Pointing Error Impairments Research Articles

Heterogeneous radio frequency/underwater optical wireless communication relaying systems with MIMO scheme

This paper studies a cooperative relay transmission system within the framework of Multiple-Input Multiple-Output Radio Frequency/Underwater Optical Wireless Communication (MIMO-RF/UOWC), aiming to establish sea-based heterogeneous networks. In this setup, the RF links obey κ-μ fading, while the UOWC links undergo the generalized Gamma fading with the pointing error impairments. The relay operates under an Amplify-and-Forward (AF) protocol. Additionally, the attenuation caused by the Absorption and Scattering (AaS) is considered in UOWC links. The work yields precise results for the Average Channel Capacity (ACC), Outage Probability (OP), and average Bit Error Rate (BER). Furthermore, to reveal deeper insights, bounds on the ACC and asymptotic results for the OP and average BER are derived. The findings highlight the superior performance of MIMO-RF/UOWC AF systems compared to Single-Input-Single-Output (SISO)-RF/UOWC AF systems. Various factors affecting the Diversity Gain (DG) of the MIMO-RF/UOWC AF system include the number of antennas/apertures, fading parameters of both links, and pointing error parameters. Moreover, while an increase in the AaS effect can result in significant attenuation, it does not determine the achievable DG of the proposed MIMO-RF/UOWC AF relaying system.

Performance Analysis of Multi-UAV Optical Communication Systems Over Foggy Channel Under Málaga Turbulence and Pointing Error Impairments

Performance Analysis of Multi-UAV Optical Communication Systems Over Foggy Channel Under Málaga Turbulence and Pointing Error Impairments

Outage Probability and Average BER of UAV-Assisted Dual-Hop FSO Communication With Amplify-and-Forward Relaying

Unmanned aerial vehicles (UAVs) can be harnessed as relay nodes in free-space optical (FSO) communication systems to realize a flexible and cost-effective approach for delivering on-demand communication in next-generation wireless communications. However, fading channels can adversely affect the performance of UAV-assisted dual-hop FSO systems owing to various factors. This paper presents a unified system performance analysis of UAV-assisted dual-hop FSO/FSO systems with the amplify-and-forward relaying protocol and intensity modulation/direct detection technique. For general applicability, the influence of the attenuation loss, atmospheric turbulence, pointing error impairments, and angle-of-arrival (AOA) fluctuations on the FSO link between the source and the UAV is considered. Meanwhile, the FSO link between the UAV and the destination is modeled using Málaga distribution channels, which are affected by atmospheric turbulence. The tight closed-form expressions for the outage probability and the average bit error rate of the dual-hop system are derived, followed with the diversity order of the system in the high signal-to-noise ratio range. Furthermore, the impact of various system parameters and channel parameters correlated with the attenuation loss, atmospheric turbulence, pointing error impairments, and AOA fluctuations on the system performance is studied, and a detailed comparative investigation of the various modulation schemes is conducted. Finally, simulation results are presented to validate the analytical results.

On Effective Secrecy Throughput of Underlay Spectrum Sharing $\alpha -\mu$/ Málaga Hybrid Model Under Interference-and-Transmit Power Constraints

The underlay cognitive radio-based hybrid radio frequency/free-space optical (RF/FSO) systems have emerged as a promising technology due to their ability to eliminate spectrum scarcity and spectrum under-utilization problems. The physical layer security of such a network with a primary user, a secondary source, a secondary receiver, and an eavesdropper is therefore examined in this work. In this network, secret communication occurs between two reliable secondary peers over the RF and FSO links simultaneously, and the eavesdropper can only overhear the RF link. In particular, the maximum transmits power limitation at the secondary user as well as the permissible interference power restriction at the primary user are also taken into consideration. All the RF and FSO links are modeled with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha$</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> fading and Málaga turbulence with link blockage and pointing error impairments. At the receiver, the selection combining diversity technique is utilized to select the signal with the best electrical signal-to-ratio (SNR). Furthermore, to examine the secrecy performance taking into account the effects of each system parameter, closed-form expressions for the secrecy outage probability and effective secrecy throughput are derived. The resultant expressions are finally verified by Monte-Carlo simulations.

