Radio Frequency Link Research Articles

Performance of an all-links MIMO-OSTBC mixed underlay cognitive RF/FSO system.

This paper presents a new model, to the best of our knowledge, of a mixed underlay cognitive radio frequency (RF)/free space optical (FSO) system in which both RF and FSO links consider multiple-input multiple-output (MIMO) orthogonal space-time block coding (OSTBC). In a dual-hop decode-and-forward configuration, the underlay cognitive radio network RF and FSO links experience κ-μ and Γ-Γ fading, respectively. For the above system model, a closed expression for the outage probability of the mixed underlay cognitive RF/FSO system with MIMO-OSTBC is derived, and the simulation results are verified using the Monte Carlo method. The results show that considering MIMO-OSTBC in all links of the mixed underlay cognitive RF/FSO system can effectively improve the communication performance of the mixed system and alleviate the degradation in the communication quality caused by the atmospheric turbulence. The communication performance of the MIMO-OSTBC mixed underlay cognitive RF/FSO system is further improved by changing the key parameters, such as peak transmit power, fading parameters, the number of secondary user transmit antennas and relay receive antennas, and relative speed of the primary user.

Performance analysis of dual-hop mixed radio frequency/free space optical transmission systems based on partially coherent beams over F-distribution channel models

The performance of dual-hop mixed radio frequency (RF)/free space optical (FSO) communication system with partially coherent beams (PCBs) is investigated on the basis of decode-and-forward (DF) protocol. Specifically, both the RF and FSO channels are assumed to follow F-distribution with proper selection of parameters. Taking the path loss, atmospheric turbulence, and pointing errors into consideration, the closed-form expressions for the probability density function (PDF) and cumulative distribution function of the received signal-to-noise ratio are derived in terms of hypergeometric function in RF and FSO links, respectively. Subsequently, the closed-form expressions for outage probability, average bit error rate, and ergodic channel capacity of this system with M-ary phase-shift keying and M-ary quadrature amplitude modulation schemes are obtained on the basis of DF protocol. The results show that shadowing in RF link as well as the pointing error and atmospheric turbulence in FSO link would result in significant performance deterioration in dual-hop mixed RF/FSO system, while PCB can bring significant performance improvement of mixed RF/FSO systems with pointing errors over weak to moderate turbulence. Furthermore, the correctness of the analytical expressions is verified by Monte Carlo simulations. This work would benefit the design and development of the mixed RF/FSO systems.

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

Recently, generalized fading distributions have gained substantial research interest due to their applicability and adaptability in practical scenarios. In this paper, we derive the exact closed-form expressions for the probability density function (PDF) and cumulative distribution function (CDF) of the maximal-ratio combining (MRC) of the free space optics (FSO) and radio frequency (RF) links over the most generalized Malaga and α-η-κ-μ fading models. Further, with the help of derived MRC channel statistics (i.e. PDF and CDF), closed-form expressions for outage probability and average symbol error rate (SER) for the hybrid FSO/RF system based on MRC and adaptive combining schemes are derived. In an adaptive combining scheme, the MRC of FSO and RF links is employed at the receiver, if the operating FSO link quality is unacceptable for transmission. Further, the derived expressions are unified for both intensity modulation/direct detection (IM/DD) and heterodyne detection (HD) techniques. We have considered the effects of atmospheric path loss, atmospheric turbulence-induced fading and non-zero boresight pointing errors in case of the FSO link. We have also determined the optimum switching threshold and beam waist values for the adaptive combining scheme to achieve optimal performance. Moreover, computationally less complex asymptotic expressions are calculated and the diversity gains of MRC and adaptive combining schemes are determined. Finally, the performances of single-link FSO, hard-switching-based hybrid FSO/RF, MRC, and adaptive-combining-based hybrid FSO/RF systems are compared and the inferences are reported under various turbulence conditions, link distances, weather conditions, and severity of the pointing errors.

