In Internet of Underwater Things (IoUT) scenarios, power-hungry devices need to periodically harvest energy to be alive. By dynamically controlling and redirecting the optical beam, reconfigurable intelligent surfaces (RIS) can enhance both the energy transfer and information reception processes, thereby improving the sustainability of low-power underwater devices. Simultaneous wireless information and power transfer (SWIPT) systems allows to harvest energy and receive data from the transmitted optical beam, thus feeding the low-power receiver (Rx) while downloading data. However, realistic underwater scenario can strongly affect the performance of such systems. This paper investigates the performance of ORIS-assisted SWIPT system under a realistic unified channel model for underwater optical wireless communications (OWC), considering both coherent and non-coherent detection schemes. Considering multiple attenuation losses, underwater turbulence, pointing error, and angle of arrival deviation, we derive the joint outage probability of an ORIS-aided SWIPT OWC system for different transmission protocols based on different resources that are allocated. Three approaches are investigatedi.e., (i)power-splitting (PS),(ii)time-switching (TS), and(iii)element-splitting (ES). While PS and TS are based on all-passive RIS elements, ES considers hybrid (active and passive) elements. Simulation results show enhanced performance obtained with ES approach for low signal-to-noise ratio (SNR), while PS outperforms ES for high SNR.

This paper proposes a novel non-terrestrial networks (NTNs) system that integrates optical intelligent reflecting surfaces (OIRS) and simultaneous transmitting and reflecting Intelligent reflecting surfaces (STAR-IRS) to address critical challenges in next-generation communication networks. The proposed system model features a signal transmitted from the optical ground station (OGS) to the earth station (ES) via an OIRS mounted horizontally on a high altitude platform (HAP). The ES uses an amplify-and-forward (AF) relay with fixed gain for signal relaying, which is then transmitted through a STAR-IRS vertically installed on a building to facilitate communication with both indoor and outdoor users. The FSO link incorporates (multiple-input multiple-output) MIMO technology, and this paper develops a channel model specifically designed for scenarios where the number of OIRS units exceeds one. For the radio-frequency (RF) link, a novel and highly precise approximation method is introduced, offering superior accuracy compared to traditional approaches based on the central limit theorem (CLT). Closed-form analytical expressions for key performance metrics, including outage probability (OP), ergodic capacity and average bit error rate (BER) are derived in terms of the bivariate Fox-H function for this novel five hops system. Asymptotic expressions at high SNR are also presented, providing insights into system diversity order.

In this work, we propose an innovative system that combines high-altitude platforms (HAPs) and optical intelligent reflecting surfaces (OIRS) to address line-of-sight (LOS) challenges in urban environments. Our three-hops system setup includes an optical ground station (OGS), a HAP, an OIRS, and a user. Signals are transmitted from the OGS to the HAP via a free space optical (FSO) link, with the HAP functioning as an amplify-and-forward (AF) relay that redirects signals through an OIRS, effectively bypassing obstacles such as buildings and trees to improve connectivity for non-line-of-sight (NLOS) User. For the OIRS link, we address key channel impairments, including atmospheric turbulence, pointing errors, attenuation, and geometric and misalignment losses (GML). An accurate approximation for the Hoyt-distributed GML model is derived, enabling us to obtain closed-form expressions for outage probability (OP) and various performance metrics, such as average bit error rate (BER) and channel capacity of the OIRS-assisted FSO link. Furthermore, we analyze the end-to-end signal-to-noise ratio (SNR) and derive closed-form expressions for OP and performance metrics. Asymptotic expressions are provided for high-SNR regimes, allowing the system’s diversity order to be calculated.