Performance Analysis of Smart Grid Wide Area Network With RIS Assisted Three Hop System

In this paper, we investigate the performance of a wide area network (WAN) with three hops over a mixed radio frequency (RF), reconfigurable intelligent surface (RIS) assisted RF and Free space optics (FSO) channel. Here RIS and decode-and-forward (DF) relays are used to improve the coverage and system performance. For general applicability, the RF and FSO links are modelled with Saleh-Valenzuela (S-V) and Gamma-Gamma distribution, respectively. In the smart grid (SG) communication network, the smart meter (SM) communicates the information from the devices to the data aggregator unit (DAU) through the RIS-assisted RF link. As all the devices are not switched on all the time, the Markov chain is used to model the number of active devices. The DAU retransmits the data to the meter data management system (MDMS) through the FSO link, where the atmospheric turbulence and pointing error impairments exist. In this context, closed-form expressions for average bit error rate (ABER) and outage probability (OP) are derived using the H-fox function and extended generalized Bi-variant H-fox function. Here the numerical results demonstrate the effect of different FSO parameters, the number of reflectors in RIS, traffic intensity, and the number of total devices in the SG network.

Dual-Hop Optical Communication Systems Over Málaga Turbulence Under Pointing Error Impairments With Decode-and-Forward Protocol

As a promising technology, free-space optical (FSO) communication plays an important role in the next generation of wireless communication. A dual-hop FSO communication system with decode-and-forward (DF) protocol is proposed in this paper, considering the influences of the atmospheric turbulence (AT), atmospheric absorption and pointing error impairments, in which the Málaga distribution model is used to characterize the fading caused by AT in each link. The end-to-end statistical expressions of our proposed dual-hop Málaga-Málaga communication system are derived, such as the cumulative distribution function (CDF), probability density function and moment generating function. Thereafter, using the above statistical results, the accurate end-to-end expressions of outage probability, average bit error rate (ABER) and ergodic capacity are deduced under the direct detection intensity modulation and heterodyne detection techniques. Furthermore, the end-to-end asymptotic expressions for CDF and the ABER at high signal-to-noise ratio as well as simplified expressions from simple basic functions are obtained. The effects of different AT and pointing error impairments conditions on the proposed system are analyzed based on theoretical and numerical results. Finally, Monte Carlo simulation results indicate that all our novel deduced expressions are basically consistent with the numerical results.

UAV-Assisted RF/FSO Relay System for Space-Air-Ground Integrated Network: A Performance Analysis

Space-air-ground integrated networks (SAGIN) within UAV-assisted free-space optical (FSO) communication systems can efficiently accommodate massive connections and provide highly reliable and seamless communications. In this work, we investigate the performance of a UAV-assisted, asymmetric, dual-hop radio frequency (RF)/FSO system with the amplified-and-forward relay protocol for the SAGIN. Specifically, the shadowed Rician fading is utilized in this study to characterize the shadowing effect on the RF signal for the satellite-to-UAV link. Meanwhile, the atmospheric turbulence effect on the optical signal for the UAV-to-terrestrial user link is modeled by the Málaga distribution fading, in view of the pointing error impairments. For comparison, the heterodyne detection technique is employed, as well as the intensity modulation with direct detection technique, in improving performance of the relay system. Thus, we derive the closed-form expressions for the cumulative distribution function, probability density function, the moment generating function, and particularly, the moments of the end-to-end RF/FSO system, in terms of Meijer’s G-function. Utilizing these derived formulae, the precise closed-form expressions for the outage probability, the average bit error rate (BER) with various modulation schemes, and the ergodic capacity are given. In specific, the tight asymptotic results for the outage probability and the average BER at the high SNR regions are derived using the asymptotic expansion of Meijer’s G-function. Furthermore, closed-form expressions are presented for the case that the FSO link experiences Gamma-Gamma distribution by changing some specific parameters. Finally, extensive numerical results validate the theoretical results with the Monte-Carlo simulation.