Efficient Hardware Architectures and Implementations of Packet-Level Erasure Coding Schemes for High Data Rate Reliable Satellite Communications

Forward error correction coding schemes have been used extensively in order to provide continuous and reliable telemetry data transfer from satellites to ground stations and guarantee reliable communications even at low signal-to-noise ratio regimes, where burst errors are dominant. Nevertheless, conventional bit-level channel coding can fail to provide downlink reliability against long error bursts in future high data rate radio frequency and optical (laser) links, especially when automatic repeat queuing strategies are either problematic or impossible due to specific service delay constraints. Packet-level erasure coding has been considered by the Consultative Committee for Space Data Systems (CCSDS) in CCSDS 131.5-O-1 experimental specification for application in high data rate near-earth and deep-space communications, since it can protect against long error bursts as they may come along with the effect of scintillation outages or transmission errors. Implementations of packet-level encoding and decoding so far exist only in software, running on a general-purpose CPU. In this article, we introduce and compare analytical descriptions of packet-level encoding algorithms suitable for hardware implementations. Based on these algorithmic descriptions, we introduce, for the first time, architectures for hardware acceleration of these functions that allow integration on a high-speed on-board data-processing chain with very low footprint. We validate our hardware implementations and demonstrate the efficiency of the proposed architectures on a Xilinx KCU105 development board, which is built around the commercial equivalent of a space-grade field-programmable gate array device. Apart from offloading packet-level encoding from the on-board embedded processor, the proposed accelerators achieve a significant speedup (over 80 times), when compared with on-board software implementations, by porting on some of the most commonly used and state-of-the-art space-qualified embedded processors.

Output Regulation With Integrated SR Switch Duty Cycle Control for Wireless Power Transfer Systems

Due to the physical separation between primary and secondary sides, achieving a good output regulation in a wireless power transfer (WPT) system is nontrivial. Typically, an additional dc–dc converter stage at the secondary side or a radio frequency (RF) communication link between the primary and secondary sides is used to regulate the output. However, these approaches are not always practical in many industrial applications where there is a size constraint, or the noise interference that tends to interrupts the RF link. In this article, an integrated secondary-side regulation scheme without extra devices or communication requirement is proposed. The duty cycle of the secondary-side synchronous rectification (SR) device is controlled to regulate the output independently of the primary. The SR duty modulation allows voltage regulation over a wide load range or current regulation over a wide input range while avoiding the additional dc–dc converter stage. Thus, the primary-side control can operate under the fixed-duty condition while maintaining zero-voltage switching (ZVS) for all switches in the entire power range. A 48-V/190-W GaN high-electron-mobility transistor (HEMT)-based prototype has been built and tested to verify the feasibility of the proposed control scheme.

Partial relay selection for two-way mixed RF/FSO DF networks in the presence of I/Q imbalance

In this paper, the study of mixed radio frequency (RF)/ free space optical (FSO) communication decode and forward (DF) two way relaying (TWR) has been presented. In fact, it has been considered that multiple relays are present out of which the best operational relay is selected as per the partial relay selection (PRS) methodology in the presence of outdated channel state information (CSI). Importantly, the relay nodes are assumed to operate in the presence of in phase (I) quadrature phase (Q) imbalance (IQI). The atmospheric turbulence on the FSO link has been modeled using the Malaga distribution with pointing errors. In addition to this, the impact of type of optical demodulation has been considered in the analysis. For the system model, outage probability expression has been derived in terms of Meijer-G and Fox’s H-functions. In addition to this, for the TWR system, the outage probability expressions have been modified to present asymptotic results in terms of elementary functions. The numerical analysis of the research work suggests that the overall mixed RF/FSO DF TWR system is impacted by the image rejection ratio (IRR) due to IQI, correlation between outdated CSI, atmospheric turbulence, pointing error and type of optical demodulation in addition to the amount of fading on the RF link.

Mixed FSO/RF Based Multiple HAPs Assisted Multiuser Multiantenna Terrestrial Communication

