Radio Frequency/free Space Optical Research Articles

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.

On secrecy analysis of wireless-powered relaying FSO-RF systems

We investigate the physical-layer security performance for a wireless-powered relaying mixed free-space optical-radio frequency (FSO-RF) system. In this system, a source transmits optical information to a relay (R), which decodes the received signal with a decode-and-forward protocol, converts it into an RF signal, and forwards it to a destination in the presence of an eavesdropper. Furthermore, we also assume that R has no extra power supply, and it only relies on harvesting energy from a power beacon to forward the information. Assuming that the FSO and all of the RF channels experience gamma-gamma and Rayleigh fading, the closed-form analytical expressions for secrecy outage probability and the probability of strictly positive secrecy capacity are derived. In addition, the effects of atmospheric turbulence, pointing errors, and detection techniques on the system performance are analyzed. Finally, we perform Monte Carlo simulations to verify the accuracy of the derived expressions.

Performance analysis of beam divergence propagation through rainwater and snow pack in free space optical communication

In the present work the future communication requirements need to fulfill with high data rate, FSO (free space optic) with it is tremendous potential is the solution. This research observed the effectiveness analysis of FSO systems by modifying one of the most important FSO parameters beam divergence, under the most affected weather attenuating condition Rainwater and snow pack. The simulation is obtained and analyzed under single channels CSRZ-FSO (carrier-suppressed return-to-zero/free space optical) systems having capacity of 40 Gbps between two transceivers with variable distance. The connection is presently under 5 meteorological turbulences (light rain, medium rain, wet snow, heavy rain and dry snow). The results show the heavy rain and dry snow have a very high attenuation carried out in terms of Q-factor. this result led us to conclude that small divergence offers significant performance improvement for FSO link and this performance decrease every time the beam divergence increase, Therefore, to build inexpensive and reliable transmission media, we go with new method that still in the experiment area called hybrid RF/FSO (radio frequency/free space optical) that compatible with atmospherically status.

Performance analysis of hybrid FSO/RF communication over generalized fading models

Hybrid free space optics/radio frequency (FSO/RF) communication has been considered as a promising solution for reliable wireless backhaul connectivity for 5G and beyond wireless communication systems. In this paper, comprehensive performance analysis of the hybrid FSO/RF system has been carried out considering single-threshold-based switching strategy for both terrestrial and satellite communication scenarios. We assume that the channel state of the FSO link follows generalized Malaga distribution with pointing errors and atmospheric attenuation. Further, the RF channel fading has been modeled using generalized α-η-κ-μ distribution, which includes most of the well-known RF fading distributions as special cases. We derive the unified closed-form expressions for outage probability, average symbol error rate (SER) and ergodic capacity of the hybrid FSO/RF system considering two types of detection techniques, i.e. heterodyne detection (HD) and intensity modulation/direct detection (IM/DD). Additionally, we determine the range of optimum switching threshold and optimum beam waist values using a numerical optimization method for achieving optimal performance. We also derive simpler asymptotic expressions in terms of elementary functions to obtain diversity gain and coding gain of the hybrid FSO/RF system. The theoretical results of the hybrid FSO/RF system, which are verified using Monte-Carlo simulations, unveil that under strong turbulence, high pointing errors, and adverse weather conditions, the performance of the hybrid FSO/RF system over FSO system is improved significantly due to RF backup link.

Performance Analysis of Two-Way Full-Duplex Relay Mixed RF/FSO System With Self-Interference

In this letter, a two-way full-duplex (FD) relay mixed radio frequency/free space optical (RF/FSO) system corrupted by self-interference is proposed and investigated. The RF/FSO channels employ Nakagami-m distribution and pointing errors combined Gamma-Gamma turbulence model, respectively. Relay selection strategies based on channel state information (CSI) are applied. Considering the effect of self-interference, the exact outage probability (OP) and upper bound achievable sum-rate (ASR) are presented and evaluated. To make the research more comprehensive, the analysis further covers the effect of residual self-interference on the considered system as well as compares the performance difference of FD relay and half-duplex (HD) relay. To get more insight, asymptotic OP is further presented.

