Radio Frequency/free Space Optical Research Articles

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.

Secrecy Performance Analysis of Mixed RF/FSO Systems Based on RIS Reflection Interference Eavesdropper

This paper proposes and studies the physical layer security of a mixed radio frequency/free space optical (RF/FSO) system based on reconfigurable intelligent surface (RIS)-aided jamming to prevent eavesdropping. This work considers Nakagami-m fading for the RF links and Málaga (M) turbulence for the FSO links. A two-hop decode-and-forward (DF) relaying method was used and the eavesdropper actively eavesdropped on the information transmitted by the RF link. The eavesdropper was thwarted by a wireless-powered jammer that transmits jamming signals, which were reflected by the RIS to the eavesdropper to ensure secure communication in the mixed RF/FSO system. The expressions of secrecy outage probability (SOP) and average secrecy capacity (ASC) of the RIS-aided mixed RF/FSO system were derived for the system model discussed above. The Monte Carlo method was utilized to verify the accuracy of these expressions. In the RIS-aided mixed RF/FSO system, the effects of the time switching factor, energy conversion efficiency, and average interference noise ratio on the system secrecy outage probability were analyzed and compared to the RIS-free mixed RF/FSO system. Meanwhile, the influence of the number of RIS reflecting elements, link distances before and after reflection, and fading severity parameter on the security performance of a mixed RF/FSO system assisted by RIS were also investigated.

Outage Performance for NOMA-Based FSO-RF Systems With a Dual Energy Harvesting Mode

Energy harvesting (EH) is an effective approach that can not only extend the service life for those energy-constraint devices, but also reduce the carbon footprint for wireless communication. Unlike the commonly used single EH scheme adopted in existing works, in this considered free space optical-radio frequency (FSO-RF) system, a dual EH mode is considered, in which an EH relay is capable of harvesting energy from both the ambient RF signal in the environment and the direct-current component separated from the FSO signal simultaneously. In addition, in this considered system, a source node plans to communicate with two users equipped with multiple antennas via an EH relay on a non-orthogonal multiple access (NOMA) basis, and all the harvested energy of the relay is used to transmit the signal to both NOMA users. Moreover, assuming a selection-combining scheme is adopted at the user side, both the analytical and asymptotic expressions of outage probability for each NOMA user are presented, and the system performance with various atmospheric turbulence, detection technique, as well as pointing error is investigated. Finally, numerical results show that our proposed dual EH mode can lead to better outage performance than that with the single EH one.

Performance analysis of mixed IRS-aided RF-FSO system with pointing errors and link blockage

This research investigates the performance of the mixed intelligent reflecting surface (IRS) aided radio frequency-free space optical (RF-FSO) communication network system. IRS-aided RF link experiences Nakagami-m distribution, while FSO link undergoes F distribution turbulence with pointing errors and link blockage. Based on amplify-and-forward (AF) relay scheme, the end-to-end signal-to-noise ratio (SNR), the probability density function (PDF) and cumulative distribution function (CDF) are obtained. Novel expressions about the performance metrics including outage probability, average bit error rate (BER) and ergodic capacity are deduced and asymptotic expression for outage probability is also given at high SNR regime. By using these expressions, the system performance influenced by the number of IRS elements, pointing errors, blocking probability, atmospheric turbulence, detection techniques are further studied. The results demonstrate that IRS can effectively improve the mixed network performance. Eventually, using Monte Carlo simulations verify the results well.

Effects of Eavesdropper on the Performance of Mixed $\eta$–$\mu$ and DGG Cooperative Relaying System

Free-space optical (FSO) channels offer line-of-sight wireless communication with high data rates utilizing unlicensed optical spectrum. The hybrid radio frequency (RF)–FSO system is currently receiving a great deal of research interest because atmospheric turbulence prevents enhanced secrecy performance. Except for the double generalized gamma (DGG) model, traditional FSO models typically are unable to show secrecy performance across all turbulence severity ranges. Due to this, we propose a dual-hop <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\eta$</tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> and unified DGG mixed RF–FSO network while taking into account the following three different eavesdropping scenarios: 1) the eavesdropper is at the RF link; 2) the eavesdropper is at the FSO link; and 3) both eavesdroppers remains active simultaneously. These proposed scenarios' security is examined in terms of the probability, of strictly positive secrecy capacity, secure outage probability and effective secrecy throughput, and further investigation is done into the implications of various system settings on the secrecy performance. The analysis shows that, when compared to the other two scenarios, the third scenario exhibits the worst secrecy condition. Intensity modulation/direct detection (IM/DD) and heterodyne detection (HD) approaches are compared, with HD outperforming IM/DD in terms of secrecy performance. Finally, Monte Carlo simulations are used to confirm all analytical findings.