In this work, a mixed free-space optics (FSO)/radio-frequency (RF) based multiple serial high altitude platforms (HAPs) assisted multiuser multiantenna terrestrial communication system is considered. For the considered multi-hop system, earth station to HAP and HAP to HAP links are assumed as FSO links, and HAP to terrestrial mobile users (MUs) link is assumed as RF link. At the FSO detector both the heterodyne detection and intensity modulation direct detection techniques are considered. Atmospheric turbulence of the ES to HAP FSO link is modeled with Gamma-Gamma fading along with the pointing error impairments. As HAP situates in the stratosphere, negligible atmospheric turbulence exists at HAP altitude (20 km), hence, only pointing error is considered for the inter HAP links. Further, the HAP consists of multiantenna array to provide high data-rates to the terrestrial users via RF links, considered to be Nakagami-m distributed. The intermediate HAPs perform selective decode-and-forward relaying and opportunistic user scheduling is performed for the best user selection. For the performance analysis, analytical expression of overall outage probability is obtained and the impact of various selection parameters like pointing error, FSO detection type, MU selection, number of antennas, and RF fading severity are observed on the outage performance. Asymptotic outage probability is also derived to obtain the diversity order of the considered communication system. Further, considering various modulation schemes, a generalized average bit-error-rate expression is derived and the impact of pointing error, FSO detection type, MU selection, number of antennas, and RF fading severity are observed on its performance. Finally, derived results are validated through Monte-Carlo simulations.

Signal Space Diversity-Based Distributed RIS-Aided Dual-Hop Mixed RF-FSO Systems

Reconfigurable intelligent surface (RIS) is a recently emerged promising technology for beyond-5G (B5G)/6G wireless networks. In this letter, we propose to employ a signal space diversity (SSD) technique to improve the performance of distributed RIS-aided dual-hop mixed radio frequency (RF)-free-space optical (FSO) communication systems. The source-relay, source-RIS, and RIS-relay links are assumed to undergo independent but not identically distributed (i.n.i.d.) 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. The destination node is equipped with multiple FSO apertures and the relay-destination links follow Gamma-Gamma (GG) distributed atmospheric turbulence (AT) model with pointing errors (PEs). Novel approximate closed-form expressions for the system’s symbol error rate (SER) are derived for both the exhaustive RIS-aided (ERA) and opportunistic RIS-aided (ORA) configurations. The numerical results show that using the SSD technique at both the RF and FSO links significantly improves the spectral efficiency and the diversity order of the system considered.

Multihop radio and optical wireless relaying systems over EGK, DGG, and CU fading channels

Optical wireless communications (OWC) systems provide higher data rates in the unlicensed spectrum bands compared to traditional radio frequency (RF) wireless communications systems. OWC systems are categorized as free space optical (FSO) communications and visible light communications (VLC) for outdoor and indoor environments, respectively. In this paper, we propose a multihop serial RF, FSO, and VLC system and investigate the link performance for five different scenarios with amplify-and-forward and decode-and-forward relaying protocols under different channel conditions. Based on the SNR statistics, we show closed-form expressions for the outage probability for all scenarios and derive their diversity orders. In addition, we examine the accuracy and correctness of the derived expressions by means of Monte Carlo simulations. We further prove that the diversity order is a function of the severity of the RF links and the shaping parameters, the atmospheric turbulence, the pointing error in FSO links, the detection method, and number of links. Finally, the presented results complement and extend several results previously reported in the literature.

TRW-MAC: A thermal-aware receiver-driven wake-up radio enabled duty cycle MAC protocol for multi-hop implantable wireless body area networks in Internet of Things

Implantable Wireless Body Area Network (IWBAN), a network of implantable medical sensors, is one of the emerging network paradigms due to the rapid proliferation of wireless technologies and growing demand of sophisticated healthcare. The wireless sensors in IWBAN is capable of communicating with each other through radio frequency (RF) link. However, recurring wireless communication inside the human body induces heat causing severe thermal damage to the human tissue which, if not controlled, may appear as a threat to human life. Moreover, higher propagation loss inside the human body as well as low-power requirement of the sensor nodes necessitate multi-hop communication for IWBAN. A IWBAN also requires meeting certain Quality of Service demands in terms of energy, delay, reliability etc. These pressing concerns engender the design of TRW-MAC: A thermal-aware receiver-driven wake-up radio enabled duty cycle MAC protocol for multi-hop IWBANs in Internet of Things. TRW-MAC introduces a thermal-aware duty cycle adjustment mechanism to reduce temperature inside the body and adopts wake-up radio (WuR) scheme for attaining higher energy efficiency. The protocol devises a wake-up estimation scheme to facilitate staggered wake-up schedule for multi-hop transmission. A superframe structure is introduced that utilizes both contention-based and contention free medium access operations. The performance of TRW-MAC is evaluated through simulations that exhibit its superior performance in attaining lower thermal-rise as well as satisfying other QoS metrics in terms of energy-efficiency, delay and reliability.

