantly raise the scintillation index, although this can be mitigated by increasing the beam divergence angle. An adaptive beam mechanism then adjusts its divergence dynamically to counteract the pointing errors experienced between optical transceivers, thereby reducing the negative effects of scintillation. The study investigates the relationships among pointing error, scintillation, and beam divergence angle and their collective impact on average spectral efficiency. We find that even marginal increases in pointing error lead to a substantial rise in the scintillation index. Nonetheless, compensating with an increased beam divergence angle alleviates scintillation effects. By dynamically adjusting the beam's divergence angle in response to the pointing error of the optical transceivers, we diminish the effects of scintillation and, consequently, enhance both the average spectral efficiency and the channel capacity. Moreover, we explore the interplay between pointing errors, scintillation, and the optimal determination of the beam divergence angle. We focus on how the angle's adjustment influences the average spectral efficiency and the channel capacity. Theoretical assessments corroborate the proposed method's capability to minimize scintillation when confronted with pointing errors. Further, by integrating adaptive beam divergence control with Orthogonal Frequency Division Multiplexing (OFDM) and adaptive modulation techniques, we achieve a marked improvement in data transmission rates. This approach aims to mitigate the scintillation effect in Unmanned Aerial Vehicle (UAV) Free-Space Optical (FSO) links, which are particularly prone to performance degradation due to atmospheric refractive index fluctuations. We have also introduced an extended discussion on the operational feasibility of UAV-FSO systems, considering the rapid deployment capabilities and spectrum availability, addressing a critical gap where existing communication infrastructures are either overwhelmed or physically untenable. Moreover, we delve into the practical applicability and adaptability of the UAV-FSO technology, addressing expedited deployment and spectrum accessibility challenges—core issues where existing infrastructure can falter under demanding circumstances. Finally, we elaborate on the scalability of proposed model, a key criterion for next-generation networks where existing state-of-the-art falls short. The framework allows for dynamic scaling, enabling efficient data transmission over various distances, which is pivotal for meeting the heterogeneous demands of evolving network topologies. The remainder of this article is organized as follows: Sect. 2 delves into the proposed architecture of the UAV-FSO OFDM communication system. Section 3 introduces the implementation and channel modeling, while Sect. 4 presents the results and discussion. Finally, the conclusion of this article can be found in Sect. 5.

The central contribution of study lies in the unveiling of a novel and comprehensive framework that integrates Orthogonal Frequency Division Multiplexing (OFDM) with Unmanned Aerial Vehicle (UAV)-supported Free Space Optical (FSO) communication systems, culminating in an unprecedented approach to enhancing wireless communication-to-ground links in the context of future network infrastructures. This pioneering integration addresses notable gaps present in current state-of-the-art models, particularly in the domain of 5G wireless communication technologies. By highlighting and mitigating limitations associated with conventional RF and FSO models, the proposed framework demonstrates enhanced performance, robustness against interference, and heightened adaptability to evolving atmospheric conditions and varying link distances. Moreover, study symbolizes a noteworthy step in advancing the theoretical understanding and practical implementation of UAV-based FSO systems, consequently contributing a critical, practical dimension to the evolving landscape of wireless communication technologies. By incorporating these explicitly addressed considerations, we aim to provide a lucid, definitive statement encapsulating paper's distinct contribution and how it successfully addresses and mitigates existing gaps in the current state-of-the-art models. Through this clarification, we underscore the significance and real-world applicability of research within the broader academic and practical realms of wireless communication systems. The graphical representation of vision for front-haul and backhaul links within the forthcoming 5G network infrastructure, as depicted in Fig. 1, showcases a UAV-ground station system based on FSO for data transmission. This proposed system serves as a versatile complement to terrestrial networks, especially in challenging situations like natural disasters that impair network integrity or during events requiring expanded coverage, such as large-scale sporting events. Moreover, it affords steadfast data transfer in areas where conventional fiber optics and microwave systems are impractical, including remote, topographically difficult, or densely urban areas. This envisioned architecture includes a network of UAVs connected at various altitudes, from several hundred meters to up to 25 km, determined by weather conditions and the required coverage area. With the ability to facilitate FSO data transmission amongst themselves, these UAVs establish point-to-point LOS communications with ground-based small-cell stations, securing precise and direct routes for consistent connectivity. We emphasize the significance of controlling and adapting each laser pulse to different focal points in the manuscript. The observation regarding the essence of OFDM is essential as it represents a multilevel amplitude-based scheme, where the modulation order relies on the number of bits for transmission. To address this critical consideration, we intend to explore adaptive modulation techniques within the framework of OFDM-based system. By implementing adaptive modulation, the goal is to dynamically adjust the modulation order to align with specific transmission requirements, allowing for efficient and flexible allocation of resources for transmitting laser pulses to specific focal points. This approach will be instrumental in optimizing the system's performance and adaptability, particularly in dynamically changing communication scenarios.

