Free Space Optical Technology Research Articles

Power-over-fiber-based optical wireless communication systems towards 6G

This paper reports two implementations of power-over-fiber (PoF) solutions applied to radio-over-fiber (RoF) and optical wireless communication (OWC) systems, in the context of an industrial environment. We employ a conventional 62.5-µm multimode fiber (MMF) to deliver optical power to different communication links based on RoF, free-space optics (FSO), and visible light communication (VLC) technologies aiming beyond 5G (B5G) and 6G applications. First, a 3.5-GHz 5G New Radio (5G NR) signal is transmitted throughout a 20-km single-mode optical fiber (SMF) link using RoF technology. Regarding the PoF system, a 5-W optical power is transmitted through a 100-m MMF link. A photovoltaic power converter (PPC) and a DC/DC converter are employed to convert the power from the optical to the electrical domain and adjust the voltage level, respectively, with the purpose of energizing a remote RoF module. The attainable optical and electrical power transmission efficiencies (OPTE and PTE) are 80% and 19%, respectively. Posterior, a second PoF system is implemented to power a hybrid RoF/FSO/VLC B5G system, comprising a 200-m MMF and an additional DC/DC converter. Over 10.5 W of optical power is transmitted to feed an electrical amplifier (EA) and a white LED from the VLC link. In this configuration, we achieve 78% and 18.5% of OPTE and PTE, respectively. Furthermore, a performance investigation based on the root mean square error vector magnitude (EVMRMS) metric is conducted to evaluate the signal using the implemented PoF systems and a conventional electrical power supply. In the first implementation, a throughput of 600 Mbps is achieved with 100-MHz bandwidth without performance degradation, when compared to the conventional-powered RoF system, whereas, in the second implementation, 60-Mbps throughput is achieved when employing the FSO and VLC technologies simultaneously, demonstrating the applicability and potential of the PoF technique for B5G and 6G industrial communications.

Demonstration of a Low-SWaP Terminal for Ground-to-Air Single-Mode Fiber Coupled Laser Links

Free space optical technology promises to revolutionize point-to-point communications systems. By taking advantage of their vastly higher frequencies, coherent optical systems outperform their radio counterparts by orders of magnitude in achievable data throughput, while simultaneously lowering the required size, weight, and power (SWaP), making them ideal for mobile applications. However, the widespread implementation of this technology has been largely hindered by the effects of atmospheric turbulence, often necessitating complex higher-order adaptive optics systems that are largely unsuitable for deployment on mobile platforms. By employing tip/tilt beam-stabilization, we present the results of a bespoke low-SWaP optical terminal that demonstrated single-mode fiber (SMF) coupling. This was achieved by autonomously acquiring and tracking the targets using a combination of aircraft transponder and machine vision feedback to a root-mean-square (RMS) tracking error of 29.4 µrad and at angular rates of up to 0.83 deg/s. To the authors’ knowledge, these works constitute the first published SMF coupled optical link to a full-sized helicopter, and we describe derived quantities relevant to the future refinement of such links. The ability to achieve SMF coupling without the constraints of complex adaptive optics systems positions this technology as a versatile quantum-capable communications solution for land-, air-, and sea-based platforms ranging across commercial, scientific, and military operators.

Tuning the parameters of a free-space optical channel using machine learning

The present work uses artificial intelligence (AI) methodology to simulate the data transmission process through free-space optical (FSO) technology. With machine learning procedures, the data are obtained by multiparametric simulation using Optisystem software. For the first simulation set, the input parameters were distance, attenuation, gain in the input signal amplifier, and gain in the output signal amplifier. For the second set, the effects of beam divergence and the receiver diameter were also evaluated. Additional sets were added to increase the data and characterize the underfitting and overfitting processes. With the data generated, artificial intelligence models were trained using decision tree regression (DTR), random forest regression (RFR), gradient boosting regressor (GBR), histogram gradient boosting regression (HGBR), and AdaBoost + deciston tree regression (ADDTR). The results showed that for the first scenario the models (DTR) and (RFR) showed an excellent estimate for the maximum quality factor (MaxQFactor), with a value of the coefficient of determination R2 above 95.00%, and, for the second scenario, the algorithms (DTR) and (RFR) also have shown excellent results, with R2 above 94.00%. The results obtained from the artificial intelligence procedures were compared graphically with the values obtained by multiparametric numerical simulation, confirming the effectiveness of the methodology used to predict the output values of the FSO channel.

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

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.