Performance of UAV-Assisted Multiuser Terrestrial-Satellite Communication System Over Mixed FSO/RF Channels

In this letter, performance of a multiantenna multiuser unmanned aerial vehicle (UAV)-assisted terrestrial-satellite communication system over mixed free space optics (FSO)/radiofrequency (RF) channels is analyzed. Downlink transmission from the satellite to the UAV is completed through FSO link which follows gamma&#x2013;gamma distribution with pointing error impairments. Both the heterodyne detection and intensity modulation direct detection techniques are considered at the FSO receiver. To avail the antenna diversity, multiple transmit antennas are considered at the UAV. Selective decode-and-forward scheme is assumed at the UAV and opportunistic user scheduling is performed while considering the practical constraints of outdated channel state information (CSI) during the user selection and transmission phase. The RF links are assumed to follow Nakagami-m distribution due to its versatile nature. In this context, for the performance analysis, analytical expressions of outage probability, asymptotic outage probability, ergodic capacity, effective capacity, and generalized average symbol-error-rate expressions of various quadrature amplitude modulation (QAM) schemes such as hexagonal-QAM, cross-QAM, and rectangular QAM are derived. A comparison of various modulation schemes is presented. Further, the impact of pointing error, number of antennas, delay constraint, fading severity, and imperfect CSI are highlighted on the system performance. Finally, all the analytical results are verified through the Monte&#x2013;Carlo simulations.

Performance of dual-hop FSO/RF systems with fixed-gain relaying over Fisher-Snedecor F and κ-μ shadowed fading channels.

We investigate the performance of a dual-hop mixed free space optical (FSO) /radio frequency (RF) system with fixed-gain relaying under direct detection and heterodyne detection techniques. The FSO link is modeled by the Fisher-Snedecor F distribution, which matches well with the experimental data under weak-to-strong turbulence regimes. The RF link experiences κ-μ shadowed fading, which unifies popular RF fading models. The κ-μ shadowed distribution is approximated by an α-μ distribution. Capitalizing on this approximation, closed-form approximate expressions for the cumulative distribution function, the average bit error rate of different modulation schemes, and the ergodic capacity are derived in terms of the bivariate Fox's H function. Moreover, asymptotic analysis is carried out at a high signal-to-noise ratio to further illustrate the obtained diversity order and the influence of system and channel parameters. Numerical results and Monte Carlo simulations are presented to validate the derived approximate expressions.

On the Performance of Mixed FSO/RF SWIPT Systems With Secrecy Analysis

In this article, we consider secure communication over asymmetric dual-hop simultaneous wireless information and power transfer system having one free-space optical (FSO) link followed by one radio frequency (RF) link. We assume that the optical transmitter conveys secure information to the RF receiver through an intermediate relay, which exploits the decode-and-forward relaying technique. The FSO link encounters Málaga (M) distributed turbulence with pointing error, and RF link suffers double shadowed Rician fading. We investigate the consequences of pointing error, atmospheric turbulence, and energy harvesting on the performance of the proposed mixed FSO/RF SWIPT framework. Specifically, we derive the analytical expressions for statistical characteristics such as probability density function, amount of fading, average bit error rate (BER), outage probability (OP), and ergodic capacity. The asymptotic expression for OP is also derived to obtain the diversity order of the system. Based on these results, the physical layer security analysis is carried out, and the analytical expressions for secure OP and strictly positive secrecy capacity are derived. Herein, the two eavesdroppers are considered. In the first scenario, it is assumed that only the FSO link is overheard, while in the second scenario, both FSO and RF links are under simultaneous eavesdropping attack.

