Performance Analysis of RF-FSO Multi-Hop Networks

On the maximal-ratio combining of FSO and RF links over generalized distributions and its applications in hybrid FSO/RF systems

In this paper, we investigate the performance of asymmetric radio frequency (RF) and free space optical (FSO) dual-hop cooperative relay network along with a direct RF link between source and destination. The FSO link experiences double generalized Gamma turbulence in the presence of the generalized non-zero boresight pointing errors, and RF links experience non-identically distributed Nakagami-m fading. Moreover, considering both cognitive radio and non-cognitive scenarios, a partial relay selection (PRS) strategy has been employed. Furthermore, both relay selection and underlay power restriction are governed with the outdated channel state information (CSI). We also assume both heterodyne and intensity modulation/direct detection methods in the FSO receiver. Under the assumption of amplify-and-forward relaying and PRS, we derive closed-form expressions for outage probability (OP) of both scenarios, while bit-error probability and ergodic capacity of the non-cognitive scenario are also obtained. The asymptotic expressions of OP and diversity order of this network are derived for both perfect CSI and outdated CSI cases. It is demonstrated that the diversity order is a function of the fading severity of the RF links, turbulence parameters of the FSO link, and pointing error, regardless of interference channel parameter of the primary user.

The generalized order user scheduling is among the efficient user selection schemes which can be used in conditions where the opportunistic scheduling could fail. The opportunistic scheduler could erroneously select a user other than the best user for data transmission or reception due to imperfect channel estimation or outdated channel information issues. In this paper, we study the performance of multiuser mixed radio frequency (RF)/free-space optical (FSO) relay network with generalized order user scheduling. Due to its importance on the system performance, the outdated channel information problem is also addressed in this paper. The considered system includes K sources or users, one decode-and-forward relay and one destination. The users are connected with the relay node through RF links, and the relay is connected with the destination through an FSO link. In the analysis, the first-hop channels are assumed to follow Rayleigh fading model and the second-hop channel is assumed to follow Gamma–Gamma fading model including the effect of pointing errors. Closed-form expressions are derived for the outage probability, average symbol error probability and ergodic channel capacity. Moreover, in order to get more insights about the system behavior, the system is studied at the high signal-to-noise ratio regime where the diversity order and coding gain are provided and studied. Monte Carlo simulations are given to validate the achieved exact and asymptotic results. Main findings illustrate that under weak atmospheric turbulence conditions, the system performance is dominated by the RF links and a diversity order of K − N + 1 is achieved by the system, whereas under sever atmospheric turbulence conditions, the system is dominated by the FSO link and the diversity order is related to the minimum value of the turbulence fading and pointing error parameters.

In this paper, we study the performance of mixed free space optical (FSO)-radio frequency (RF) multi-user relay network with aperture selection and opportunistic user scheduling in the presence of Poisson field interference. The considered system includes multiple optical sources (apertures), one amplify-andforward (AF) relay, and multiple users. The source is connected with the relay node through FSO links and the relay is connected with the users through RF links with a Poisson field interference at the users. The FSO-RF channels are assumed to follow Málaga-M/shadowed κ-μ fading models with pointing errors on the FSO links. Exact closed-form expressions are derived for the outage probability and average symbol error probability (SEP). Moreover, the system performance is studied at the high signal-to-noise ratio (SNR) regime, where an approximate expression for the system outage probability is derived, in addition to deriving the system diversity order and coding gain. Then, the optimum source and relay transmission powers and relay position are determined to minimize the system asymptotic outage probability. The Monte-Carlo simulations are provided to validate the achieved exact and asymptotic results.

In this paper, the physical layer security (PLS) analysis for a decode-and-forward (DF) protocol based multi-hop hybrid radio frequency (RF)/ free space optics (FSO) is presented. Herein, two different scenarios are considered for detecting the secured information at the destination. In the first scenario, at each hop, the received signals with higher secrecy capacity are selected. However, in the second scenario, the RF and FSO signals are simply decoded and forwarded to the next hop and selection is done at the last hop only. Each node in the system is connected to its subsequent node through parallel RF and FSO links. The FSO links and RF links are characterized by 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> ) 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> composite distributions respectively. For both scenarios, the secrecy outage probability (SOP), strictly positive secrecy capacity (SPSC), intercept probability (IP), and effective secrecy throughput (EST) are obtained as performance metrics. These performance metrics are obtained by considering pointing error, different optical signal detection methods, turbulence effect, and the RF link fading parameter ( <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> ). Using the results, the security and reliability trade-off analysis is discussed for two scenarios. In addition, the asymptotic results are also obtained to study the system in depth. Further, the secrecy diversity order (SDO) and secrecy coding gain (SCG) are obtained to analyse the system in depth. Finally, Monte-Carlo Simulation is performed to verify the obtained results. The results manifest that the scenario with selection at each hop provides better secrecy performance than the scenario with selection at the last hop.