The proposed UAV-to-ground terminal Free-Space Optics (FSO) transmission link utilizing Orthogonal Frequency Division Multiplexing (OFDM) signals. This method is intended to enable high-speed data transmission in scenarios where Line of Sight (LOS) communication is not feasible. The process outlined in Fig. 2 involves the generation of a 20 Gbps binary data signal using a Pseudo-Random Binary Sequence (PRBS), followed by its mapping onto 4-Quadrature Amplitude Modulation (QAM) symbols at a 2-bits-per-symbol rate. The 4-QAM signal then undergoes processing via an OFDM modulator, which includes several signal processing functions such as serial-to-parallel conversion, Inverse Fast Fourier Transform (IFFT) algorithm utilization, addition of a cyclic prefix (CP), parallel-to-serial conversion, digital-to-analog conversion, and signal filtering. The specific OFDM configuration comprises 1024 IFFT points, 512 subcarriers, an average power of 15 dBm, and a 32-cyclic prefix. Subsequently, the resultant electrical OFDM signal undergoes in-phase quadrature modulation at a frequency of 7.5 GHz before being optically modulated over a laser beam from a continuous wave laser through a Mach–Zehnder modulator (MZM). The optical beam, carrying the information, is then transmitted to the ground station through the free-space channel. At the ground station, data retrieval is achieved via direct detection, where the optical beam is transformed into an electrical signal using a PIN-photodetector, and subsequently demodulated through OFDM and quadrature demodulation stages. Finally, the information is decoded utilizing a 4-QAM sequence decoder. The selected laser power values and distances align with practical deployment scenarios for OWC applications involving UAVs. This includes discussions on factors such as atmospheric conditions, hardware constraints, power consumption, and regulatory considerations, ensuring that our choices are congruent with the operational parameters feasible within the UAV OWC domain. The optical laser power level has been carefully selected to ensure an adequate signal-to-noise ratio (SNR), guaranteeing that the transmitted signal maintains high fidelity and integrity over the free-space optical channel. This is especially important in the presence of atmospheric attenuation and other environmental factors. To accommodate the transmission distance and varying atmospheric conditions caused by weather dynamics and turbulent atmospheric conditions, we have chosen power values that align with the design specifications of the transmitter and receiver components. This takes into account their sensitivity, dynamic range, and optical link budget to ensure robust communication over extended ranges. Furthermore, the power values have been selected to align with permissible limits and safety regulations, ensuring compliance with legal requirements and operational safety. Adherence to regulatory guidelines and safety standards is imperative. It is essential that the chosen power values comply with safety regulations and standards. Optimal power values have been chosen to minimize energy usage while maintaining effective communication, especially in the context of UAV platforms where power efficiency is critical. By thoroughly considering these criteria, the selection of optical laser power values can be justified in a way that demonstrates their alignment with the system's operational parameters and performance objectives.