A Weather-Dependent Hybrid RF/FSO Satellite Communication for Improved Power Efficiency

Recent studies have shown that satellite communication (SatCom) will have a fundamental role in the next generation non-terrestrial networks (NTN). In SatCom, radio frequency (RF) or free-space optical (FSO) communications can be used depending on the communication environment. Motivated by the complementary nature of RF and FSO communication, we propose a hybrid RF/FSO transmission strategy for SatCom, where the satellite selects RF or FSO links depending on the weather conditions obtained from sensors and used for the context-awareness. To quantify the performance of the proposed network, we derive the outage probability (OP) expressions by considering different weather conditions. Moreover, asymptotic analysis is conducted to obtain the diversity order. Furthermore, we investigate the impact of non-zero boresight pointing errors and illustrate the benefits of the aperture averaging to mitigate the effect of misalignment and atmospheric turbulence. Finally, we suggest effective design guidelines that can be useful for system designers. The results have shown that the proposed strategy performs better than the dual-mode conventional hybrid RF/FSO communication in terms of OP offering some power gain.

Self-Optimizing Data Offloading in Mobile Heterogeneous Radio-Optical Networks: A Deep Reinforcement Learning Approach

In addition to the exploration of more spectrum at high-frequency bands, next-generation wireless networks will witness an intelligent convergence of radio frequency (RF) and non-RF links such as optical and visible light communication. Optical attocell (OAC) networks provide an additional layer to RF-based wireless networks with gigabit-per-second data transmission rate and centimeter-level location accuracy. However, the directionality, line-of-sight constraints, as well as strong sensitivity to the location and orientation of user terminals challenge the stringent requirements for throughput and latency. In this article, we consider mobile heterogeneous networks (HetNets) incorporating indoor OAC with femtocells and macrocells to provide a low-cost and energy-efficient solution. The HetNets solution satisfies diverse service requirements in terms of user-experienced data rate, mobility, latency, accuracy, and security in the Internet of Things. To support seamless connectivity and optimal resource allocation in the proposed HetNets with mobility awareness, handover in dynamic environments needs to be addressed efficiently. Incorporating rich environmental parameters into such a decision making problem facilitates the self-optimization process, but extensively expands the state space. To achieve a fast convergence speed, a deep reinforcement learning approach is proposed to optimize the handover parameters (e.g., time-to-trigger and hysteresis margin). This is a model-free and off-policy reinforcement setting that trains and employs a deep neural network to predict future rewards for successions of states and actions. Thus, the optimal parameters are obtained by selecting the best actions to take. Through numerical simulation and performance analysis, we discover the gain from enriching the state space and the adaptability of the system to dynamic environments.

On the performance of a relay assisted hybrid RF-NLOS UVC system with imperfect channel estimation

Non-line-of-sight (NLOS) ultraviolet communication (UVC) is emerging as an attractive optical wireless communication technology that enables wireless connectivity in radio-frequency (RF) prohibited areas with no LOS availability. NLOS UVC, however, suffers from a very high path loss, thereby restricting its usage to smaller link distances. In this paper, we address the challenge of providing long-distance wireless connectivity to RF prohibited areas by mixing NLOS UVC with RF communication using a decode-and-forward relay. The RF link is modeled using Rayleigh distribution, and the NLOS UV link is modeled using lognormal distribution under weak turbulence conditions. A framework for analytical expressions of the outage probability and probability density function (PDF) of the end-to-end signal-to-noise ratio is presented by considering the practical scenario of imperfect channel state information (CSI) at the receiver. Subsequently, a PDF based novel closed-form analytical expression of the average symbol error rate is deduced for spectrally efficient higher-order modulation schemes, including rectangular quadrature modulation (RQAM), square QAM (SQAM), cross-QAM (XQAM), and hexagonal QAM (HQAM). Numerical investigations are conducted, and the impact of CSI imperfections on the system performance is evaluated. It is shown that the RF link is more vulnerable to channel estimation error (CEE) than the NLOS UV link. Further, it is illustrated that for constellation sizes greater than four, HQAM always performs better than the RQAM, SQAM, and XQAM schemes, irrespective of the amount of CEE present. Furthermore, it is shown that an elevation angle of 70° or less in the NLOS UVC link results in better outage performance. Correctness of the derived analytical expressions is supported through extensive Monte Carlo simulations.

