This paper investigates a depth-layered acoustic Underwater Wireless Sensor Network (UWSN) architecture, to be used for environment monitoring. The underwater scenario is populated by sensor nodes and is partitioned into several horizontal layers. For each layer, we analytically determine the optimal number of sensor nodes required to ensure sensing coverage, while maintaining long-term energy sustainability. To address the severe energy constraints inherent to underwater environments, a mobile charger (MC) is introduced to periodically replenish node energy. The MC’s trajectory, charging locations, and charging durations are jointly optimized to maximize the harvested energy and minimize network outage probability. A comprehensive underwater acoustic communication model and a detailed node energy consumption model are developed to capture the unique characteristics of underwater propagation. Based on these models, outage probability (OP) is analytically derived, and the overall system design is formulated as a mixed-integer optimization problem. Simulation results demonstrate that the proposed depth-layered design and MC optimization significantly improve energy balance, reduce OP, and extend network lifetime, as compared to a non-optimized deployment and charging strategies.

Underwater wireless optical communication (UWOC) enables high-speed, low-latency links for the Internet of Underwater Things (IoUT), but remains highly vulnerable to complex and dynamic channel impairments. In practice, slant transmission paths frequently occur due to spatial offsets between underwater sensors and marine surface vehicles (MSVs), where link quality is jointly affected by MSV-induced misalignment, depth-dependent attenuation, and oceanic turbulence. Most existing models address these factors independently or under static assumptions, limiting their applicability. To address this challenge, this paper proposes a novel modeling and simulation framework that jointly captures the coupled effects of dynamic posture, stratified absorption and scattering, and spatially varying turbulence along slant UWOC paths. A parallelized wave optics simulation (WOS) platform is developed using Argo float data to reflect realistic ocean conditions. Leveraging this dataset, a CNN-based Fast Channel Simulation Method (CNN-FCSM) is designed, achieving over 98% prediction accuracy of received optical intensity, while reducing runtime by more than 90% compared to WOS. Comprehensive evaluations demonstrate that the proposed approach enables accurate and scalable estimation of average bit error rate (ABER) and outage probability (OP) without relying on predefined statistical fading models, offering a robust tool for dynamic UWOC analysis and IoUT system design in complex underwater environments.

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 presents a novel transmission protocol for simultaneous transmission and reflection reconfigurable intelligent surface (STAR-RIS) that jointly exploits traditional time switching and mode splitting protocol. The proposed approach, namely time-switching-variable mode splitting (TVM), allows to dynamically allocate STAR-RIS resources (i.e., time slot duration, and number of reflecting and transmitting elements) based on variable environment conditions. Energy harvesting (EH) and information transfer (IT) phases alternate to each other in downlink according to a variable time frame structure, while the energy harvested in downlink is exploited for IT in uplink. The elements of the STAR-RIS are then dynamically switched from transmission to reflection in different time slots based on the demand of the network, allowing the transmission users (TUs) and the reflection users (RUs) to be connected to the base station (BS) simultaneously. The proposed approach is investigated under the hypothesis of a non-orthogonal multiple access (NOMA) network, and the Nakagami-mdistributed channel model with attenuation loss. The analytical performance metrics are obtained considering dynamic time slots and link duration for both IT and EH downlink phases, as well as a variable number of transmission and reflection elements for each time slot. The assessment of the proposed TVM protocol is carried out by means of numerical and Monte Carlo (MC) simulation results of the joint outage probability, the sum throughput, and the age of information in case of multiple reflecting and transmitting users.

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.

In underwater environment, wireless signal propagation is very challenging, and strongly affected by absorption- and scattering-induced attenuation losses. Optical wireless communications (OWC), due to huge bandwidths, can provide high data rates and medium propagation ranges, representing a viable technology for underwater scenarios. There are several statistical distributions that model the effect of underwater turbulence on OWC, and how to model the underwater OWC (UOWC) channel with higher accuracy is still a topic to investigate. In this paper, we investigate the use of intelligent reflecting surfaces (IRSs) for enhancing performance in underwater OWC systems. In this regard, we present a unified channel model for UOWC, working both for heterodyne and intensity modulated/direct detection (IM/DD) schemes. The analytical expressions of average bit-error-rate (BER), outage probability, and channel capacity are derived including the combined effect of (i) attenuation, (ii) turbulence, (iii) pointing error, and (iv) angle-of-arrival (AOA) fluctuations to ensure the comprehensive characterization of underwater optical wireless communication channel. To make the analysis and results more realistic, the practical scenarios and parameters are considered. The probability density function (PDF) and the cumulative distribution function (CDF) of the underwater OWC channel are obtained analytically. Simulation results are presented for various parameters of underwater channel and communication systems. It is observed that the application of IRS remains as an important tool in terms of mitigating the overall fading effect caused by the combination of previous phenomena in underwater wireless channel. Also, the benefits of heterodyne detection over IM/DD is evinced.