Reconfigurable intelligent surfaces-assisted terahertz communications

Reconfigurable intelligent surfaces (RIS) are considered for Terahertz (THz) communication system in this paper. The THz channel is assumed to follow κ−μ−shadowed fading model and performance analysis for the system under study are performed in terms of outage probability, ergodic capacity and average bit error rate (ABER). As well, asymptotic analysis at high but pragmatic signal to noise ratio (SNR) values are obtained for these metrics. In addition, the impact of the channel parameters, κ, μ and the shadowing ms, along with the number of reflecting elements and misalignment error (i.e., pointing error impairments) in RIS is investigated and thoroughly discussed. It is revealed that increasing the number of reflecting surfaces and the channel parameter μ enhances the overall performance; whereas higher ms, κ, operating frequency and misalignment error is shown to severely degrades the performance. It is also reported that the system diversity gain is related to the number of reflecting surfaces, μ, and the pointing error impairment.

Pulse Jamming in Aperture-Averaged FSO Receiver Over Exponentiated Weibull Fading Channel

In this paper, we study the pulse jamming effect thoroughly in aperture-averaged free space optical (FSO) systems. The FSO fading channels are considered to be Exponentiated Weibull (EW) distributed due to its excellent fit for all aperture sizes over the ranges of atmospheric turbulence conditions. It is inferred from the theoretical analysis that random jamming behaviour is impulsive and it reveals a destructive property over aperture-averaged FSO receivers. Analytical expressions of the average bit error rate (ABER) and outage probability (OP) are formulated for the considered FSO system in the presence of random jammer. The effects of pulse jamming on ABER and OP performances are shown. Additionally, the asymptotic ABER analysis is performed in order to investigate the response of the considered FSO system in high transmit power region and to analyze the diversity order under the pulse jamming effect. Pointing error impairments, being another important performance degrading parameter of the FSO communication system, are studied along with the pulse jamming effect. Also, asynchronous jamming effect over the ABER performances is described by both analytically and graphically.

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.

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.

Performance Analysis of Terahertz Communications in Random Fog Conditions With Misalignment

In this letter, we consider Terahertz (THz) communications in the presence of pointing error impairments (misalignment) and under random fog conditions. We derive closed-form expressions, in terms of I-function, for the probability density function (PDF) and the cumulative distribution function (CDF). Then, the PDF and the CDF are employed to derive closed-form expressions for the outage probability, average symbol error rate (SER), and ergodic channel capacity. Additionally, very tight simple approximations in the high signal-to-noise ratio for the outage probability and average SER are provided. The derived expressions are validated through numerical and Monte-Carlo simulation results.

Average symbol error probability and channel capacity of the underwater wireless optical communication systems over oceanic turbulence with pointing error impairments.

The influence of oceanic turbulence and pointing error impairments on the underwater wireless optical communication (UWOC) systems is considered in this study. We propose a generalized fading model, which comprises the path loss due to the absorption and scattering, the oceanic turbulence (modeled by Málaga distribution), and the pointing error impairments resulting from ocean movements. Thereafter, closed-form expressions of the average symbol error probability (SEP) and average channel capacity are proposed for optical waves propagate in oceanic turbulence with the M-ary pulse position modulation (PPM) and under the constraints of the limited average-power and peak-power. The Monte Carlo simulations are conducted to validate the analytical results and demonstrate that the fading parameters, including the mean-squared temperature, the salinity-temperature contribution factor, jitters, and water conditions, significantly affect the system performance. Moreover, the thermal noise and quantum noise in ocean environment have more serious impact than the background noise. Finally, we prove that the UWOC systems with the pure peak-power constraint performs better than that limited by average-power and peak-power.