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.

Spectral-Efficient Network Design for High-Altitude Platform Station Networks With Mixed RF/FSO System

Integrating terrestrial networks with burgeoning high-altitude platform stations (HAPSs) will be a disruptive challenge for beyond-5G systems provisioning large-scale three-dimensional connectivity. Here, we study the problem of forwarding packets between terrestrial terminals and backhaul through multi-HAPS relaying. Considering the limited wireless backhaul, which is the practical constraint of HAPS relaying, dual-hop mixed radiofrequency/free-space optical (RF/FSO) networks are investigated, where backhaul-to-relay and relay-to-user communications employ FSO and RF links, respectively. To maximize the end-to-end network throughput, including downlink and uplink rate, we formulate the optimization problem for variables; the association between aerial and terrestrial terminals, transmit power, and deployment of multiple HAPSs, respectively. We tackle this problem using an iterative algorithm with proposed surrogate functions to efficiently obtain the locally optimal solution. Simulation results corroborate that our proposed optimal method achieves up to 11.3% spectral efficiency compared to the conventional heuristic method. Furthermore, we answer three questions; 1) what the wireless backhaul requirements are, 2) how the number of HAPSs and terrestrial terminals impact the network, and 3) what if a certain user terminal has a particular demand on data rate.

Investigation of Machine Learning Methods for Prediction of Measured Values of Atmospheric Channel for Hybrid FSO/RF System

This research paper addresses the problems of fiberless optical communication, known as free space optics, in predicting RSSI (Received Signal Strength Indicator) parameters necessary for hard switching in a hybrid FSO/RF (Free Space Optics/Radio Frequency) system. This parameter is used to determine the intensity of the transmitted signal (in our case, a light beam) from one FSO head to another. Since we want to achieve almost 100% reliability, it is important to know the parameters of the transmission environment for the FSO and RF lines. Each of them has its limitations and, as a result, a weather monitoring station is required. The FSO is mostly affected by fog and the concentration of particles in the air, while the RF line is affected by rain and snow. It is precisely due to these influences that it is necessary (based on the mentioned RSSI parameter) to switch using the hard switching method from the primary FSO line to the backup RF line by correctly predicting this value. If the value of the RSSI parameter falls below the critical level—42 dBm—the system automatically switches to the backup RF line. There are several ways we can predict this parameter. One of them is machine learning methods such as decision trees. Our research focused on the prediction of the RSSI parameter, the methods of decision trees and decision trees using the AdaBoost regressor. Since we want to correctly predict the RSSI parameter, it is also necessary to choose the right way to predict it based on the recorded weather conditions. If we want to correctly use the hard switching method in hybrid FSO/RF systems, it is necessary to choose the correct method of predicting the RSSI parameter, which serves as an indicator for switching from the primary FSO line to the secondary RF line. Therefore, we decided to investigate methods of machine learning—the decision tree and the decision tree with the use of an AdaBoost Regressor. The main benefit of this paper is the improvement of existing machine learning methods (decision trees and decision trees using the AdaBoost regressor) for the correct prediction of the RSSI parameter for the needs of hard switching in a hybrid FSO/RF system. The method chosen in this manuscript has very good results. As can be seen in the attached graphs, over a longer period and using correctly selected training data, it is possible to achieve ideal results for the prediction of the RSSI parameter. The tables also show the effectiveness of the prediction, and the fact that it is best to train on either the first- or third-minute data. In the future, it would be appropriate to implement weather prediction or to consider other methods, such as random forests or neural networks.

