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With the completion of the standardization of fifth-generation (5G) networks, the researchers have begun visioning sixth-generation (6G) networks that are predicted to be human-centric. Hence, similar to 5G networks, besides high data rates, providing secrecy and privacy will be the center of attention by the wireless research community. To support the visions beyond 5G and 6G, in this paper we propose a secure radio frequency (RF)-free space optical (FSO) mixed framework under the attempt of wiretapping by an eavesdropper at the RF hop. We assume the RF links undergo α - η - μ fading whereas the FSO link exhibits a unified Málaga turbulence model with pointing error. The secrecy performance is evaluated by deducing expressions for three secrecy metrics i.e. average secrecy capacity, secrecy outage probability, and probability of non-zero secrecy capacity in terms of univariate and bivariate Meijer's G and Fox's H functions. We further capitalize on these expressions to demonstrate the impacts of fading, atmospheric turbulence, and pointing errors and show a comparison between two detection techniques (i.e. heterodyne detection (HD) and intensity modulation with direct detection (IM/DD)) that clearly reveals better secrecy can be achieved with HD technique relative to the IM/DD method. The inclusion of generalized fading models at the RF and FSO hops offers the unification of several classical scenarios as special cases thereby exhibiting a more generic nature relative to the existing literature. Finally, all the analytical results are corroborated via Monte-Carlo simulations. [ABSTRACT FROM AUTHOR]

A less-complex and efficient time-hysteresis switching algorithm assisted hybrid free-space optics (FSO)/millimeter wave (mm)-radio frequency (RF) system is presented in the paper. A time-hysteresis switching algorithm is utilized for link transient for selection of FSO or RF links. For improving the long-haul communication efficiency and coverage a selective decode-and-forward relay scheme is blended with hybrid FSO/RF system along with maximal-ratio-combining (MRC) for signal reconstruction at the receiver end. However, the pointing errors due to misalign apertures and building sway, which have the potential to suppress the benefits of the relay assisted hybrid system. The impact of pointing errors and path loss attenuation are modeled with Gamma–Gamma distribution and generalised Nakagami-m is used to model the RF channel fading. The exact closed-form expressions of average symbol-error-rate (ASER) and outage-probability (OP) have been derived for the performance evaluation of the proposed system with and without pointing errors. We also present an asymptotic-analysis of the ASER and outage with lower computational-complexity for the proposed system and obtain the diversity order. To achieve target ASER value for the proposed switching scheme, the optimum-threshold signal-to-noise ratio (SNR) value has been obtained numerically in this work. The derived theoretical outcomes have been validated with Monte-Carlo simulation. In addition, we present comparative results of ASER and OP performance for proposed system with Gamma–Gamma and generalised Málaga distribution under various adverse weather conditions. It is observed that the proposed hybrid FSO/RF system outperforms a cooperative single FSO system as well as single-hop hybrid FSO/RF system on account of diversity as well as switching technique benefits. The presented hybrid FSO/RF system achieves ASER of 10–7 in the lower SNR regimes. [ABSTRACT FROM AUTHOR]

In this paper, investigation of mixed radio frequency (RF)/ free space optical (FSO) decode and forward (DF) two way relaying (TWR) has been performed. It has been assumed that the relay hardwares involved are prone to aggregated hardware impairments and operate near the vicinity of multiple co-channel interferers (CCIs). In order to combat the impact of hardware impairment and interference, double relay selection scheme has been employed in the system model. Therefore, the cooperation between the RF and FSO source nodes takes place in two different paths to establish TWR. The fading characteristics of the RF and CCI links have been modeled using the Rayleigh distribution. The atmospheric turbulence on the FSO link has been assumed to undergo Malaga irradiance model. For the considered system, analysis of outage probability has been performed. In fact, the closed-form for the outage probability has been derived in terms of Meijer-G function. The derived expressions are then approximated for the high signal to noise ratio (SNR) by exploiting the asymptotic analysis of outage probability. Results are present to confirm that the reliability of the communication channel is impacted by number of nodes available for selection, type of optical demodulation, atmospheric turbulence, pointing error, interference and hardware impairment. The analytical results are confirmed using the Monte-Carlo simulations. [ABSTRACT FROM AUTHOR]

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.) $\alpha -\mu$ distribution is considered for each branch in the RF link, whereas the FSO link is assumed to experience Málaga ($\mathcal {M}$) 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. [ABSTRACT FROM AUTHOR]