Performance analysis of a hybrid optical amplifier based 480-Gbps DWDM-FSO system under the effect of different atmospheric conditions

Free space optical (FSO) systems offer a desirable and affordable way of providing communication services in remote locations. They provide secure wireless transmission without the requirement for licensing and with lower implementation costs. However, due to the impact of atmospheric turbulences on the effectiveness of FSO systems, their implementation faces significant challenges. Dense wavelength division multiplexing (DWDM) has shown promise in addressing the growing bandwidth needs in optical networks. This paper introduces a hybrid optical amplifier based system, combining DWDM and FSO technology. Energy conservation in such a system involves optimizing various aspects to reduce power consumption while maintaining or improving performance. The proposed DWDM-FSO system operates at a rate of 480 Gbps and consists of 12 channels, each capable of transmitting data at 40 Gbps. The system's performance is evaluated and compared by determining the Q-factor and bit error rate (BER) for both the cases when hybrid amplifier is employed and with no hybrid amplifier within the C-band, specifically focusing on wavelengths around 1550 nm. Moreover, the length of the FSO link is increased to assess the corresponding Q-factor and BER. Resultantly, the maximum distance for the FSO link is determined, ensuring that it remains within acceptable Q-factor and BER thresholds. Furthermore, the FSO system's effectiveness is assessed and compared across various atmospheric conditions. The findings reveal that, in clear weather conditions, the FSO system achieves a maximum distance of 510 meters while maintaining satisfactory Q-factor and BER values when not utilizing a hybrid amplifier. However, by integrating a hybrid amplifier, the system's reach significantly extends to 1700 m under clear weather conditions, still maintaining acceptable Q-factor and BER values.

Performance Analysis of Multi-Hop Hybrid FSO/mm Wave Communication System for Next-Generation Wireless Networks

Next-generation wireless networks are facing increasing demand for high data rates, low latency, and seamless connectivity. To address these challenges, a multi-hop hybrid communication system integrating Free Space Optics (FSO) and millimeter wave (mm Wave) technologies for backhaul communication is proposed. This system combines the advantages of FSO, such as high bandwidth and low latency, with the robustness and reliability of mm Wave technology. The multi-hop architecture enables the formation of a network of interconnected nodes, providing improved coverage and flexibility. Each node is equipped with FSO and mm Wave transceivers, allowing for seamless handovers and adaptive routing. For the proposed system, analytical formulas for outage probability (OP) and average bit error rate (ABER) are derived and validated using simulations. The Monte Carlo simulations provide evidence of enhanced performance in the proposed Multi-Hop Hybrid FSO/mm Wave (MHFM) system compared to the FSO system.

A comprehensive review of performance analysis of RF-FSO hybrid communication systems

The combination of radio frequency (RF) and free-space optical (FSO) communication has grown in importance as a study area for wireless communication systems in recent years. This technology has the ability to address the rising need for high-speed, high-capacity wireless communication in a variety of applications by successfully integrating the benefits of both RF and FSO communication. The performance analysis of various communication systems based on RF-FSO hybrid technology is covered in detail in this research. An overview of the RF-FSO hybrid technology and its many architectures follows the introduction of the key features and benefits of RF and FSO communication. The performance analysis of several RF-FSO hybrid communication systems, including relay-assisted RF-FSO systems, cooperative diversity RF-FSO systems, and dual-hop RF-FSO systems, are discussed in detail. The performance metrics, such as bit error rate (BER), outage probability, and channel capacity, is analyzed and compared for these systems. Based on the analysis of existing literature, several key conclusions are drawn. First, RF-FSO hybrid technology can provide higher data rates and greater reliability than traditional communication systems. Secondly, the performance of the RF-FSO hybrid system, which combines the advantages of RF technology and FSO technology, has bright development prospects. Finally, the RF-FSO hybrid system can also be combined with other systems for better performance and wider application scenarios.

A comprehensive survey of free-space optical communication – modulation schemes, advantages, challenges and mitigations

Abstract The demand for large bandwidth and high data rates in communication systems has become the main cause of the upgrade of traditional networks into free space optical (FSO) technology. FSO technology has gained significant popularity due to its easy deployment, high data rates, abundant bandwidth, enhanced security, and license-free spectrum utilization. However, the performance of FSO communication systems can be affected by certain limiting factors, such as changes in weather conditions during data transmission. To overcome these challenges and improve FSO performance, various modulation techniques are employed. This article presents a concise overview of the FSO communication system, highlighting different modulation techniques used to enhance its performance, as well as discussing its advantages, applications, and existing challenges. Some advanced modulation formats which are recently introduced in the field of FSO communication such as QPSK, DP-QPSK, QAM, and OFDM are also made part of this paper.