Free-space optical (FSO) communication systems enhanced with optical intelligent reflecting surfaces (OIRS) offer promising solutions to overcome line-of-sight limitations and mitigate atmospheric turbulence effects. This paper presents a comprehensive analysis of OIRS-assisted FSO systems with phase errors utilizing both heterodyne and intensity modulation with direct detection (IM/DD) methods, which provides a more tractable alternative to existing methods. We first derive exact closed-form formulas for the mean and variance of the squared sum of products of independent, identically distributed Gamma-Gamma random variables, incorporating both pointing errors and IRS phase errors. Based on these results, we approximate the square envelope of the OIRS-assisted FSO channel with a Gamma distribution, with shape and scale terms determined from the derived mean and variance. This approximation allows us to obtain closed-form expressions for key performance metrics, including the average bit-error rate for different modulations, the outage probability, and the ergodic capacity, expressed using the Meijer’s G function. Our approach demonstrates excellent agreement with Monte Carlo simulations while offering superior computational efficiency compared to methods based on multivariate Fox’s H functions. Furthermore, we conduct asymptotic analysis at high signal-to-noise ratios and investigate the system’s diversity order for both heterodyne and IM/DD techniques.

The automotive industry has seen rapid advancements with the integration of artificial intelligence (AI), particularly in autonomous driving. While significant progress has been made, navigating complex traffic scenarios, such as intersections, remains challenging due to their dynamic nature. Traditional rule-based systems often struggle to adapt, prompting the need for more advanced solutions. This study aims to enhance autonomous driving at intersections by integrating bird’s eye view (BEV) fusion and reinforcement learning (RL). Using the CARLA simulator, we fine-tune the UNetXST model to fuse multiple camera perspectives into a BEV representation, providing a holistic view of the vehicle’s surroundings. This comprehensive view serves as input for the RL agent, which is trained using the proximal policy optimization (PPO) algorithm to learn optimized driving strategies, avoid collisions, and ensure efficient navigation. Our results show that the proposed framework outperforms baseline techniques.

This article presents a novel technique for the simultaneous wireless information and power transfer (SWIPT) toward low-power devices, by means of reconfigurable intelligent surfaces (RISs). Our approach is based on a Reflect-and-Amplify protocol meaning that, based on the RIS beam-steering functionality and assuming different reflection coefficients for the RIS elements, it allows to reflect and amplify the impinging wireless signal from the transmitter toward the receiver, which will be able to both collect energy and download data. Specifically, by considering a hybrid RIS comprised of both passive and active elements, the proposed SWIPT approach is defined active/passive switching (APS). The reflection of the wireless beam toward the receiver is accomplished by means of passive RIS elements, while an amplification of the wireless signal is provided through active RIS elements. Such amplification allows the receiver to convert the incoming wave power into a DC voltage, but also causes an increase of power consumption due to active elements. To assess the performance of the proposed SWIPT approach, we derive the closed-form expression of the joint data transmission and energy harvesting outage probability (OP) in case of both a single and multiple users. We compare the proposed technique to other SWIPT approaches from the literature, such as the power splitting and the time switching, under power fairness conditions. Our results validate the benefits of using hybrid RISs for SWIPT systems, as compared to other existing approaches showing reduced performance, also in case of asymptotic analysis for high power and high number of RIS elements.

This paper presents a comprehensive performance study for the underwater optical wireless communication (UOWC) channel, accounting for the effects of attenuation, turbulence, pointing errors, and fluctuations in the angle-of-arrival (AOA), modeled by the Beer-Lambert model, the Fisher-Snedecor$\mathcal {F}$turbulence, Hoyt, and Rayleigh distributions, respectively. Considering the stochastic nature of UOWC, closed-form expressions for outage probability, average bit-error-rate (BER), and ergodic capacity are derived. The results revealed that increasing the field-of-view (FoV) reduces the impact of AOA fluctuations. Asymmetric two-dimensional beam misalignment exhibits better outage probability performance compared to symmetric misalignment. Furthermore, heterodyne detection consistently outperforms direct detection. When employing heterodyne detection, M-quadrature amplitude modulation (M-QAM) achieves superior BER performance. Additionally, as absorption increases with longer wavelengths, BER performance deteriorates. Narrower beamwidths improve ergodic capacity by enhancing received signal power.