The reliability and performance of the FSO channel linking the UAV and the ground terminal are primarily influenced by attenuation caused by external weather dynamics and turbulent atmospheric (AT) conditions (Elsayed et al. 2022a, b, 2018; Hayal et al. 2023; Yousif and Elsayed 2019; Yousif et al. 2019). The path loss \({h}_{l}^{p}\) can be estimated using exponential Beer-Lambert Law (Alkholidi and Altowij 2014):where \(\sigma\) signifies the specific coefficient of attenuation for external climate conditions and \({L}_{p}\) is the transmission distance from the UAV and ground station, which is given as (Austin 2010; Vu et al. 2018), and (Hayal et al. 2023):where \({H}_{p}\) signifies the height of the UAV from sea level while \({\zeta }_{p}\) denotes the zenith angle which measures the angle between the UAV and the ground station (Austin 2010; Vu et al. 2018), and (Hayal et al. 2023). AT \({h}_{a}^{p}\) is a random occurrence caused by fluctuations in the atmospheric refractive index structure (RINS) parameter. The Gamma-Gamma turbulence model has been utilized in the referenced work for this analysis (Andrews et al. 1999; Elsayed et al. 2022a, b, 2018; Hayal et al. 2023; Yousif and Elsayed 2019; Yousif et al. 2019; Elsayed and Yousif 2020a, b; El-Mottaleb et al. 2021; Abd et al. 2020):where \({K}_{\alpha -\beta }\)(.) signifies the modified-Bessel function having \((\alpha -\beta\)) order, \(\Gamma\) signifies the Gamma function, \(\alpha\) and \(\beta\) signifies the number of large and small-scale eddies given as (Elsayed et al. 2018, 2022a, b; Hayal et al. 2023; Yousif and Elsayed 2019; Yousif et al. 2019; Elsayed and Yousif 2020a, b; El-Mottaleb et al. 2021; Abd et al. 2020), respectively (Petkovic and Dordevic 2014; Elsayed et al. 2018, 2022b, c; Yousif and Elsayed 2019; Yousif et al. 2019; Elsayed and Yousif 2020a, b, c; El-Mottaleb et al. 2021; Abd et al. 2020; Singh et al. 2022; Elfikky and Rezki 2024):

where \({\sigma }_{R}^{2}\) signifies the Roytov variance and is expressed as (Ma et al. 2015; Hayal et al. 2023; Elsayed et al. 2022b, 2018; Yousif and Elsayed 2019; Yousif et al. 2019; Elsayed and Yousif 2020a, b; El-Mottaleb et al. 2021; Abd et al. 2020):where k signifies the wavenumber,\({h}_{0}\) represents the height of the ground terminal, \({C}_{n}^{2}\) signifies the RINS parameter, which can be expressed using the Hufnagel-Valley model as (Epple 2010):where \(\omega\) signifies RMS value of air velocity, \(h\) signifies the height from the sea level, \({C}_{n}^{2}\left(0\right)\) is the ground value of \({C}_{n}^{2}\) which is \(1.7 \times {10}^{-14} {m}^{-2/3}\).

The study conducted a comprehensive evaluation of the proposed system, with a focus on its capacity to enhance range, power, receiver diameter, and attenuation. The system and link parameters used in the simulations were sourced from specified references (Alzenad et al. Jan. 2018; Kumar 2013). As illustrated in Fig. 3, the transmission performance of the proposed system demonstrates an expanding transmission range and presents the bit error rate (BER) at different laser input power levels. At a laser power of 15 dBm, the log (BER) values for distances of 18 km, 21.5 km, and 25 km are − 2.22, − 1.16, and − 0.65, respectively. Conversely, for a laser power of 20 dBm, the log (BER) values at the same distances are − 4.21, − 2.32, and − 1.24. The findings suggest that as the distance increases, the signal's BER rises due to increased attenuation and turbulence losses (Kumar 2013; Elsayed et al. 2022a, b, 2018; Hayal et al. 2023; Yousif and Elsayed 2019; Yousif et al. 2019; Elsayed and Yousif 2020a, b). Furthermore, an increase in laser power leads to a higher BER. Subsequently, Fig. 4 illustrates the constellation plots (CPs) for various laser power (LP) levels at a distance of 20 km. We emphasize the significance of controlling and adapting each laser pulse to different focal points in the manuscript. To address the impact of channel turbulence, including attenuation, pointing error, and angle of arrival fluctuations on system performance, a multi-faceted strategy is adopted. Firstly, adaptive transmission techniques are implemented to dynamically adjust transmission parameters to accommodate changing channel conditions. Additionally, advanced error correction coding is incorporated to mitigate the effects of attenuation and pointing errors, thus enhancing the system's resilience to channel turbulence. Furthermore, beamforming strategies are explored to optimize signal reception and minimize the impact of angle of arrival fluctuations. By integrating these approaches, the robustness and overall performance of the system is improved under challenging channel conditions.