Study of Nonlinearities in FSO Communication System: A Detailed Review

In the world of today where growing demand for a fast, reliable and secured communication system for the growing population of the world is in dire need, the Free Space Optics (FSO) provides a very good alternative to the conventional RF communication. When compared in terms of free space communication it is better than RF communications in some aspects. Also in regard to optical mode of communication, it is superior to fiber optics when it comes to surpassing the geographical installation related issues and being the “last mile” solution to the impeding problem of providing access of communication link to remote areas across the world. FSO also offers a good access for local operation usage in civil and military areas. Considering such wide usage and promising future applications of FSO technology, this paper brings out various challenges presently faced in the usage of FSO communication system. In the present paper study of non linearities faced and a brief discussion regarding possible mitigation techniques has been carried out.

A Secure Optical Body Area Network Based on Free Space Optics and Time-Delayed 2D-Spectral/Spatial Optical CDMA

Free space optics (FSO)-based optical body area networks (OBANs) are receiving massive attention as an opportunity to address the limitations of their radio frequency (RF)-based counterparts. This boom in research interests is primarily due to multitude of benefits, including high capacity, immunity to electromagnetic interference (EMI), rapid installation, cost efficiency, and license-free use of spectrum. Securing the transmission of patient health data against interception in OBANs using insecure FSO channels is a challenging task. Therefore, we propose a low-cost, flexible, and secure OBAN based on FSO technology and a time-delayed two dimensional (2D) spectral/spatial optical code-division multiple access (OCDMA) system. The proposed architecture consists of eight sensors attached to the bodies of patients. The sensors operate at a rate of 50 kbps. Electrical data generated from each sensor are used to modulate an optical carrier and then encoded using 2D-spectral/spatial double weight–zero cross correlation (DW-ZCC) code. The 2D encoded optical signals are then time delayed to eliminate the multiple parallel FSO channels between the transmitter and medical center. The combined optical signal consists of eight 2D-encoded time-delayed optical signals transmitted towards a remote medical center over an FSO channel with a range of 1 km. The received signal is decoded and the data from each sensor are recovered after photodetection at the medical center for further analysis. The overall performance of the sensors is analyzed using bit-error rate (BER) and quality factor (Q-factor) plots for different weather conditions and lengths of the FSO channel, considering the log-normal channel model. The capital expenditure (CAPEX) of the proposed architecture is analyzed and compared with the conventional 2D-spectral/spatial FSO system to determine the overall impact of introducing time delay units on the cost of implementation.

Asymptotic Ergodic Capacity Analysis for FSO Communication between Mobile Platforms in Maritime Environments

With the rapid development of wireless communication technologies, free-space optical (FSO) communication has gained much attention in recent years. The FSO technology, which is usually used for communication in terrestrial environments, is expanding its regime to communication in maritime environments. In this study, we analyze the ergodic capacity of FSO communication between mobile platforms in maritime environments under foggy conditions, pathloss, and pointing error. More specifically, based on the moment expressions, we derive the closed-form expression of asymptotic ergodic capacity using conventional detection techniques (i.e., heterodyne and intensity modulation/direct detection). The derived analytical results are cross-verified with simulation results via Monte Carlo simulations. The results show that the combined effects of fog with pathloss and pointing error degrade the FSO performance and that it is important to apply the appropriate detection technique according to the communication environment.

Beyond 5G Fronthaul Based on FSO Using Spread Spectrum Codes and Graphene Modulators.

High data rate coverage, security, and energy efficiency will play a key role in the continued performance scaling of next-generation mobile systems. Dense, small mobile cells based on a novel network architecture are part of the answer. Motivated by the recent mounting interest in free-space optical (FSO) technologies, this paper addresses a novel mobile fronthaul network architecture based on FSO, spread spectrum codes, and graphene modulators for the creation of dense small cells. The network uses an energy-efficient graphene modulator to send data bits to be coded with spread codes for achieving higher security before their transmission to remote units via high-speed FSO transmitters. Analytical results show the new fronthaul mobile network can accommodate up to 32 remote antennas under error-free transmissions with forward error correction. Furthermore, the modulator is optimized to provide maximum efficiency in terms of energy consumption per bit. The optimization procedure is carried out by optimizing both the amount of graphene used on the ring resonator and the modulator's design. The optimized graphene modulator is used in the new fronthaul network and requires as low as 4.6 fJ/bit while enabling high-speed performance up to 42.6 GHz and remarkably using one-quarter of graphene only.