On the Capacity Performance of Hybrid FSO/RF System With Adaptive Combining Over Generalized Distributions

This paper investigates the ergodic capacity performance of hybrid free space optics (FSO)/ radio frequency (RF) system by employing an adaptive-combining-based switching scheme. In the adaptive combining scheme, the FSO and RF links are combined using the maximal-ratio combining (MRC) at the output, when the instantaneous signal-to-noise ratio (SNR) of the FSO link drops below a predefined threshold SNR value. The FSO link experiences the atmospheric turbulence, which is modeled using the generalized Malaga distribution, non-zero boresight misalignment or pointing errors, and atmospheric attenuation, under two types of detection techniques, i.e. heterodyne detection (HD) and intensity modulation/direct detection (IM/DD). Similarly, the fading of the RF link is characterized using the generalized <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\kappa$</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> distribution. In particular, we derive the probability density function (PDF) of the instantaneous SNR of the adaptive combining scheme. Capitalizing on the SNR statistics, we obtain the unified closed-form ergodic capacity expressions for the adaptive-combining-based hybrid FSO/RF system. In addition, the asymptotic expressions for the ergodic capacity of the system at the high-SNR region are presented. All the derived analytical expressions are validated by using Monte-Carlo simulations. Numerical results and discussions are provided to compare the ergodic capacity performance of the adaptive-combining-based hybrid FSO/RF system with the existing system models such as single-link FSO, MRC-based hybrid FSO/RF, and hard-switching-based hybrid FSO/RF systems.

Secure Transmission for Energy Efficient Parallel Mixed FSO/RF System in Presence of Independent Eavesdroppers

In this paper, the physical layer security (PLS) challenges for a decode and forward (DF) based dual hop mixed free space optical (FSO) / radio frequency (RF) communication system in the presence of multiple eavesdroppers has been investigated. In order to improve the PLS of the energy efficient co-operative communication system, diversity combining techniques have been introduced at the destination node. In addition to this, collusion and non collusion eavesdropper scenarios are considered to make the system more practical. The FSO and RF links are assumed to experience independent 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> fading distributions, respectively. Additionally, the FSO link model considers the impact of boresigt and non-zero boresight pointing errors and type of optical demodulation scheme. To investigate the security performance, the lower bound for the secrecy outage probability (SOP) and effective secrecy throughput (EST) have been obtained in terms of Meijer’s G function. Thereafter, asymptotic expressions have been derived for the considered system and the numerical results have been verified by the Monte-Carlo simulation method. Finally, we have analyzed the secrecy energy efficiency (SEE) based on secrecy rate ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$R_s$</tex-math></inline-formula> ) of the considered system to provide practical insights into the effect of major parameters on the energy efficient secure transmission design.

Secure reconfigurable intelligent surface aided heterogeneous VLC–RF cooperative NOMA networks

In this paper, we investigate the physical layer security for a reconfigurable intelligent surface (RIS)-aided heterogeneous visible light communication (VLC) and radio-frequency (RF) network with two legitimate user equipments (UEs) in cooperative nonorthogonal multiple access (NOMA). An external eavesdropper (Eve) tries to eavesdrop the information from the cooperative channel. Since VLC is mainly available for line-of-sight path, and the UEs and Eve under the worktable cannot directly receive signals from the optical access point, a device is naturally employed to perform optical–electrical conversion and RF transmission. Further the device and RIS are jointly employed to eliminate blind spots and extend communication coverage. With these settings, the statistical characteristics of the VLC link and RF link are determined based on the feature of the optical channel and the RIS-aided wireless channel, respectively. The close-form expression of the secrecy outage probability for the proposed RIS-aided heterogeneous VLC–RF cooperative NOMA network is derived. Simulation results show the secrecy outage performance of the proposed network depends on not only the reflecting meta-surface number of the RIS, the power allocation based on NOMA, but also the tradeoff design between the optical and electrical parameters.

Hybrid FSO-RF Turbulent Link Performance under Misalignment Aperture Error

A novel hybrid Free Space Optics (FSO)/Radio Frequency (RF) model is proposed which is based on the single decision threshold feedback bit switching technique. The FSO and proposed hybrid FSO-RF links performance are investigated under strong turbulence and misalignment aperture error in terms of the average symbol error rate (ASER) and outage probability. 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 the outage probability and ASER have been derived for various 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 with the hybrid model whereas the obtained results are validated through MC simulations and results are found in good agreement.