Secrecy performance analysis for a NOMA based FSO-RF system with imperfect CSI

In this paper, we consider a non-orthogonal multiple access (NOMA) based mixed free space optical–radio frequency (FSO-RF) system with imperfect channel state information (CSI) and evaluate the secrecy performance in terms of secrecy outage probability (SOP). Specifically, a source ( S ) first forwards an optical NOMA signal to a relay ( R ) via an FSO link, and then R performs an optical–electrical conversion and broadcasts the converted signal to users over the RF links. Both the FSO and RF links are incorporated with the NOMA technique, and the CSI imperfection in both links is considered. Assuming both users have multiple antennas, and adopting the maximal ratio combining diversity schemes, we derive the analytical and asymptotic expressions for SOP when the FSO link undergoes gamma–gamma fading with pointing error and the RF links are subject to Rayleigh fading. In addition, both decode-and-forward and amplify-and-forward protocols are taken into account and compared. Monte Carlo simulations are also used to verify the accuracy of the derived results.

Performance Analysis of Dual-Hop RF/FSO Relaying Systems With Imperfect CSI

This paper proposes a unified performance analysis framework of asymmetric dual-hop radio-frequency/free-space optical (RF/FSO) transmission systems. The generalized independent and identically distributed (i.i.d.) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha -\mu$</tex-math></inline-formula> distribution is considered for each branch in the RF link, whereas the FSO link is assumed to experience Málaga ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathcal {M}$</tex-math></inline-formula> ) fading with pointing error impairments. With amplify-and-forward (AF) and decode-and-forward (DF) relaying, the imperfect channel state information (CSI) of both links is considered. For the considered system, we first derive exact and novel analytical expressions for the probability density function (PDF) and cumulative distribution function (CDF) of the end-to-end signal-to-noise ratio (SNR). Furthermore, we obtain key performance metrics such as outage probability (OP), average bit-error-rate (ABER), and effective capacity (EC). The important effects of channel parameters on the system are disclosed from mathematical analysis and are further validated by Monte-Carlo simulations.

On Secrecy Performance of Mixed RF-FSO Systems With a Wireless-Powered Friendly Jammer

This paper presents a study of the physical layer security performance of a mixed radio frequency-free space optical (RF-FSO) system with a wireless-powered friendly jammer. The RF links undergo the Nakagami-m fading, and the FSO link experiences the Exponentiated Weibull (EW) distribution. A two-hop decode-and-forward (DF) relay is present in the system. A single-antenna eavesdropper located on the RF link attempts to eavesdrop on the channel transmission information. There is a nearby multi-antenna jammer that can be charged wirelessly, and a save-then-transmit (ST) protocol is introduced in the jammer. The closed expressions for the secrecy outage probability (SOP) and the average secrecy capacity (ASC) of the mixed RF-FSO system are derived, and the correctness of the expressions is verified using the Monte-Carlo method. The influence of various key factors on the secrecy performance of the system is analyzed by simulations. The results show that increasing the average interference noise ratio, the number of interferer antennas, the time block allocation factor, and the size of the receiving aperture has a significant effect on the secrecy performance. This study provides a new system structure and a good theoretical basis for evaluating the physical layer security performance of mixed RF-FSO systems.

On the Design of Rate Adaptation for Relay-Assisted Satellite Hybrid FSO/RF Systems

This paper addresses the design of hybrid free-space optical/radio frequency (FSO/RF) systems for a high-altitude platform (HAP)-aided relaying satellite communication for mobile networks supported by unmanned aerial vehicle (UAV). While prior work primarily focused on fixed-rate design, which frequently switches between FSO and RF lead reduced the system performance, we propose a rate adaptation design that gradually adjusts the data rate in each link when its channel state fluctuates, under a target bit error rate (BER) constraint. The proposed design’s downlink performance is analyzed, taking into account many challenging issues, including beam spreading loss, cloud attenuation, statistical behaviors of the atmospheric turbulence in the dual-hop channel, and pointing misalignment due to the UAV hovering. Different performance metrics are analytically derived based on channel modelings, including outage probability, average transmission rate, and achievable spectrum efficiency. In addition, the average system BER, which satisfies the design constraint, is also numerically obtained. The results quantitatively confirm the effectiveness of our proposed system under the impact of UAV hovering misalignment and atmospheric-related issues like clouds and turbulence. Finally, Monte-Carlo simulations validate the accuracy of theoretical results.