A Location-Aware Resource Optimization for Maximizing Throughput of Emergency Outdoor–Indoor UAV Communication with FSO/RF

In emergency communication scenarios, unmanned aerial vehicles (UAVs) can be used as an air relay to provide higher-quality communication for indoor users. When bandwidth resources are scarce, the use of free space optics (FSO) technology will greatly improve the resource utilization of the communication system. Therefore, we introduce FSO technology into the backhaul link of outdoor communication, and use free space optical/radio frequency (FSO/RF) technology to realize the access link of outdoor indoor communication. The deployment location of UAVs will affect not only the through wall loss of outdoor-indoor communication but also the quality of FSO communication, and, therefore, it needs to be optimized. In addition, by optimizing the power and bandwidth allocation of UAVs, we realize the efficient utilization of resources and improve the system throughput on the premise of considering information causality constraints and user fairness. The simulation results show that, by optimizing the location and power bandwidth allocation of UAVs, the system throughput is maximized, and the throughput between each user is fair.

Future Communication Systems for Railway: the AB4Rail project in H2020 Shift2Rail Programme

In the last few years, the railway community has been working on the replacement of the GSM-R technology used in ERTMS/ETCS system. It is crucial to identify alternative communication technologies able to replace GSM-R guaranteeing the same requirements of QoS/QoE, security, and safety. For this purpose, the two most promising approaches are based on FRMCS and ACS systems. The Eu-Rail JU H2020 AB4Rail project (Alternative Bearer For Rail) aims to investigate the possibility of extending the set of communication bearers that can be integrated into the FRMCS/ACS systems. In this paper, the authors describe the main results obtained in AB4Rail research activities and adopted methodologies. From AB4Rail project investigations, it has emerged that the most promising candidates are: Free Space Optics (FSO) and innovative aerial communication technologies including LEO HTS satellites and HAPS.

Error Probability of a Coherent M-ary PSK FSO System Influenced by Phase Noise

In this paper, we aim to develop an analytical framework for design and analysis of new generation mobile networks fronthaul/backhaul links based on the application of free-space optical (FSO) technology. Taking the receiver hardware imperfections into account, we present an efficient analytical approach in analyzing average symbol error probability (SEP) of the coherent FSO system employing M-ary phase-shift keying (PSK). Optical signal transmission is influenced by pointing errors and atmospheric turbulence. The signal intensity fluctuations caused by atmospheric turbulence are modeled by general Málaga (M) distribution, which takes into account the effect of multiple scattered components. We estimate the range of the signal-to-noise ratio at which the SEP floor appears, as well as the value of this non-removable error floor. The results illustrate that the effect of imperfect phase error compensation on the SEP is more critical under weaker turbulence conditions and for higher order modulation formats. Based on the analytical tools presented here, it is possible to estimate tolerable value of standard deviation of phase noise for the given value of SEP. This value of standard deviation is an important parameter in designing the phase-locked loop filter in the receiver.

Efficient deployment of aerial relays in FSO-based backhaul networks

Using network flying platforms (NFPs) as aerial relays has been emerging as a promising solution for mobile backhaul frameworks in fifth generation and beyond wireless networks. Moreover, free space optics (FSO) technology has become the alternative to radio frequency technology for backhaul links due to its advantages of a high data rate, long transmission range, and negligible interference. In an FSO-based NFP relay backhaul network, due to the fragility of FSO links, how to deploy NFPs to satisfy network traffic demands is a challenging problem. Moreover, the establishment of non-essential FSO links leads to the extra deployment of NFPs due to the limited number of transceivers carried by each NFP, which makes the NFP deployment problem more complex. To solve these issues, we jointly consider the deployment of NFPs and the formation of network topology, the targets of which are to minimize the number of deployed NFPs and network congestion. The problem is formulated as mixed-integer nonlinear programming, which is hard to solve directly, and we linearize it based on transmission range simplification. A heuristic algorithm is proposed to solve the problem for large-scale network scenarios. Simulation results show that the joint optimization scheme of NFP deployment and network topology formation surpasses the single optimization scheme, and the proposed heuristic algorithm can solve the joint optimization problem efficiently in differently scaled networks.