The integration of non-terrestrial networks (NTNs), which include high altitude platform (HAP) stations and intelligent reflecting surfaces (IRS) into communication infrastructures has become a crucial area of research to address the increasing requirements for connectivity and performance in the post-5G era. This paper presents a comprehensive performance study of a new NTN architecture, which enables communication from the optical ground station (OGS) to end users through the utilization of HAP and terrestrial IRS nodes. In this configuration, the HAP acts as an amplify-and-forward (AF) relay terminal between the free-space optical (FSO) link and the RF links. Specifically, the RF links are modeled using the Shadowed Rician and the generalized Nakagami-m models, where the FSO link is characterized by the Gamma-Gamma distribution with generalized pointing errors. The FSO system operates under either intensity modulation with direct detection or heterodyne detection. Using the mixture Gamma model, we approximate the non-centered chi-square distribution that describes the total fading of the RF link, and we assess the performance of the end-to-end system by analyzing the ergodic capacity, the average bit-error rate (BER), and the outage probability, calculated using the bivariate Fox-H function. We also provide simple asymptotic expressions for the average BER and the outage probability at high signal-to-noise ratio (SNR). Finally, the proposed analysis is validated with numerical and Monte-Carlo simulation results, showing an exact match.

In this work, we present a unified framework for the performance analysis of dual-hop underwater wireless optical communication (UWOC) systems with amplify-and-forward fixed gain relays in the presence of air bubbles and temperature gradients. Operating under either heterodyne detection or intensity modulation with direct detection, the UWOC is modeled by the unified mixture Exponential-Generalized Gamma distribution that we have proposed based on an experiment conducted in an indoor laboratory setup and has been shown to provide an excellent fit with the measured data under the considered lab channel scenarios. More specifically, we derive the cumulative distribution function (CDF) and the probability density function of the end-to-end signal-to-noise ratio (SNR) in exact closed-form in terms of the bivariate Fox's H function. Based on this CDF expression, we present novel results for the fundamental performance metrics such as the outage probability, the average bit-error rate (BER) for various modulation schemes, and the ergodic capacity. Additionally, very tight asymptotic results for the outage probability and the average BER at high SNR are obtained in terms of simple functions. Furthermore, we demonstrate that the dual-hop UWOC system can effectively mitigate the short range and both temperature gradients and air bubbles induced turbulences, as compared to the single UWOC link. All the results are verified via computer-based Monte-Carlo simulations.

Due to recent advances in laser satellite communications technology, free-space optical (FSO) links are presented as an ideal alternative to the conventional radio frequency (RF) feeder links of the geostationary satellite for next generation very high throughput satellite (VHTS) systems. In this paper, we investigate the performance of multibeam VHTS systems that account for nonlinear high power amplifiers at the transparent fixed gain satellite transponder. Specifically, we consider the forward link of such systems, where the RF user link is assumed to follow the shadowed Rician model and the FSO feeder link is modeled by the Gamma-Gamma distribution in the presence of beam wander and pointing errors where it operates under either the intensity modulation with direct detection or the heterodyne detection. Moreover, zero-forcing precoder is employed to mitigate the effect of inter-beam interference caused by the aggressive frequency reuse in the user link. The performance of the system under study is evaluated in terms of the outage probability, the average bit-error rate (BER), and the ergodic capacity that are derived in exact closed-forms in terms of the bivariate Meijer's G function. Simple asymptotic results for the outage probability and the average BER are also obtained at high signal-to-noise ratio.

A unified statistical model is proposed to characterize turbulence-induced fading in underwater wireless optical communication (UWOC) channels in the presence of air bubbles and temperature gradient for fresh and salty waters, based on experimental data. In this model, the channel irradiance fluctuations are characterized by the mixture exponential-generalized gamma (EGG) distribution. We use the expectation-maximization algorithm to obtain the maximum likelihood parameter estimation of the new model. Interestingly, the proposed model is shown to provide a perfect fit with the measured data under all channel conditions for both types of water. The major advantage of the new model is that it has a simple mathematical form making it attractive from a performance analysis point of view. Indeed, we show that the application of the EGG model leads to closed-form and analytically tractable expressions for key UWOC system performance metrics such as the outage probability, the average bit-error rate, and the ergodic capacity. To the best of our knowledge, this is the first-ever comprehensive channel model addressing the statistics of optical beam irradiance fluctuations in underwater wireless optical channels due to both air bubbles and temperature gradient.