Secrecy Performance Analysis of Mixed α − μ and Exponentiated Weibull RF-FSO Cooperative Relaying System

Increasing concerns regarding wireless systems’ security are leading researchers to exploit the physical properties of a medium while designing any secured wireless network. The secrecy performance of a mixed radio frequency-free space optical (RF-FSO) system with a variable gain relaying scheme is investigated in this paper under the attempt of wiretapping by an eavesdropper. We assume that the eavesdropper can intrude the target data from the RF link only. Both the RF links (main and eavesdropper) undergo the $\alpha -\mu $ fading statistics and the FSO link experiences the exponentiated Weibull fading statistics. Exploiting the amplify-and-forward (AF) relaying scheme while considering two detection techniques (i.e. heterodyne detection and intensity modulation/direct detection) with pointing error impairments, the mathematical formulations of the unified probability density function and cumulative distribution function are performed for the equivalent signal-to-noise ratio of the considered dual-hop RF-FSO link. Closed-form analytical expressions for average secrecy capacity, secrecy outage probability, and the probability of non-zero secrecy capacity are derived in terms of Meijer’s $G$ and Fox’s $H$ functions to quantify the system performance. Capitalizing on these expressions, the secrecy performance is further analyzed for various channel parameters of RF links, aperture sizes of the receiver, pointing errors, and atmospheric turbulence severity. The results reveal that aperture averaging can improve the secrecy performance remarkably by suppressing the effects of turbulence. Monte Carlo simulations are provided to justify the accuracy of the proposed model.

Physical Layer Security Performance Analysis of Hybrid FSO/RF Communication System

In this paper, the secrecy performance of the physical layer security (PLS) of the hybrid free-space optical/radio frequency (FSO/RF) communication system is analyzed. The transmission protocol of the considered system is performed under the eavesdropper’s attempt to overhear the RF link between the transmitter and legitimate receiver of the hybrid system. The FSO link is characterized by Malaga-M distribution while the RF links are modeled by Nakagami- $m$ distribution. The two practical eavesdropping modes considered in this paper include: colluding and non-colluding. Exact closed-form expressions for the system’s secrecy outage probability (SOP), the asymptotic of the SOP (SOP $^{\infty }$ ), the probability of strictly positive secrecy capacity (SPSC), the average secrecy capacity (ASC), and the asymptotic of the ASC (ASC $^{\infty }$ ), are specifically derived under the influence of both eavesdropper modes. Our derived analytical expressions present an efficient tool to investigate the impact of some channel parameters on the secrecy performance, namely the fading severity of the RF links, atmospheric turbulence severity, pointing error of the FSO link, number of eavesdroppers, and the power of the eavesdropper links. The results show that the increase of the eavesdroppers’ number under both modes profoundly degrades the considered system secrecy performance. The accuracy of the numerical results obtained is validated by Monte-Carlo simulations.

Outage probability of multi-aperture and multi-antenna wireless-powered relaying assisted FSO-RF systems with a nonlinear energy harvester.

We consider the outage performance of a mixed free-space optical-radio frequency (FSO-RF) communication system, which consists of a source (S), a relay (R), a destination (D), and a power beacon (PB). In particular, S communicates with D through a multi-aperture and multi-antenna decode-and-forward (DF) relay, which is assumed to harvest energy from the PB under a nonlinear energy harvesting mode. Considering an equal gain combining scheme and a transmit antennas selection scheme, the exact expressions of probability density function (PDF) and cumulative distribution function (CDF) for instantaneous signal-to-noise ratio are first obtained, and then based on those PDF and CDF expressions, the analytical and asymptotic expressions for outage probability are derived. Finally, Monte Carlo simulation results are used to verify the accuracy of the derived analytical expressions.

Impact of RF I/Q Imbalance on Interference-Limited Mixed RF/FSO TWR Systems With Non-Zero Boresight Error

In this letter, we investigate a generic model assessing the effect of in-phase/quadrature-phase imbalance (IQI) on an asymmetric dual hop radio frequency/free space optical (RF/FSO) two-way relay (TWR) system in the presence of multiple co-channel interferers (CCIs) at the relay. The fading on the RF and FSO links have been modeled using K-distribution and double generalized Gamma (D-GG) turbulence model, respectively. The impact of non-zero boresight pointing error and type of optical demodulation schemes have been considered on the FSO link. To this end, a closed-form probability density function (PDF) has been derived for the FSO link undergoing D-GG irradiance with non-zero boresight pointing error. Furthermore, the exact and high signal-to-noise (SNR) asymptotic expression for outage probability have been presented. To gain insights into the throughput of the system, approximate and asymptotic expressions for the achievable sum rate (ASR) have been derived. The results show dependency of reliability and throughput offered on IQI at the RF front-end and strength of interference for the considered TWR system.