Manifold Learning Inspired Dynamic Hybrid Precoding With Antenna Partitioning Algorithm for Dual-Hop Hybrid FSO-RF Systems

The decode-and-forward (DF) based free-space optical-radio frequency (FSO-RF) hybrid system combines the advantages of both FSO links and RF links, which makes the system easy to be deployed and enables the extended transmission coverage. In the case of multiple users, the unreasonable dynamic antenna selection algorithm will lead to multi-user interference (MUI) and reduce the spectral efficiency of the system. Therefore, we propose hybrid precoding based on manifold learning with the antenna partitioning algorithm for dual-hop hybrid FSO-RF systems. In the hybrid precoding of dynamic subarray structure, the high-dimensional channels are embedded into the low-dimensional manifolds by the optimized multidimensional scaling (MDS). The potential spatial correlations of the high-dimensional channel are preserved by the scaling by majorizing a complicated function (SMACOF) algorithm. Through proper user clustering, the hybrid precoding is investigated for the sum-rate maximization problem by manifold quasi-conjugate gradient methods. Meanwhile, an antenna subarray partitioning algorithm is proposed, so that each antenna unit can be assigned to an RF chain based on the increment of the user’s maximum signal to interference noise ratio (SINR). By calculating the simulated equivalent channel SINR for the selected users, the antenna division can greatly reduce the computational complexity and the size of the search space, and ensure fairness among users. Simulation results show that this method can obtain almost the best summation rate and higher spectral efficiency compared with the conventional method.

Secrecy performance of mixed single-input multiple-output-FSO/single-input multiple-output-RF networks with energy harvesting.

In this paper, we study the secrecy performance of an energy-harvested relaying assisted free space optical-radio frequency (FSO-RF) network. In particular, a single-beam source sends an optical signal to a multi-aperture relay over an FSO channel, and then the relay converts it into an RF signal and adopts a decode-and-forward protocol to forward it to a multi-antenna destination under the wiretapping of a multi-antenna eavesdropper. We assume that the relay is power constrained, and it requires to harvest energy from a power beacon to assist the information transmission. It is also assumed that the FSO link and all RF links are subject to gamma-gamma fading with atmospheric turbulence and Rayleigh fading, respectively. Analytical expressions for secrecy outage probability (SOP) and the probability of strictly positive secrecy capacity are derived. In addition, the asymptotic expression for SOP is obtained to present insight into secrecy performance at high signal-to-noise ratio regimes. Finally, Monte Carlo simulations verify the correctness of the theoretical derivations.

Outage probability analysis in multi-user FSO/RF and UAV-enabled MIMO communication networks

This paper considers a mixed free-space optical/radio frequency (FSO/RF) system, which facilitates the communication between a ground central unit (CU) and multiple ground users (GUs) via a hovering unmanned aerial vehicle (UAV) acting as a decode-and-forward (DF) aerial relay. It is considered that the CU employs multiple transmit apertures and is interconnected with the aerial relay via an FSO link with transmit aperture selection (TAS), whereas the relay communicates with the GUs over multiple-input multiple-output (MIMO) RF links with orthogonal space–time block coding (OSTBC) transmission. It is assumed that the optical channels are mainly affected by atmospheric attenuation as well as geometric and misalignment loss (GML). Besides, the atmospheric turbulence is assumed weak for short-range links. Also, fully correlated shadowed conditions are considered in the RF links and the MIMO channels are modeled using the Nakagami/Inverse Gamma (IG) composite fading distribution. Opportunistic GU scheduling is applied in the downlink and novel closed-form mathematical formulas for the outage probability (OP) are derived. The results demonstrate the theoretical derivations and the performance benefits of the proposed approach.

Performance Analysis of RIS-Assisted Mixed Dual-Hop FSO-RF Communication Systems over Exponentiated Weibull Channels

Information security is the basis of economic development, information construction, and the protection of people’s and national interest. In this paper, the performance of reconfigurable intelligent surface (RIS)-assisted mixed dual-hop free-space optical-radio frequency (FSO-RF) communication systems is studied and analyzed to ensure the safe transmission of information. In this scheme, it is assumed that the atmospheric turbulence in the fading of the FSO channel follows the exponentiated Weibull distribution model and the fading of the RF channel follows the Rayleigh distribution model. Based on the mathematical function, the probability density function and cumulative distribution function of RIS-assisted mixed dual-hop FSO-RF communication systems are derived. Then, the expression of outage probability and the bit error rate of the system are derived from the channel statistics. Finally, the analysis results are verified by Monte Carlo simulation, and the effects of different parameters on the system performance are analyzed. The simulation results show that the atmospheric turbulence parameters, channel distance of FSO, intensity of atmospheric turbulence, and number of reflectors all affect the system performance.