Hybrid radio frequency (RF)/free space optical (FSO) systems are among the candidate enabling technologies for the next generation of wireless networks since they benefit from the advantages of both the FSO subsystem, e.g. high data rates, and the RF subsystem, e.g. high reliability in terms of link connectivity. In this paper, we focus on the problem of throughput maximization in the parallel hybrid RF/FSO relay channel. In the parallel hybrid RF/FSO relay channel, a source node sends its data to a destination node with the help of multiple relay nodes. Thereby, the source-relay and the relay-destination FSO links are orthogonal with respect to each other due to the narrow beam employed for FSO transmission whereas the RF links are half duplex with respect to each other due to the broadcast nature of RF signals. We derive the optimal relay selection policies for transmission and reception for the RF and FSO links and the optimal time allocation policy to the RF relay reception and transmission links. Simulation results demonstrate that a considerable gain can be achieved by the proposed adaptive protocol in comparison with benchmark schemes from the literature.

In this paper a novel hybrid Free Space Optical (FSO)-Radio Frequency (RF) model is proposed which is based on the single decision threshold feedback bit technique. The FSO and proposed hybrid FSO-RF link performance is investigated under strong turbulence and misalignment aperture error or pointing error in terms of the average symbol error rate (ASER) and outage probability. Atmospheric turbulence and misalignment aperture are foremost factors that affect the performance of FSO link which entails the requirement of an RF link as a backup to enhance the system performance. The FSO-RF link is preferred in this work over the normal FSO link due to ease in switching among the link established with the help of decision threshold during failure of FSO link. In this proposed hybrid model, Gamma-Gamma distribution is considered as free-space optical turbulence channel and Nakagami-m model is for RF link. The analytical expressions for outage probability, ASER have been derived and results are given for various Subcarrier Intensity Modulation—M-Phase Shift Keying (PSK) schemes. A performance comparative study is reported for FSO link and hybrid FSO-RF link. The obtained result indicates the enhanced system performance whereas the obtained results are validated through Monte-Carlo simulations and results are found in good agreement.

Free-space optical (FSO) links offer an attractive alternative to the radio frequency channel for the purpose of transmitting data at very high rates. A major impairment in FSO links is the turbulence-induced scintillations of an optical wave. Spatial diversity, which involves the deployment of multiple laser transmitters/receivers, can be used over FSO links to reduce the turbulence-induced scintillations. In this paper, the channel of a multiple-transmitter-multiple-receiver FSO link considering the spatial correlation between the constituent sub-channels was mathematically modeled and then the joint distribution function of the channel gain coefficients was derived. Base on the joint distribution function, the model for the outage probability of a FSO link with spatial diversity was established. With Monte Carlo method, simulations for the outage probability were performed. The results show that spatial correlations have a significant influence on the outage probability, and the diversity-induced performance improvement reduces to zero as the spatial correlation coefficient increases to 1.

A three-hop long range communication system with mixed fading for smart grid (SG) wide area network (WAN) is proposed. The three hops proposed here are power line communication (PLC), radio frequency (RF) and free space optical (FSO) in cascade. The smart meter (SM) transfers the data to the access point (AP) through a PLC link. The AP is acting as a decode-and-forward (DF) relay. It retransmits the data to data aggregator unit (DAU) through RF link. The DAU also acts as a DF relay and retransmits the information to meter data management system (MDMS) through FSO link. The PLC, RF, and FSO links are distributed with Log-Normal, Nakagami-m, and Gamma-Gamma distribution, respectively. The modulation scheme considered here is binary phase shift keying (BPSK). A closed form expression for average bit error probability (ABEP) is obtained. Numerical and Monte-Carlo simulation results demonstrate the effect of impulsive noise scenario in the PLC channel, fading severity in Nakagami-m channel, and different FSO parameters in SG communication system.

In this work, the performance of a dual-hop variable gain amplify-and-forward (AF) mixed radio frequency (RF)/free space optics (FSO) system is analyzed in details. As variable gain AF relay network is considered, processing delay results in outdated channel state information (CSI) during amplification at the relay. For general applicability, the RF link is modeled with the generalized Nakagami-m fading. The FSO link is modeled with the Gamma-Gamma distribution which is affected with the atmospheric turbulence and pointing error impairments, and both the intensity modulation with direct detection (IM/DD) and heterodyne detection are employed at the FSO receiver. In this context, analytical expressions of outage probability and ergodic capacity are derived in terms of Meijer-G function and extended generalized bivariate Meijer-G function. For diversity order, asymptotic outage probability expression is also derived. Further, cumulative distribution function based generalized average symbol error rate expressions for various quadrature amplitude modulation (QAM) schemes such as hexagonal QAM, rectangular QAM, and cross QAM are derived in terms of Meijer-G function. Furthermore, a detailed comparative study of various modulation schemes is presented and the impact of pointing error, atmospheric turbulence, outdated CSI, and Nakagami-m parameter are highlighted on the system performance. Finally, all the analytical results are verified through Monte-Carlo simulations.