In this paper, we perform the statistical modeling for turbulence-induced intensity fluctuations with optical waves propagating in turbulent ocean vertically. To accurately model the turbulence-induced optical wavefront distortion in vertical direction, we construct the wave optics simulation (WOS). In WOS procedure, we use the general oceanic turbulence spatial power-law spectrum and Argo experimental data to produce depth-dependent random phase screens. We specify the number and interval of phase screens based on Fourier optics theory so that we can determine the placement depth of phase screens reasonably. By fitting the WOS statistical results under various typical sea areas and system configurations, we propose the mixture Weibull-generalized Gamma (WGG) distribution to model oceanic turbulence-induced intensity fading. We then develop the analytical expression of the outage probability (OP) and average bit-error rate (BER) for vertical underwater wireless optical communication (UWOC) systems by use of the WGG distribution. The analytical results demonstrate the non-negligible effects of water depths on the performance of vertical UWOC systems.

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.

Leveraging on the multiple-input multiple-output (MIMO) application for enabling high data rates and ensuring reliable communication between underwater platforms connected to underwater sensor network, this paper presents a comprehensive analysis of the performance of optical wireless MIMO communication systems depending on the various phenomena. The outage performance of an underwater optical wireless communication (UOWC) is obtained in analytical form using Meijer-G function. Aysmptotic expression of outage probability is also derived for high signal-to-noise ratio (SNR) regime. Results show that the destructive combined effect of the underwater turbulence, pointing error, attenuation and angle-of-arrival (AOA) on the outage performance can be reduced by applying the MIMO spatial diversity technique.

This study presents new results for characterization of optical communication links through seawater-to-air interaction by taking into account the effects of pitching angle, attenuation and underwater turbulence. Pitching angle variation is assumed to present Beckmann distribution while underwater turbulent channel is modeled by Lognormal distribution. The attenuation effect covers both absorption and scattering effects and is modeled by Beer-Lambert law. Probabilistic behavior of seawater-to-air wireless communication channel is obtained with the analytical form expressions of the probability density function (PDF) and cumulative distribution function (CDF). Then, outage performance of optical wireless link is investigated. Results show that moving seawater yields a certain effect on the performance of optical wireless communication systems.

This paper investigates the role of reconfigurable intelligent surface (RIS) for enhanced handover management in 6G networks with overlapping wireless network layers i.e., ground, aerial, and space. A high altitude platform system (HAPS) is equipped with RIS and the comparison of direct and RIS-assisted link performance is presented. To do this, the average bit error rate, the outage probability (OP), and the channel capacity of direct and HAPS-assisted links are obtained in closed form expressions. By estimating the link error probability, a novel handover mechanism exploits the use of RIS, resulting in enhanced connectivity management. Hard and soft handover modes are defined depending on the radio frequency (RF) connectivity by means of link error threshold. The decision for the handover execution is based on selecting the optimal link among multiple available ones from ground, aerial, and space layers. The performance of RIS-assisted handover mechanism has been compared to traditional hard and soft handover procedure, as well as link performance achieved with relay node. Numerical and simulation results reveal that the connectivity of a user equipment can be maintained through RIS-aided links in case of any performance degradation in the direct RF links. Particular scenarios highlight that the use of RIS for handover is preferred to traditional handover procedures, as well as relay nodes.

This paper investigates the performance of high altitude platforms high altitude platforms (HAPS) based free-space optical (FSO) communication links including HAPS-to-ground station (downlink), ground-to-HAPS (uplink) and HAPS-to-HAPS (horizontal link) communications. The effects of attenuation loss, atmospheric turbulence, pointing error and angle-of-arrival (AOA) are taken into account. Also, the application of adaptive optics correction, one of the most effective turbulence mitigation techniques, is analyzed using the Zernike polynomials representation. Analytical expressions are obtained for probability density function (PDF), cumulative distribution function (CDF), Rytov variance, adaptive optics filter function and outage probability mainly in terms of Meijer’s G functions when both no adaptive optics correction is used and adaptive optics correction is applied. Some selected results are presented depending on the various parameters such as the HAPS altitude, the ratio of vertical and horizontal deviations, beam waist, Zenith angle, height of ground station, receiver aperture diameter, channel state threshold and wind speed. The performance improvement with adaptive optics correction is investigated by removing different Zernike modes. We show that (i) the downlink outperforms the uplink, (ii) the performance of the horizontal link sharply increases above a certain altitude, and, (iii) the communication links benefit from the adaptive optics correction up to a certain level in terms of performance improvement.

On-axis slant path uplink transmittance (used in short as transmittance throughout the text) for multi Gaussian optical beam in Kolmogorov atmospheric turbulent medium is investigated. It is observed that for both the flat-topped and the annular beams, as the propagation distance, wind speed and the zenith angle increase, the transmittance decreases. The transmittance of flat-topped beams increases when the number of beams, source size or the wavelength increases. For the annular beam, when the outer/inner beam size ratio is kept constant, larger source sizes yield larger transmittance values. Transmittance of the thicker annular beams is found to be larger than the transmittance of the thinner annular beams.