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.

On the Performance of Mixed FSO-UWOC Dual-Hop Transmission Systems

In this letter, a performance study of a mixed dual-hop communication system consisting of free-space optical/underwater wireless optical communication (FSO-UWOC) links is presented. In particular, considering both heterodyne detection and intensity modulation with direct detection (IM/DD), we assume that the FSO channel suffers a Gamma-Gamma fading in the presence of pointing error impairments, while the UWOC channel experiences a mixture Exponential-Generalized Gamma (EGG) fading with pointing errors. Assuming a fixed-gain amplify-and-forward (AF) protocol, closed-form expressions for the outage probability, average bit-error-rate, and average capacity in terms of the bivariate Fox’s H-function are derived. To verify the derived analytical expressions, numerical results are provided to verify the accuracy of our results.

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.

Performance Analysis of Dual-Hop Wireless Systems Over Mixed FSO/RF Fading Channel

This paper investigates the performance of a dual-hop mixed free-space optical (FSO)/radio frequency (RF) system. To this end, the FSO link is subject to the Gamma-Gamma distribution effects that account for pointing error impairments and under both heterodyne detection and intensity modulation with direct detection (IM/DD) methods, while the RF link undergoes the α- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> distribution fading. For amplify-and-forward (AF) and decode-and-forward (DF) relaying, we derived the closed-form expressions of various important performance metrics by using bivariate Fox's H-functions, such as the average bit-error rate (BER), end-to-end outage probability (OP), effective capacity, and ergodic capacity. We unveil the effects of system and channel parameters on performance. Furthermore, we provide an asymptotical analysis to discuss significant diversity gains. Note that our results include previous works as special cases, which provides a framework. Finally, our derived expressions have confirmed the correctness via exactly matching Monte Carlo simulation results.

Performance Analysis of FSO Communications Over F Turbulence Channels With Pointing Errors

Recently, the Fisher-Snedecor F distribution was proposed to model the turbulence in free-space optical (FSO) communications. However, the existing model does not consider pointing error impairment. To fill this gap, in this letter, we derive novel closed-form expressions for the probability density function (PDF) and cumulative distribution function (CDF) for irradiance fluctuations in the presence of pointing error impairments. Subsequently, the PDF and CDF of the received signal-to-noise ratio (SNR) are derived and employed to obtain novel closed-form expressions for the outage probability, average bit error rate, and average ergodic capacity. To gain more insight into the impact of system and turbulence channel parameters, simple and accurate asymptotic expressions are further derived. Our analytical results are supported by Monte-Carlo simulations to validate the analysis.

Unified Performance Analysis of Multi-Hop FSO Systems Over Double Generalized Gamma Turbulence Channels With Pointing Errors

Free space optical (FSO) communication systems provide high bandwidth in unregulated spectrum and act as a powerful line-of-sight wireless connectivity solution. The performance of FSO systems can be seriously impaired by fading as a result of atmospheric turbulence and/or pointing errors due to misalignment. In the context of FSO systems, relaying was proposed as an effective fading mitigation technique due to the fact that the variance is distance-dependent in turbulence channels. In this article, we present a unified performance analysis of multi-hop FSO systems over Double Generalized Gamma (DGG) turbulence channels with pointing error impairments. We assume amplify-and-forward relaying and consider both heterodyne detection and intensity modulation with direct detection. We derive tight closed-form expressions for the outage probability and bit error probability of both fixed-gain and channel state information (CSI)-assisted relaying in terms of the bivariate Fox-H functions and Fox-H functions, respectively. We further analyze asymptotic behavior of the outage probability in terms of simple elementary functions and obtain the achievable diversity orders. Diversity gain is found to be a function of atmospheric turbulence parameters, pointing error, detection type and the number of hops. Monte Carlo simulation results are further provided to verify the accuracy of the derived expressions.

On the Performance Analysis of Higher Order QAM Schemes Over Mixed RF/FSO Systems

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.