Capacity and outage performance analysis of secure cooperative RF-FSO relaying system in the presence of multiple eavesdroppers

This paper presents the performance analysis of the cooperative radio frequency-free space optical (RF-FSO) relaying system in the presence of multiple eavesdroppers. In this work, the RF link is modeled with Hyper-Gamma (HG) distribution and the FSO link is modeled with unified Gamma–Gamma (ΓΓ) distribution. The RF-FSO combined channel is linked together through a decode-and-forward (DF) based relaying scheme. It is assumed that unknown number of eavesdroppers try to wiretap the confidential data sent from the source by using HG fading channels in the physical layer. The key concern of this work is to ensure safe data transmission through the proposed RF-FSO channel by defending them against passive eavesdropping activities. This work offers the mathematical closed-form expressions of the probability of strictly positive secrecy capacity (PSPSC), secure outage probability (SOP), and average secrecy capacity (ASC) in respect of Meijer’s G function. To acquire better insights into the proposed model, the asymptotic expression of SOP is also provided. Moreover, the impacts of receiver detection type, atmospheric turbulence, pointing error, and eavesdropper signal-to-noise ratio (SNR) on the secrecy performances are investigated in the derived outcomes. The demonstration of the supremacy of the proposed model is done based on the generic nature HG fading channel. Finally, all analytical outputs are checked through Monte Carlo simulations for validation purpose.

Performance analysis of an asymmetric two-hop amplify-and-forward relaying RF–FSO system in a cognitive radio with partial relay selection

In this paper a performance analysis is presented for an asymmetric two-hop amplify-and-forward (AF) relaying system with multiple relays in an underlay cognitive radio network (CRN), where a partial relay selection (PRS) is used to select the best relay for the signal transmission. In the considered system, the two hops employed are hybrid and asymmetric, which include the source-to-relay (SR) and the relay-to-destination (RD) links. The first hop (SR link) is a radio frequency (RF) channel and the second hop (RD link) is a free space optical (FSO) channel. The RF and FSO channels follow Rayleigh, and Gamma–Gamma distribution, respectively, where some practical concerns such as atmospheric turbulence and pointing errors are taken into consideration. To start the performance evaluation and analysis, the cumulative-distribution-function (CDF) of the received signal-to-noise-ratio (SNR) at the destination side is mathematically derived. Then, by means of the derived CDF, closed-form expressions are derived for the outage performance, average BER and the average channel capacity of the underlying system. Finally, the numerically evaluated results, validated by computer simulations, provide insights about system performance under various parameters and practical situations.

Outage probability of wireless-powered multi-relaying MIMO FSO-RF systems

This paper investigates a wireless-powered multi-relaying multiple-input multiple-output free space optical-radio frequency (FSO-RF) system, which consists of a source (S) with N (N≥1) laser beams, N single-antenna and single-aperture wireless-powered relaying nodes, a multi-antenna destination (D), and a single-antenna power beacon. Specifically, each beam at S is directed to the receiving aperture of a particular relay, and only one beam based on a transmit laser selection scheme is used for information transmission. Considered a nonlinear energy harvesting mode at the relays, a partial relay selection scheme based on the first hop, and a maximal ratio combining scheme at D, analytical and asymptotic expressions for outage probability are derived, which are also validated with Monte-Carlo simulations.

On the effect of I/Q imbalance in mixed RF/FSO cooperative relaying systems

In this paper, the impact of in-(I) phase/quadrature-(Q) phase imbalance (IQI) in mixed radio frequency/free space optical (RF/FSO) cooperative relaying systems is studied. The RF link has been assumed to undergo Nakagami-m distributed fading, while the FSO link is under the influence of atmospheric turbulence modeled by the Malaga irradiance model with pointing errors. The FSO model accounts for both kinds of optical demodulation schemes, i.e., intensity modulation direct detection (IM/DD) and coherent demodulation. For the considered system, the impact of hardware IQI has been taken into account in order to derive closed-form expressions for the outage probability and ergodic capacity of the amplify-and-forward (AF) relaying system. These expressions have been further presented in terms of elementary functions for high signal-to-noise ratio (SNR) region of operation. The presented analytical framework has been verified by Monte-Carlo simulations to illustrate the impact of various parameters on the performance of the considered system.