Reconfigurable intelligent surfaces (RISs) are expected to play crucial roles in enhancing energy and spectral efficiencies, as well as the security of 6G networks. However, regular RIS, where elements are arranged regularly over RIS grids, is not spectral efficient in dynamic channels due to the high overhead, especially in the case of a large number of RIS elements. In this article, we introduce a new multilayered irregular RIS (MLI-RIS) architecture with a corresponding design optimization framework to reduce the required overhead, in addition to optimizing the spectral and energy efficiency of the wireless network. The proposed MLI-RIS design strategically disperses elements irregularly across multiple layers, harnessing additional spatial degrees of freedom to amplify signal power and optimize system capacity. To simplify the optimization problem and reduce required overhead, the proposed framework decouples the optimization problem into the MLI-RIS topology design problem, which is based on channel state information, and the elements’ phase and precoding optimization problem. Advanced optimization techniques, such as modified tabu-hybrid sophisticated neighbor search-simulated annealing and neighborhood exploration and refinement-cross entropy are utilized to optimize these two problems. Results of extensive simulations demonstrate that the proposed MLI-RIS exhibits significant superiority over both irregular RIS setups and conventional multilayer regular RIS architectures. These findings highlight MLI-RIS as a transformative solution, positioning it as a cornerstone in the evolution toward future wireless communication paradigms.

Twisted light, characterized by its Orbital Angular Momentum (OAM) and helical phase front, holds great promise for advancing Free-Space Optical (FSO) communication systems by significantly increasing both capacity and spectral efficiency. However, Atmospheric Turbulence induces power fluctuations and wavefront distortions, degrading system performance. To address these challenges, we propose a Multi-Layered Irregular-Reconfigurable Intelligent Surface (MLI-RIS) architecture designed to enhance data rates and mitigate the detrimental effects of turbulence. To optimize system performance while minimizing computational complexity, a two-stage optimization framework is introduced: (1) MLI-RIS topology design, and (2) joint phase-shift and precoding optimization. This framework leverages a Diversification Tabu–Advanced Neighborhood Search (DT-ANS) algorithm, along with a Neighborhood Exploration Strategy integrated with Simulated Annealing (NES-SA). Furthermore, a compressive sensing-based correction method using regular RIS is implemented to compensate for wavefront distortions and enhance the purity of OAM modes. Numerical simulations validate the effectiveness of the proposed approach in reducing error rates, mitigating wavefront distortions and crosstalk, and enhancing overall system performance. The results show that the proposed system achieves a Forward Error Correction level with a 1.4 dB loss compared to the ideal case, whereas the best of the other approaches considered in this paper achieve this level with an 18 dB loss compared to the ideal case.

Enabling high-speed internet access in rural areas is vital for advancing education, healthcare, environmental protection, and economic growth. However, the cost of backhaul poses a significant barrier to achieving broadband connectivity in these regions. Optical satellite-ground feeder links offer an affordable solution for rural backhauling, delivering high-speed capabilities. Nevertheless, cloud-induced attenuation poses a major challenge, often leading to substantial degradation or complete disruption of such optical satellite feeder links. To address this issue, we propose an optical feeder link utilizing fiber tethered aerostats. Our study focuses on evaluating the performance of this proposed approach in Canada's expansive northern geography, where we analyze the statistical distributions of cloud-induced attenuation using ERA5 data. We compare the availability of the proposed approach with traditional ground diversity approaches. The numerical results demonstrate that even during the warm months when liquid clouds cause significant attenuation, the proposed approach achieves a link availability of over 95%.

The automotive industry has witnessed flourishing development in the last few years with the hope of achieving fully autonomous driving. In parallel, dual-function radar-communication (DFRC) systems are being developed to tackle the spectrum scarcity challenge and integrate radar and communication transceivers into a unified system. However, with a DFRC system, critical information may be intercepted by unauthorized people in the target vehicle. Therefore, the physical layer security of the DFRC system needs to be given considerable attention. To improve the security of information transmission, artificial noise (AN) is used to interfere with eavesdroppers. The main objective of this work is to maximize the secrecy rate of vehicles subject to both radar signal-to-interference-plus-noise ratio (SINR) and communication transmit power constraints. To overcome the difficulty caused by the existence of the nonconvex Shannon capacity formula in the formulated problem, we convert the secrecy rate optimization to an equivalent bi-convex optimization problem. Furthermore, due to the nonconvexity of the formulated design problem, we exploit an inexact block coordinate descent (IBCD) algorithm based on semi-definite Programming (SDP) to obtain the optimal solution. Moreover, to reduce the complexity of the SDP-based IBCD algorithm, a bi-search-based IBCD algorithm is employed for single target scenarios. Finally, numerical simulations are used to demonstrate the superiority of using the AN approach to enhance the performance of the DFRC system in terms of the secrecy rate.