A novel statistical model is proposed to characterize turbulence-induced fading in underwater wireless optical channels in the presence of air bubbles for fresh and salty waters, based on experimental data. In this model, the channel irradiance fluctuations are characterized by the mixture Exponential-Gamma distribution. We use the expectation maximization (EM) algorithm to obtain the maximum likelihood parameter estimation of the new model. Interestingly, the proposed model is shown to provide a perfect fit with the measured data under all the channel conditions for both types of water. The major advantage of the new model is that it has a simple mathematical form making it attractive from a performance analysis point of view. Indeed, the application of the Exponential-Gamma model leads to closed-form and analytically tractable expressions for key system performance metrics such as the outage probability and the average bit-error rate.

In this paper, we analyze the end-to-end performance of dual-hop free-space optical (FSO) fixed gain relaying systems under heterodyne detection and intensity modulation with direct detection techniques in the presence of atmospheric turbulence as well as pointing errors. In particular, we derive the cumulative distribution function (cdf) of the end-to-end signal-to-noise ratio (SNR) in exact closed form in terms of the bivariate Fox's H function. Capitalizing on this cdf expression, novel closed-form expressions for the outage probability, the average bit-error rate (BER) for different modulation schemes, and the ergodic capacity of dual-hop FSO transmission systems are presented. Moreover, we present very tight asymptotic results for the outage probability and the average BER at high SNR regime in terms of simple elementary functions, and we derive the diversity order of the considered system. By using dual-hop FSO relaying, we demonstrate a better system performance as compared with the single FSO link. Numerical and Monte Carlo simulation results are provided to verify the accuracy of the newly proposed results, and a perfect agreement is observed.

In this paper, we analyze the end-to-end performance of dual-hop free-space optical (FSO) fixed gain relaying systems in the presence of atmospheric turbulence as well as pointing errors. More specifically, an exact closed-form expression for the outage probability is presented in terms of the bivariate Fox's H function that accounts for both heterodyne detection as well as intensity modulation with direct detection. At high signal-to-noise ratio (SNR) regime, we provide very tight asymptotic result for this performance metric in terms of simple elementary functions. By using dual-hop FSO relaying, we demonstrate a better system performance as compared to the single FSO link. Numerical and Monte-Carlo simulation results are provided to verify the accuracy of the newly proposed results, and a perfect agreement is observed.

This paper presents novel results for the performance analysis of dual-hop free-space optical/radio frequency (FSO/RF) transmission systems where the FSO link is modeled by the Gamma-Gamma distribution with pointing error impairments and under both heterodyne detection and intensity modulation with direct detection (IM/DD), and the RF link experiences the generalized Nakagami-$m$ fading. Using amplify-and-forward fixed-gain relaying as well as channel-state-information(CSI)-assisted relaying, we derive closed-form expressions for the outage probability, the average bit-error rate (BER), and the ergodic capacity in terms of the bivariate H-Fox function. For a special case, we obtain simplified results for Nakagami-$m$ fading channels in the RF link. Furthermore, new asymptotic results for the outage probability and the average BER at high signal-to-noise ratio (SNR) regime are presented in terms of simple functions. Numerical and Monte-Carlo simulation results are provided to verify the accuracy of the newly proposed results, and a perfect agreement is observed.