We study a dual-hop relaying system, composed of mixed radio frequency (RF)/free-space optical (FSO) links. Partial amplify-and-forward relay selection is assumed, which is based on outdated channel state information (CSI). The RF link is subject to Rayleigh fading, while the FSO link is affected by atmospheric turbulence, modeled by Gamma–Gamma distribution. Novel analytical expressions for the outage probability and average bit error rate (BER) are derived. Numerical results are presented and validated by Monte–Carlo simulations. The outage probability and average BER dependence on turbulence strength, average signal-to-noise ratio (SNR) over RF hop, average electrical SNR over FSO hop, number of relays, and rank of selected relay, are also investigated, while the effect of time correlation between current source-relay channel and its outdated CSI, is considered. The results illustrate that the outdated CSI used for the relay selection has a strong effect on the system performance, especially in weak turbulence conditions of the FSO hop. Furthermore, the improvement of the system performance with an increase of the number of the relays is highly dependent on the FSO channel state.

Space–air–ground integrated network (SAGIN) is considered an enabler for sixth-generation (6G) networks. By integrating terrestrial and non-terrestrial (satellite, aerial) networks, SAGIN seems to be a quite promising solution to provide reliable connectivity everywhere and all the time. Its availability can be further enhanced if hybrid free space optical (FSO)/radio frequency (RF) links are adopted. In this paper, the performance of a hybrid FSO/RF communication system operating in SAGIN has been analytically evaluated. In the considered system, a high-altitude platform station (HAPS) is used to forward the satellite signal to the ground station. Moreover, the FSO channel model assumed takes into account the turbulence, pointing errors, and path losses, while for the RF links, a relatively new composite fading model has been considered. In this context, a new link selection scheme has been proposed that is designed to reduced the signaling overhead required for the switching operations between the RF and FSO links. The analytical framework that has been developed is based on the Markov chain theory. Capitalizing on this framework, the performance of the system has been investigated using the criteria of outage probability and the average number of link estimations. The numerical results presented reveal that the new selection scheme offers a good compromise between performance and complexity.

This paper investigates a novel method for boosting the performance of parallel-relaying decode-and-forward (DF) cooperative free space optical (FSO) networks. The proposed solution takes advantage from the presence of the radio frequency (RF) links that are often established to backup the FSO links in practical systems. In this context, the low speed RF links are used for carrying the information packets that have not been delivered along the direct FSO link for the sake of sharing these packets among the relays in a simple and efficient manner. This packet sharing enhances the chances for correct detection at the relays thus increasing the number of relays that will participate in the relay-destination transmission phase. An exact outage probability analysis is carried out in the case of gamma-gamma FSO fading channels with pointing errors and of Rician/Rayleigh RF fading channels. At a second time, the cut-set method is applied for deriving a simple and tractable upper-bound that is useful for evaluating the asymptotic performance and diversity gain.

We propose a framework for providing instantaneous backup to traffic in a hybrid RF/FSO mesh network. Free space optical (FSO) links have high bandwidth and security, making them suitable for use in backbone networks. RF links have low bandwidth, but offer high reliability in conditions where FSO links are obscured. Thus, RF links are primarily used to provide instantaneous backup to traffic flowing on FSO links. We propose a framework to model FSO link failures due to obscuration, and propose algorithms for integrated topology control of RF links and routing for maximizing the backup provided. We do extensive simulations to demonstrate that the performance of RF topology and routing computed using our algorithms is much better than the case of having the same RF and FSO topologies and routes.

In this paper, we aim at tackling the security performances of dual-hop mixed free-space optical (FSO)/radio frequency (RF) cooperative with the non-orthogonal multiple access (NOMA) technique. Particularly, we consider a downlink communication scenario, where a base station (BS) is commu-nicating with remote users in the existence of an eavesdropping attack. The hop from the BS to the relaying system is considered an FSO link, where the atmospheric turbulences are described by Málaga- <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathcal{M}$</tex> . Besides, the RF links between the relay and users/attacker are assumed to follow the Double <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\kappa-\mu$</tex> Shadowed fading since it unifies a wide set of special-case fading models. Moreover, we derive the closed-form expressions of the outage probability at the 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> legitimate user and at the attacker, as well as the secrecy outage probability (SOP) in terms of the multivariate Fox-H function. Finally, we investigate the impact of the RF fading parameters, path-loss, power ratio, and pointing error on the secrecy performance.