Throughput Maximization of Mixed FSO/RF UAV-Aided Mobile Relaying With a Buffer

In this paper, we investigate a mobile relaying system assisted by an unmanned aerial vehicle (UAV) with a finite size of the buffer. Under the buffer size limit and delay constraints at the UAV relay, we consider a dual-hop mixed free-space optical/radio frequency (FSO/RF) relaying system (i.e., the source-to-relay and relay-to-destination links employ FSO and RF links, respectively). Taking an imbalance in the transmission rate between RF and FSO links into consideration, we address the trajectory design of the UAV relay node to obtain the maximum data throughput at the ground user terminal. Specifically, we classify two relaying transmission schemes according to the delay requirements, i.e., i) delay-limited transmission and ii) delay-tolerant transmission. Accordingly, we propose an iterative algorithm to effectively obtain the locally optimal solution to our throughput optimization problems and further present the complexity analysis of this algorithm. Through this algorithm, we present the resulting trajectories over the atmospheric condition, the buffer size, and the delay requirement. In addition, we show the optimum buffer size and the throughput-delay tradeoff for a given system. The numerical results validate that the proposed buffer-aided and delay-considered mobile relaying scheme obtains 223.33% throughput gain compared to the conventional static relaying scheme.

Threshold optimization for modified switching scheme of hybrid FSO/RF system in the presence of strong atmospheric turbulence

In hybrid free space optical/radio frequency (FSO/RF) systems, switching between the optical and RF links depends on the optical and RF signal to noise ratio (SNR) threshold levels; therefore, the correct choice of these threshold levels is essential to make best use of the complementary nature of optical and RF links. Optimal selection of these threshold levels is the prime objective of this paper. A hybrid FSO/RF system using modified switching scheme with two optical and one RF threshold is considered. Here, we propose an algorithm to obtain the optimized thresholds, which yield minimum bit error rate (BER) for a given outage probability under a constraint on maximum transmitted power. The optimized thresholds are dependent on the average optical and RF SNR. Hybrid FSO/RF system use both optical as well as RF power. The average system SNR is computed by considering the actual optical and RF power consumption. Also the performance of the hybrid FSO/RF system is studied with respect to average system SNR using optimized optical and RF thresholds. This analysis reveals the existence of a critical operating point of average optical SNR, beyond which the system should be preferably operated to ensure minimum BER.

Outage analysis of wireless-powered relaying FSO-RF systems with nonlinear energy harvesting

In this paper, the outage analysis of a wireless-powered relaying based dual-hop mixed free space optical-radio frequency (FSO-RF) system, which consists of a source (S), a relay (R), a destination (D), and a power beacon (PB), is studied. Specifically, we consider the case where S sends an optical signal to R, R processes the received signal, converts it into a RF signal, and transmits it to D with the energy harvested from PB. Assuming R is with a nonlinear energy harvester, the effects of atmospheric turbulence, two types of detection techniques, pointing error, and saturation threshold of the energy harvester on the outage performance are studied. Specifically, the analytical and asymptotic expressions for outage probability as well as the diversity order are derived. In addition, Monte Carlo simulation results are used to verify the accuracy of the derived expressions.

Channel characterization and performance analysis of unmanned aerial vehicle‐operated communication system with multihop radio frequency–free‐space optical link in dynamic environment

SummaryUnmanned aerial vehicles (UAVs) have been widely used in wireless communication in recent years and are a promising component of future wireless systems. UAVs are easily deployable and can provide reliable, robust, and cost‐effective communication to any desired location. However, the performance parameters of UAV cannot attain the desired level due to additional path loss caused by diffraction in non‐line of sight (NLOS) propagation during atmospheric turbulence. In this paper, UAV operated communication system with multihop radio frequency (RF)–free‐space optical (FSO) link is proposed. In order to provide better network connectivity and coverage, relays which also act as low altitude platform are configured between source and destination. These relays are modeled using RF and FSO technologies and also maintain decode and forward protocol and linear network coding. Based on this setup, communication channel is classified into three segments namely, ground to UAV, UAV to UAV, and UAV to ground channel. Initially, the optimal altitude of UAVs is derived using RF and FSO channel parameters for reliable system performance. The proposed model is verified using Monte Carlo simulation method, and the performance of the model is measured in terms of outage probability and symbol error rate, assuming that coherent detection takes place at receiver which employs M‐ary phase shift keying modulation techniques. Numerical results demonstrate that the proposed multihop RF–FSO link outperforms the relevant benchmark schemes for a given signal to noise ratio value.