Jamming Threats in Free-Space Optics

Free space optical (FSO) technology has recently found a strong foothold in the telecommunication market due to its technical superiority and high-end applicability. Its technological advancement has caused rising implementation of FSO systems in defence, aerospace, and telecommunication industries. Although it is cutting edge, the fundamental benefits of this technology may be adversely affected by several foreign elements such as atmospheric turbulence, physical obstructions, and weather conditions. Along with these natural disrupting phenomena, for the deployment of FSO systems in security-constrained sectors like military and defence units, where extra data protection mechanism is needed, fabricated adversaries like jamming become feasible which is concerning for FSO applicability. It therefore becomes a challenging task to decipher legitimate FSO data in the presence of jammers and to secure an FSO system from jamming activities. On this account, a detailed discussion on jamming effects in FSO communication systems is showcased in this article. Also, the potential efforts to mitigate jamming in FSO networks are highlighted.

Nano-electromechanical spatial light modulator enabled by asymmetric resonant dielectric metasurfaces

Spatial light modulators (SLMs) play essential roles in various free-space optical technologies, offering spatio-temporal control of amplitude, phase, or polarization of light. Beyond conventional SLMs based on liquid crystals or microelectromechanical systems, active metasurfaces are considered as promising SLM platforms because they could simultaneously provide high-speed and small pixel size. However, the active metasurfaces reported so far have achieved either limited phase modulation or low efficiency. Here, we propose nano-electromechanically tunable asymmetric dielectric metasurfaces as a platform for reflective SLMs. Exploiting the strong asymmetric radiation of perturbed high-order Mie resonances, the metasurfaces experimentally achieve a phase-shift close to 290∘, over 50% reflectivity, and a wavelength-scale pixel size. Electrical control of diffraction patterns is also achieved by displacing the Mie resonators using nano-electro-mechanical forces. This work paves the ways for future exploration of the asymmetric metasurfaces and for their application to the next-generation SLMs.

Modulating Retroreflector Based Free Space Optical Link for UAV-to-Ground Communications

Weight reduction and low power consumption are key requirements in the next generation of unmanned aerial vehicle (UAV) networks. Employing modulating retro-reflector (MRR)-based free space optical (FSO) technology is an innovative technique for UAV-to-ground communication in order to reduce the payload weight and power consumption of UAVs which leads to increased maneuverability and flight time of UAV. In this paper, we consider an MRR-based FSO system for UAV-to-ground communication. We will show that the performance of the considered system is very sensitive to tracking errors. Therefore, to assess the benefits of MRR-based UAV deployment for FSO communications, the MRR-based UAV FSO channel is characterized by taking into account tracking system errors along with UAV’s orientation fluctuations, link length, UAV’s height, optical beam divergence angle, effective area of MRR, atmospheric turbulence and optical channel loss in the double-pass channels. To enable effective performance analysis, tractable and closed-form expressions are derived for probability density function of end-to-end signal to noise ratio, outage probability and bit error rate of the considered system under both weak-to-moderate and moderate-to-strong atmospheric turbulence conditions. The accuracy of the analytical expressions is verified by extensive simulations. Analytical results are then used to study the relationship between the optimal system design and tracking system errors.

Beamforming Design and Performance Analysis for Satellite and UAV Integrated Networks in IoRT Applications

Satellite and unmanned aerial vehicle (UAV) integrated networks (SUINs) are considered as a promising method to offer various Internet of Remote Things (IoRT) applications. In this article, we investigate the downlink transmission of SUINs where the satellite-to-UAV link uses the free-space optical (FSO) technology with an equal gain combining (EGC) scheme while the links from UAV to IoRT devices exploit radio frequency (RF) with the space-division multiple access (SDMA) technique. Specifically, considering that only statistical channel state information (CSI) is available, we first formulate an optimization problem to maximize the ergodic sum rate (ESR) of the system, which is constrained by the total transmit power budget and IoRT devices’ rate requirements. Then, a beamforming (BF) scheme based on the alternating direction method of multipliers (ADMM) is proposed to solve the nonconvex problem. Furthermore, a zero-forcing (ZF)-based suboptimal approach is also presented to reduce the implementation complexity. Finally, by assuming that the FSO link and RF links are subject to Gamma–Gamma fading and Nakagami- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$m$ </tex-math></inline-formula> fading, respectively, we derive closed-form ESR expressions for the considered network with the proposed BF schemes. Simulation results are provided to confirm the accuracy of the theoretical analysis. Moreover, it is revealed that our proposed EGC scheme for FSO communication and BF schemes for RF transmission can both achieve better performance than the existing works.