Figure 5 demonstrates how the proposed system's transmission performance changes as attenuation increases across different laser input power levels. The log values for bit error rate (BER) are examined in relation to rising attenuation coefficients and input power levels (Elsayed and Yousif 2020b, c, d; El-Mottaleb et al. 2021; Abd et al. 2020; Elsayed et al. 2022c; Singh et al. 2022; Elfikky and Rezki 2024). This analysis offers valuable insights into the system's performance in varying conditions. Specifically, when the laser power (LP) is at 15 dBm, the log (BER) values are − 2.60, − 1.01, and − 0.47. Meanwhile, for a laser power of 20 dBm, the log (BER) values are − 4.21, − 2.03, and − 0.79, corresponding to atmospheric attenuation coefficients of 0.8, 1.15, and 1.5 dB/km, respectively. It's important to note that as the attenuation coefficient increases, so does the BER (Elsayed and Yousif 2020b, c, d; El-Mottaleb et al. 2021; Abd et al. 2020; Elsayed et al. 2022c; Singh et al. 2022; Elfikky and Rezki 2024). Figure 6 showcases Cps at a 1 dB/km attenuation coefficient for varying LP levels. The significance of Fig. 6 a, b lies in the insights they provide regarding the performance of the communication system under different scenarios. From the simulation results, it derives benefits such as determining the robustness of the system under various power levels, identifying potential error rates or signal degradation, and optimizing the system for efficient and reliable communication (Elsayed and Yousif 2020d; Prabu et al. 2018; Aarthi and Ramachandra Reddy 2017, 2018; Navas et al. 2012; Aarthi et al. 2017). Ultimately, Fig. 6 conveys valuable insights into the system's performance and aids in making informed decisions about the design and operation of the communication system.

In Fig. 7, the performance of the proposed system in terms of transmission is displayed as the beam divergence angle (BDA) increases for different sizes of receiver antenna apertures. For a 10 cm aperture diameter, the log (BER) outcomes are − 2.01, − 0.59, and − 0.41. Contrastingly, with a 20 cm aperture diameter, the log (BER) values are − 4.21, − 1.24, and − 0.70 at divergence angles of 0.4, 1.2, and 2 mrad, respectively. It is apparent that an increase in the BDA corresponds to an increase in the BER. The beam divergence angle has a significant effect on channel capacity in free space optical communication systems. As the beam divergence angle increases, the transmitted optical signal spreads over a larger area at the receiver end. This dispersion reduces the received power, leading to potential signal degradation and ultimately impacting the overall channel capacity (Aarthi et al. 2017; Hu et al. 2023; Adardour et al. 2023; Wu et al. 2023; Lastname, et al. 2024; Gupta et al. 2023; Wang et al. 2023). Therefore, understanding and quantifying the impact of the beam divergence angle is crucial in optimizing the design and performance of free space optical communication systems, especially in scenarios requiring high channel capacity and reliable data transmission (Aarthi et al. 2017; Hu et al. 2023; Adardour et al. 2023; Wu et al. 2023; Lastname, et al. 2024; Gupta et al. 2023; Wang et al. 2023). Figure 8 presents constellation plots at a 0.8 mrad divergence angle for the receiver aperture diameter (RAD). Based on the simulation results, Fig. 8 illustrates the spread and distortion of signals, allowing for the assessment of potential signal quality degradation. Additionally, it can aid in optimizing the system design by helping to identify the most effective combinations of divergence angles and aperture diameters for reliable communication. Considering the evolving nature of UAV technology, we recognize the potential impact of dynamic UAV mobility on the deployment and performance of our proposed OFDM-UAV free space optical transmission system for Wireless Communication-to-Ground Links. While the current study does not explicitly address dynamic UAV mobility, we emphasize the importance of considering this factor in deploying UAV communication systems effectively. To enrich the scope of our study, we intend to highlight the significance of UAV maneuverability in future work, particularly in the context of practical deployment and operational feasibility. This consideration will illuminate the pivotal role of dynamic UAV mobility in shaping the performance and adaptability of the OFDM-UAV-FSO framework within real-world scenarios.