Secrecy Analysis for Multi-Relaying RF-FSO Systems With a Multi-Aperture Destination

In this paper, we investigate the physical layer security for an asymmetric dual-hop radio frequency-free space optical (RF-FSO) communication system, in which a source transmits information to a multi-aperture destination (D) through a best relay under the wiretapping of a multi-aperture eavesdropper (E). The best relay is selected by a partial relay selection scheme in terms of maximum instantaneous channel gain for the RF link. Moreover, equal gain combining scheme is also used to process the multiple copies of receiving signal at D and E. Assuming all the RF channels are subject to independent and identical distributed (i.i.d.) Nakagami-m fading and all the FSO channels are modeled by i.i.d. Gamma-Gamma process, the secrecy performance in terms of secrecy outage probability and the probability of strictly positive secrecy capacity are studied, and the accuracy of the analytical results is verified by Monte-Carlo simulations.

Secrecy performance of cognitive underlay hybrid RF/FSO system under pointing errors and link blockage impairments

In this paper, we present the secrecy performance of a cognitive underlay hybrid radio frequency/free space optical (RF/FSO) system, where the licensed spectrum of a primary user (PU) is shared with the secondary user (SU) and an Eavesdropper attempts to intercept the information through the SU RF sub-link. At the SU receiver, the selection combining scheme with heterodyne detection is considered as signal reception. In order to ensure the required quality of service at the PU, the transmit power constraint of SU RF-sub link is assumed to depend only on the interference level arriving at the PU. It is also assumed that all the system RF links follow the Nakagami-m distribution except the Eavesdropper link that is subjected to Rayleigh fading. The SU FSO link is assumed to undergo Gamma–Gamma distribution with pointing error and link blockage. Thus, the cumulative distribution function (CDF) of the SU RF sub-link is obtained and used to derived the system equivalent CDF. By utilizing system equivalent CDF, the closed-form expressions for the secrecy outage probability and average secrecy capacity are derived through the lower bound. In addition, Monte-Carlo simulation is provided to verify the accuracy of the derived expressions. The results demonstrate the influence of atmospheric turbulence, pointing error, link blockage and average signal to noise ratio at the Eavesdropper on the system performance.

Statistical Analysis of Noise Propagation Effect for Mixed RF/FSO AF Relaying Application in Wireless Sensor Networks.

In this paper, we investigate the so-called noise propagation effect in a mixed radio-frequency/ free-space optical (RF/FSO) amplifying-and-forwarding (AF) relaying system that is applied for data transmission in wireless sensor networks. The noise propagation could be essentially severe when battery-charged sensor nodes have very limited transmit power. We provide an exact expression on the cumulative distribution function (CDF) of end-to-end signal-to-noise power ratio (SNR) for a dual-hop mixed RF/FSO AF relaying system. We assume a tightly power-constrained amplifying gain at the relay, which has been usually ignored in existing performance studies for the mixed RF/FSO AF system. It however should be considered to properly evaluate the noise propagation effect especially if the relaying power is not infinite or the sensor has a poor budget in transmit power. We apply the derived exact CDF to evaluate the system performances such as outage probability, average bit-error rate, and ergodic capacity. Numerical investigation is used to justify that the proposed analysis is exactly matched with the simulation and shows that the performance gap caused by the inclusion of the noise propagation effect is significant (about 2–12%) especially when the SNR per hop is in the medium- or the low-SNR ranges (i.e., at 10–20 dB).