Free-space optics (FSO) communication is often unreliable, especially for long distances, as it suffers from attenuation due to foggy weather, atmospheric turbulence, and pointing errors. To avoid communication link failure, radio frequency (RF) communication, which is unaffected from these effects, is utilized when FSO communication is in outage. Diversity is known to counteract the adverse fading effects, therefore, this paper presents a transmit aperture/antenna selection (TAS) and selection combining (SC)-based multiple-input-multiple-output (MIMO) hybrid FSO/RF system. The proposed system consists of a MIMO FSO and RF sub-systems. The MIMO FSO sub-system is given higher priority to transmit and receive using the selected best link. The best link of the MIMO RF sub-system is used when the FSO sub-system is in outage. The probability density function (PDF) and cumulative distribution function (CDF) expressions for TAS/SC-based MIMO FSO and RF sub-systems are derived. The derived PDF and CDF expressions are used further to derive the exact and asymptotic expressions for outage probability, average symbol error rate (SER), and capacity of the hybrid system. From the asymptotic SER expressions, the diversity order of the proposed system is determined. The optimum values for the threshold SNR and transmit beam waist, which minimize the average SER, are obtained. Further, the obtained optimal values using analytical expressions are verified using numerical optimization technique. The effect of multiple apertures/antennas, pointing errors, and atmospheric attenuation over the system performance is analyzed extensively by varying the FSO and RF channel parameters. The results show that the proposed hybrid FSO/RF system is more reliable compared to the individual MIMO FSO and RF systems and achieves higher capacity compared to the MIMO RF system. [ABSTRACT FROM AUTHOR]

In this work, we provide a framework analysis of dual-hop hybrid Millimeter Wave Radio Frequency (RF)/Free Space Optical (FSO) MIMO relaying system. The source is equipped with multiple antennas and employs conjugate beamforming while the destination consists of multiple apertures with selection combining. The system also consists of a relay operating at amplify-and-forward mode. The RF channels are subject to Nakagami-m fading while the optical links experience the Málaga distribution. In addition, we introduce the impairments to the relay and receiver. In fact, the relay is impaired by the High Power Amplifier (HPA) nonlinearities while the receiver suffers from the In phase and Quadrature Imbalance. Moreover, we assume two types of HPA nonlinearities impairments called Soft Envelope Limiter (SEL) and Traveling Wave Tube Amplifier (TWTA). Closed-forms of the outage probability, the bit error probability, and the ergodic capacity are derived. Capitalizing on these performances, we derive the high SNR asymptotes to unpack insightful metrics such as the diversity gain. We also address the impacts of some key factors on the system performance such as the impairments, the interferers, the number of antennas and apertures and the pointing errors, etc. Finally, the analytical expressions are confirmed by Monte Carlo simulation. [ABSTRACT FROM AUTHOR]

In this work, we analyze the secrecy outage performance of a dual-hop relay system composed of multiple-input-multiple-output radio-frequency (RF) links and a free-space optical (FSO) link while a multiple-antenna eavesdropper wiretaps the confidential information by decoding the received signals from the source node. The channel state information (CSI) of the RF and FSO links is considered to be outdated and imprecise, respectively. We propose four transmit antenna selection (TAS) schemes to enhance the secrecy performance of the considered systems. The secrecy outage performance with different TAS schemes is analyzed and the effects of misalignment and detection technology on the secrecy outage performance of mixed systems are studied. We derive the closed-form expressions for probability density function (PDF) and cumulative distribution function (CDF) over Málaga channel with imperfect CSI. Then the closed-form expressions for the CDF and PDF of the equivalent signal-to-noise ratio (SNR) at the legitimate receiver over Nakagami- $m$ and Málaga channels are derived. Furthermore, the bound of the effective secrecy throughput (EST) with different TAS schemes are derived. Besides, the asymptotic results for EST are investigated by exploiting the unfolding of Meijer’s $G$ -function when the electrical SNR of FSO link approaches infinity. Finally, Monte-Carlo simulation results are presented to testify the correctness of the proposed analysis. The results illustrate that outdated CSI shows a strong effect on the secrecy performance of the mixed RF-FSO systems. In addition, increasing the number of antennas at the source cannot significantly enhance the secrecy performance of the considered systems. [ABSTRACT FROM AUTHOR]

We investigate the performance of mixed free-space optical (FSO)/millimeter-wave (mmWave) relay networks with interference at the destination. The FSO/mmWave channels are assumed to follow Málaga- $\mathcal {M}$ /generalized- $\cal K$ fading models with pointing errors in the FSO link. The H-transform theory, wherein integral transforms involve Fox’s H-functions as kernels, is embodied to unifying the performance analysis framework that encompasses closed-form expressions for the outage probability, the average bit error rate (BER), and the average capacity. By virtue of some H-transform asymptotic expansions, the high signal-to-interference-plus-noise ratio (SINR) analysis reduces to easy-to-compute expressions for the outage probability and BER, which reveals inside information for the system design. We finally investigate the optimal power allocation strategy that minimizes the outage probability. [ABSTRACT FROM AUTHOR]