Figure 9 illustrates the average spectral efficiency (ASE) concerning the channel capacity relative to the average transmitted optical power under strong turbulence (ST), both with and without pointing errors (PEs) of the beam divergence angle for the coherent FSO OFDM optical wireless communication (OWC) setup in the UAV wireless communications to ground links (Aarthi et al. 2017; Hu et al. 2023; Adardour et al. 2023; Wu et al. 2023; Lastname, et al. 2024; Gupta et al. 2023; Wang et al. 2023). Our numerical results indicate that incorporating spatial coherence diversity and adaptive modulation OFDM in the coherent OWC can improve the ASE (Prabu et al. 2018; Aarthi and Ramachandra Reddy 2017). Based on the data in Fig. 9, we observe ASE values 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 (Aarthi and Ramachandra Reddy 2017, 2018; Navas et al. 2012).

Comparing our proposed method in Fig. 9 to the findings in a previous works (Elsayed and Yousif 2020a) (Aarthi et al. 2017), our approach yields ASE performances of 49 bits/s/Hz and 34 bits/s/Hz at an average transmitted optical power of 5 dBm for the same aperture diameter (Aarthi et al. 2017). This represents an improvement over the results in the earlier work (Aarthi et al. 2017; Hu et al. 2023; Adardour et al. 2023; Wu et al. 2023; Lastname, et al. 2024; Gupta et al. 2023; Wang et al. 2023). Specifically, when comparing the coherent OWC-FSO UAV OFDM technique with and without PEs, we achieve an ASE of 53 bits/s/Hz and 37 bits/s/Hz at an average transmitted optical power of 10 dBm for the same aperture diameter, showing progress of 4 bits/s/Hz and 3 bits/s/Hz, respectively, over the prior findings.

In Fig. 9, the average spectral efficiency is depicted in relation to the pointing error and each beam divergence angle as they pertain to channel capacity relative to average transmitted optical power under ST. This encompasses scenarios both with and without PEs of the beam divergence angle for the coherent FSO OFDM OWC setup in UAV wireless communications to ground links (Park et al. 2023). Our 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 (Park et al. 2023). 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. Wider beam divergence angles lead to a decrease in transmit gain, and the determination of the beam divergence angle depends on its interaction with pointing error. Figure 9 showcases the use of orthogonal frequency-division multiplexing to control the adaptive beam divergence angle, effectively mitigating scintillation effects due to pointing errors and improving the average spectral efficiency and channel capacity. This method's performance is evaluated using OFDM for free-space optical communication in unmanned aerial vehicle scenarios. 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 (Park et al. 2023). The performance achieved in atmospheric turbulence, when compared with (Hayal et al. 2023), is notably high. Introducing a novel framework that integrates orthogonal frequency division multiplexing with unmanned aerial vehicle-supported free space optical communication systems allows for a comparison with (Hayal et al. 2023). Addressing notable gaps in current state-of-the-art models, particularly within the domain of 5G wireless communication technologies, mitigates limitations associated with conventional RF and FSO models through the proposed integrated framework. Insights into the robustness and performance of the communication system under various power levels, antenna apertures, and atmospheric conditions are provided. Additionally, a foundation for future studies and practical deployment considerations is suggested by recognizing the potential impact of dynamic UAV mobility on the proposed communication system.

Based on the analysis of bit error rate (BER) for different ambient noise levels, the practical implications and recommendations for selecting an appropriate modulation format for UAV free space optical transmission systems are significant. It provides insights into identifying modulation formats that offer improved robustness and reliability in the presence of varying ambient noise levels. The analysis can guide the selection of modulation schemes that are resilient to noise, ensuring higher data transmission accuracy and system performance. Consequently, this data can influence the design and implementation of UAV Free Space Optical Transmission systems, aiding in the selection of modulation formats best suited for the wireless communication-to-ground links while considering real-world ambient noise scenarios. This study is important for future work as it contributes to a deeper understanding of coding mechanisms within dynamic underwater visible light communication systems in free space optics with UAV-OFDM adaptive modulation and coding techniques, including all dynamic mobility effects (Elfikky et al. 2024). This aligns with OWC UAV-based FSO OFDM for wireless communications to ground links.