On Secrecy Performance of Mixed Generalized Gamma and Málaga RF-FSO Variable Gain Relaying Channel

The emergence of an array of new wireless networks has led researchers to evaluate the prospect of utilizing the physical properties of the wireless medium in order to design secure systems. In this paper, the physical layer secrecy performance of a mixed radio frequency-free space optical (RF-FSO) system with variable gain relaying scheme is investigated in the presence of an eavesdropper. We assume that the eavesdropper can wiretap the transmitted confidential data from the RF link only. It is further assumed that the main and eavesdropper RF links are modeled as generalized Gamma (GG) fading channel, and the free space optical (FSO) link experiences Malaga turbulence with pointing error impairment. Our primary concern is to protect this confidential information from being wiretapped. Besides pointing error, the atmospheric turbulence and two types of detection techniques (i.e. heterodyne detection and intensity modulation with direct detection) are also taken into consideration. Utilizing amplify-and-forward (AF) scheme, the novel mathematical closed-form expressions for average secrecy capacity, lower bound of secrecy outage probability, and strictly positive secrecy capacity are derived. As both the links (RF and FSO) undergo generalized fading channels, the derived expressions are also general. We present a unification of some existing works utilizing the proposed model to better clarify the novelty of this work. Finally, all the derived expressions are justified via Monte-Carlo simulations.

Secrecy Performance Analysis of Mixed Hyper-Gamma and Gamma-Gamma Cooperative Relaying System

In this paper, we investigate a secure dual-hop radio frequency-free space optical (RF-FSO) mixed variable gain relaying framework in the presence of a single eavesdropper. The RF and FSO links are modeled with hyper Gamma (HG) and Gamma-Gamma (ΓΓ) fading channels, respectively. We assume that the eavesdropper utilizes another HG fading channel to wiretap the transmitted confidential data from the RF link. Our key concern is to defend this information against passive eavesdropping. We carry out the secrecy measurements by deriving closed-form mathematical expressions of average secrecy capacity (ASC), secure outage probability (SOP), and strictly positive secrecy capacity (SPSC), all in terms of Meijer's G function. Capitalizing on the derived expressions, we analyze the impacts of atmospheric turbulence and pointing errors on the secrecy capacity and outage performance of the proposed scenario. For gaining more insights, we also analyze the asymptotic outage behaviour for high signal-to-noise ratio. Two detection techniques i.e. heterodyne (HD) and the intensity modulation with direct detection (IM/DD) are taken into consideration and our results demonstrate that HD technique notably outperforms the IM/DD scheme. The supremacy and novelty of the model is demonstrated via utilizing generic properties of the HG fading channel. Finally, we provide a justification of the derived expressions via Monte-Carlo simulations.

On the Effect of Incorrect Channel Condition Information on Modified Switching Scheme of Hybrid FSO/RF System

In hybrid free space optical/radio frequency (FSO/RF) communication, the optical link as well as the RF link is switched ON and/or OFF based on the feedback of the channel state information (CSI). However, feedback of full CSI is computationally complex and bandwidth inefficient. A bandwidth efficient form of channel state information which is referred to as Channel Condition Information (CCI) is used as a feedback to decide the modes of transmission/operation in accordance with the switching scheme that is used. Here, modified switching scheme is used, which has four operating modes. CCI is a combination of few bits containing information that is required to identify the mode of transmission of the hybrid FSO/RF system. Incorrect reception of CCI at the transmitter due to noise and fading in the feedback RF link may lead to an erroneous switching between modes of operation. These errors in the mode of transmission/operation lead to degradation in the system performance. In this paper, we have studied the impact of incorrect reception of CCI on the performance of the hybrid FSO/RF system under strong turbulence and fading conditions. The closed form expressions for outage probability and BER are derived.

Wireless Sensor Networks: Towards Resilience Against Weather-Based Disruptions

Abstract The article discusses vulnerability of wireless sensors networks to weather-based disruptions considering the opinions of different experts published in a range of scientific materials. The introduction provides a brief overview of wireless signals in real world conditions focusing on how weather affects signals (rain, fog and clouds, snow, hail, lightning, wind, bodies of water, trees and physical obstruction). Information about the effects of weather on wireless sensor networks using Free Space Optical / Radio Frequency (FSO/RF) communication is then provided. Finally, the impact of weather conditions on MANET routing protocols is considered theoretically, and experimental simulations are performed by comparing the sustainability of different protocols to different weather conditions. After analysis of experiment results, ideas on how to decrease vulnerability of wireless networks to weather-based disruptions are discussed.