This article presents accurate approximation expressions for the outage and secrecy outage probabilities of relay-assisted free-space optical (FSO) communication links utilizing unmanned aerial vehicles (UAVs). We consider the effects of weather attenuation and random fluctuations of UAVs’ orientations and positions. The obtained expressions are applicable to different types of intensity modulation direct detection techniques. We utilized$L$-ary multipulse pulse-position modulation$(L$MPPM) scheme and optimized link performance by adjusting the$L$MPPM settings using our derived expressions. We conduct numerical analyses to evaluate the impact of system settings, including beam divergence and modulation settings, on outage and secrecy outage probabilities. Our results demonstrate that adaptive$L$MPPM can improve average spectral efficiency, particularly in time-varying UAV-based FSO channels, while maintaining the same outage and secrecy outage probabilities as ordinary multipulse pulse-position modulation. Moreover, we investigate the effect of an eavesdropper’s location on the probability of a secure channel between legitimate users. We validate the accuracy of our expressions by comparing them with Monte Carlo (MC) simulation results under different channel conditions. Derived expressions provide efficient and accurate evaluation results to optimize the performance of UAV-based relay-assisted FSO communication systems.

Relaying is a key technology for millimeter wave communications to extend the range and to route around blockages. In practical implementation of relaying systems, probing is required to identify proper neighbor terminals which will serve as relays. There is however an inherent trade-off between relay probing and required overhead. In this paper, we consider WiGig (IEEE 802.11ad) devices, which are multiband capable with Wi-Fi support, and propose a Wi-Fi assisted relay probing for WiGig device-to-device networks. In the proposed scheme, Wi-Fi received signal strengths are used to pre-select the WiGig relays expected to maximize the spectral efficiency of the overall system. Then, only these pre-selected relays are used in online relay probing step. Simulation analysis demonstrate improvements in throughput and energy consumptions over existing relay probing schemes.

In line with the increasing number of high speed train (HST) commuters and their demand for mobile data, railways operators expore means of providing high speed onboard wireless access. The data from all train cars are sent to a gateway terminal mounted at the roof of the train and an efficient backhaul solution is required to connect the train to the nearest base station. In this paper, we propose the use of unmanned aerial vehicles (UAVs) equipped with free space optical (FSO) communication terminal to provide backhauling for HSTs. We present a composite fading model for UAV-to-HST channel where both atmospheric turbulence and pointing errors due to position/orientation deviation are considered. Based on the derived fading model, we present closed form expressions for the bit error and outage probability. We further investigate the effect of several parameters such as beam divergence angle, displacement deviation variance, coverage distance and UAV operation altitude on system performance.

In this paper, we present a polarization-assisted L-ary differential phase-shift keying multipulse pulse-position modulation (PA.LDPSK-MPPM) technique that is secured in the physical layer by a discrete-chaos system. The all-optical PA.LDPSK-MPPM scheme benefits from the polarization as an additional degree of freedom which greatly reduces the system complexity relative to prior implementations. The discrete-chaos scrambling is based on a message-seeded two-dimensional chaotic map tailored for independent perturbation of the occupied time-slot positions (MPPM information) and their relative phase shift (LDPSK information). Synchronized and non-synchronized implementations of the chaotic PA.LDPSK-MPPM technique are proposed with expressions for the corresponding spectral efficiencies being determined and compared with prior LDPSKMPPM setups. The performance of PA.LDPSK-MPPM under gamma-gamma (GG) free-space optical (FSO) fading channels is analytically verified to outperform the prior designs for different FSO channel states which is supplemented by Monte Carlo (MC) simulations. The system security is numerically examined against various types of attacks, including brute-force, differential, and statistical attacks.

A hybrid two-level multipulse pulse-position modulation-M-ary differential phase-shift keying (2L-MPPM- MDPSK) technique is proposed which achieves high power and spectral efficiencies. The proposed 2L-MPPM-MDPSK scheme is shown to have the highest spectral efficiency in comparison with other hybrid modulation schemes. Additionally, the proposed technique has the advantage of removing the complex variable delay-line required for the direct- detection of other hybrid MPPM-MDPSK systems. The performance of 2L-MPPM-MDPSK is quantified in free-space optical (FSO) channels modeled with an exponentiated Weibull (EW) fading distribution. Closed-from expressions for both bit-error rate (BER) and outage probability upper-bound are derived and verified using Monte Carlo (MC) simulation. At the same data rate and bandwidth, 2L-MPPM-MDPSK techniques outperform traditional MPPM-MDPSK scheme by approximately 2dB at a BER of 10-4.