Figure 10 portrays the relationship between beam divergence angle and pointing error in comparison to scenarios with and without scintillation, concerning coherent FSO OFDM optical wireless communication in UAV wireless links to ground stations. Figure 10 presents a comparison between scenarios with and without scintillation effects. It demonstrates the correlation between beam divergence angle and pointing errors, with plotted markers indicating the optimal beam width for various pointing errors. Specifically, there is a noted increase in required divergence, going from 6 \(\mu\) radians in pointing error to 10 \(\mu\) radians. In scenarios devoid of scintillation, the optimal divergence angle varies from 27 to 35 \(\mu\) radians, whereas it stretches from 29 to 37 \(\mu\) radians when scintillation is a factor (Park et al. 2023). Moreover, an exponential increase in scintillation effects is directly associated with the presence of pointing errors. However, when no pointing errors are present, the scintillation effect remains minimal, as the Gaussian beam transitions into spherical waves along the axis. Additionally, the impact of scintillation is negligible during downlinks regardless of the beam's divergence angle, due to the beam's transformation to a spherical wave form after encountering atmospheric turbulence (Mai and Kim 2021, 2019).

The paper presents a pioneering framework for 4-QAM-OFDM-FSO optical wireless transmission in vertical front-haul and backhaul networks, enabling high-speed 5G wireless transmission from UAVs to ground stations. The results showcase successful transmission of 20 Gbps data over distances ranging from 18 to 25 km. This proposed setup holds promise for facilitating high-speed front-haul and backhaul network traffic transmission in the future. In addition to these findings, future work may involve further optimizing the transmission technology to support longer distances and higher data rates. This could entail exploring methods to mitigate atmospheric effects and enhance overall link performance. Furthermore, the development of robust and efficient protocols for seamless integration of this technology into existing or future 5G networks could be a key focus for upcoming research. These advancements would contribute to the continual evolution of high-speed traffic transmission, ensuring its sustained effectiveness in progressively demanding wireless communication environments. In this research, we have conducted a comprehensive exploration of the integration of orthogonal frequency division multiplexing with unmanned aerial vehicle-based free space optical communication systems, particularly in the context of wireless network infrastructures. We introduced a pioneering 4-QAM-OFDM-FSO framework tailored for UAV-to-ground communication, effectively addressing critical challenges and laying the groundwork for a resilient and high-capacity wireless communication paradigm. Through extensive simulations and analytical evaluations, the study has revealed the system's outstanding performance in countering atmospheric turbulence, signal integrity optimization, and adaptability over varying link distances. This research stands as a seminal contribution, paving the way for the practical implementation of OFDM-UAV-FSO optical wireless communication systems, thus catalyzing a fundamental shift toward agile and robust wireless connectivity, especially within the realms of 5G networks. Moving forward, our research opens avenues for subsequent investigations to delve into practical deployment strategies, scalability studies, and real-world field tests aimed at validating the efficacy and practical adaptability of the proposed OFDM-UAV-FSO optical wireless framework. Subsequent research endeavors should focus on refining system parameters, such as dynamic mobility adaptations for UAV platforms, power management strategies, and optimization of the link establishment process. Additionally, efforts to integrate advanced signal processing techniques and dynamic network reconfiguration capabilities would further enhance the robustness and resilience of the system in varied environmental conditions. 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. Moreover, future work should encompass extensive field trials and collaboration with industry partners to effectively bridge the gap between theoretical underpinnings and practical implementations, thereby ushering in a new era of wireless communication with far-reaching implications for modern network infrastructures. In future work, we will emphasize the importance of potential challenges and uncertainties associated with dynamic UAV mobility in practical deployment scenarios, which was not directly addressed in the current study. By conducting further research in this area, we aim to gain a comprehensive understanding of how dynamic UAV movements, influenced by factors such as weather, network load, and service coverage, impact the effectiveness and reliability of the proposed communication system in real-world settings. Moreover, we aim to evaluate the implications of both manual and autonomous UAV control on the performance of the system, ultimately striving to bridge the gap between theoretical framework and practical deployment considerations. Through this exploration, the aim is to offer valuable insights that can contribute to the robustness and adaptability of the framework in dynamic operational environments. In the future work, utilizing machine learning, especially deep learning algorithms, to accurately model the communication channel and ambient conditions for mitigating signal interruptions in free space optical (FSO) communication is crucial. Deep learning algorithms have the potential to effectively model dynamic environmental factors and channel behavior, enabling proactive adaptation to varying conditions and minimizing signal disruptions. This insight will serve as a guiding principle for our ongoing exploration, which aims to enhance the reliability and robustness of FSO communication systems.