In this work, we carry out a unified performance analysis of a dual-hop amplify-and-forward fixed gain relay system over asymmetric links composed of both radio-frequency (RF) and unified free-space optics (FSO) under the effect of pointing errors. The RF link is modeled by the Nakagami-m fading channel and the FSO link by the Gamma-Gamma fading channel subject to both types of detection techniques (i.e. heterodyne detection and intensity modulation with direct detection (IM/DD)). In particular, we derive new unified closed-form expressions for the cumulative distribution function, the probability density function, the moment generating function, and the moments of the end-to-end signal-to-noise ratio of these systems in terms of the Meijer's G function. Based on these formulas, we offer exact closed-form expressions for the outage probability, the higher-order amount of fading, and the average bit-error rate of a variety of binary modulations in terms of the Meijer's G function. Further, an exact closed-form expression for the end-to-end ergodic capacity for the Nakagami-m-unified FSO relay links is derived in terms of the extended generalized bivariate Meijer's G function. All the given results are verified via computer-based Monte-Carlo simulations.

In this work, we carry out a unified performance analysis of a dual-branch transmission system composed of a direct radio-frequency (RF) link and a dual-hop fixed gain relay over the asymmetric links composed of both RF and unified free-space optics (FSO) under the effect of pointing errors. RF links are modeled by the Nakagami-m fading channel and the FSO link by the Gamma-Gamma fading channel subject to both types of detection techniques (i.e. heterodyne detection and intensity modulation with direct detection (IM/DD)). Selection combining (SC) and maximum ratio combining (MRC) diversity schemes are investigated. More specifically, for the SC method, we derive new unified closed-form expressions for the cumulative distribution function (CDF), the probability density function (PDF), the moment generating function (MGF), the moments, the outage probability (OP), the average bit-error rate (BER) of a variety of binary modulations, and the ergodic capacity for end-to-end signal-to-noise ratio (SNR). Additionally, using the MGF-based approach, the evaluation of the OP, the average BER, and the ergodic capacity for the MRC diversity technique can be performed based entirely on the knowledge of the MGF of the output SNR without ever having to compute its statistics (i.e. PDF and CDF). By implementing SC or MRC diversity techniques, we demonstrate a better performance of our system relative to the traditional RF path only. Also, our analysis illustrates MRC as the optimum combing method. All the analytical results are verified via computer-based Monte-Carlo simulations.

In this paper, we carry out a unified performance analysis of hybrid automatic repeat request (HARQ) with incremental redundancy (IR) from an information theoretic perspective over a point-to-point free-space optical (FSO) system. First, we introduce a novel unified expression for the distribution of a single FSO link modeled by the Gamma fading that accounts for pointing errors subject to both types of detection techniques at the receiver side (i.e. heterodyne detection and intensity modulation with direct detection (IM/DD)). Then, we provide analytical expressions for the outage probability, the average number of transmissions, and the average transmission rate for HARQ with IR, assuming a maximum number of rounds for the HARQ protocol. In our study, the communication rate per HARQ round is constant. Our analysis demonstrates the importance of HARQ in improving the performance and reliability of FSO communication systems. All the given results are verified via computer-based Monte-Carlo simulations.

In this paper, we study the performance of hybrid automatic repeat request (HARQ) with incremental redundancy over double Rayleigh channels, a common model for the fading amplitude of vehicle-to-vehicle communication systems. We investigate the performance of HARQ from an information theoretic perspective. Analytical expressions are derived for the e-outage capacity, the average number of transmissions, and the average transmission rate of HARQ with incremental redundancy assuming a maximum number of HARQ rounds. Moreover, we evaluate the delay experienced by Poisson arriving packets for HARQ with incremental redundancy. We provide analytical expressions for the expected waiting time, the packet's sojourn time in the queue, the average consumed power, and the energy efficiency. In our study, the communication rate per HARQ round is adjusted to the average signal-to-noise ratio (SNR) such that a target outage probability is not exceeded. This setting conforms with communication systems in which a quality of service is expected regardless of the channel conditions. Our analysis underscores the importance of HARQ in improving the spectral efficiency and reliability of communication systems. We demonstrate as well that the explored HARQ scheme achieves full diversity. Additionally, we investigate the tradeoff between energy efficiency and spectral efficiency.