Outage Probability Research Articles

Unmanned-Aerial-Vehicle-Assisted Secure Free Space Optical Transmission in Internet of Things: Intelligent Strategy for Optimal Fairness

In this article, we consider an UAV (unmanned aerial vehicle)-assisted free space optical (FSO) secure communication network. Since FSO signal is impossible to detect by eavesdroppers without proper beam alignment and security authentication, a BS employs FSO technique to transfer information to multiple authenticated sensors, to improve the transmission security and reliability with the help of an UAV relay with decode and forward (DF) mode. All the sensors need to first send information to the UAV to obtain security authentication, and then the UAV forwards corresponding information to them. Successive interference cancellation (SIC) is used to decode the information received at the UAV and all authenticated sensors. With consideration of fairness, we introduce a statistical metric for evaluating the network performance, i.e., the maximum decoding outage probability for all authenticated sensors. In particular, applying an intelligent approach, we obtain a near-optimal scheme for secure transmit power allocation. With a well-trained allocation scheme, approximate closed-form expressions for optimal transmit power levels can be obtained. Through some numerical examples, we illustrate the various design trade-offs for such a system. Additionally, the validity of our approach was verified by comparing with the result from exhaustive search. In particular, the result with DRL was only 0.3% higher than that with exhaustive search. These results can provide some important guidelines for the fairness-aware design of UAV-assisted secure FSO communication networks.

Relay‐assisted D2D communication over novel multicluster two‐wave and multi‐cluster fluctuating two‐ray channels

AbstractA decode and forward relay assisted device‐to‐device (D2D) communications over a novel multicluster two‐wave (MTW) fading is analysed with Terahertz (THz) channel conditions. Co‐channel interference (CCI) over a recently proposed multi‐cluster fluctuating two‐ray (MFTR) fading is also present at relay and receiver. Maximal ratio combining (MRC) and power harvesting based on power splitting are present at relay and receiver. A characteristic function based approach is used here to derive success probability, outage probability and capacity with outage expressions. The expressions depend on THz channel parameters, distances between various nodes in D2D system, power harvesting parameter and fading channel conditions. The system is numerically analysed under various D2D network conditions.

Performance analysis of UAV-enabled dual-hop mixed RF-FSO communication systems with user scheduling

Abstract Using UAVs as relays in wireless communication offers several advantages like flexibility, coverage, line-of-sight communication, cost effectiveness, adaptability to changing network demands, etc. UAVs can perform surveillance, delivery, mapping, and disaster response. They offer real-time data collection, high maneuverability, and access to remote areas. In this manuscript, we consider a UAV-enabled dual hop mixed RF/FSO wireless communication system. The system includes a UAV acting as a decode-and-forward relay, facilitating communication from the base station (B) to multiple mobile users. The opportunistic scheduling scheme is used to select the user with the best instantaneous channel conditions, typically based on the highest signal-to-noise ratio (SNR) or lowest outage probability, to maximize throughput and ensure more reliable communication. The first hop from B to the UAV relay R, employs a free space optics (FSO) link modeled with gamma–gamma fading. The second hop, from R to multiple users, is a radio frequency (RF) link modeled with Rayleigh fading. The direct communication between base station and mobile users is not possible due to the blockage by various natural and man-made obstacles. We analyze the outage probability at high SNR values to gain further insights into system performance. Additionally, closed-form analytical expressions for the outage probability and average bit error rate (ABER) of the proposed system are derived.

Outage Probability Analysis and Altitude Optimization of a UAV-Enabled Triple-Hop Mixed RF-FSO-Based Wireless Communication System

In this paper, we present the evaluation of network parameters for the unmanned aerial vehicle (UAV)-enabled triple-hop mixed RF/FSO-based wireless communication system, where one relay is a fixed relay and the second relay is a UAV which acts as decode-and-forward relay for communication from the base station (B) to the multiple mobile users (Mi), i∈{1,2⋯N}. The first hop from B to fixed relay (R1) is the radio frequency (RF) link modeled using α−κ−μ fading distribution, the second hop from the relay R1 to the UAV relay (R2) is a free space optics (FSO) link modeled using Gamma-Gamma fading, and the third hop from R2 to the Mi is, again, an RF link modeled using the Rayleigh fading model. The direct communication between the B and the Mi is not feasible due to the very large distance. We derive the closed form analytical expression for the outage probability of the proposed system and find the effect of the base system parameters on the performance of the system. We also analyze the outage probability of the system at high SNR values to get further insights of the system performance. In addition, altitude optimization of UAV is carried out to know the optimal elevation angle in correspondence with UAV’s optimal altitude in order to maximize performance of the system.

Performance Analysis of Multiple UAV-Based Hybrid Free-Space Optical/Radio Frequency Aeronautical Communication System in Mobile Scenarios

Free-space optical (FSO) communication with unmanned aerial vehicles (UAVs) as relays is a promising technology for future aeronautical communication systems. In this paper, a multiple UAV-based aeronautical communication system is proposed, wherein a hybrid FSO/radio frequency (RF) link is established to connect the Airborne Warning and Control System (AWACS) with the mobile ground station (GS). Initially, we consider the velocity variance of both AWACS and the mobile GS, along with the influence of the Doppler effect. Furthermore, four relay selection modes are proposed, and exact closed expressions are derived for the end-to-end outage probability and bit error rate (BER) of the considered system. Numerical simulations demonstrate the impact of velocity variance variation on system performance. Additionally, we analyze the applicability of these four relay modes under different platform mobility characteristics through simulation results, while discussing the optimal numbers and the deployment altitude of UAVs. Finally, effective design guidelines that can be useful for aeronautical communication system designers are presented.

Performance Analysis of Radio Frequency/Underwater Wireless Optical Communication System With Fog Conditions Based on NOMA

ABSTRACTThis paper investigates the performance of cooperative radio frequency/underwater wireless optical communication (RF/UWOC) systems based on a downlink non‐orthogonal multiple access (NOMA), where amplify‐and‐forward (AF) protocols are considered. We assume that there is no direct connection between the transmitter and the receiver. The unified PDF for RF/UWOC systems was derived using atmospheric and underwater turbulence with non‐zero average unilateral pointing error under foggy conditions. Based on the analysis results, the expression of outage probability (OP) is obtained. Simulation results show that NOMA can provide fairness, then the increase of the number of channels and the change in turbulence parameters will affect all aspects of system performance.

Performance Analysis of Reconfigurable Intelligent Surface (RIS)-Assisted Satellite Communications: Passive Beamforming and Outage Probability

Reconfigurable intelligent surfaces (RISs), which consist of numerous passive reflecting elements, have emerged as a prominent technology to enhance energy and spectral efficiency for future wireless networks. RISs have the capability to intelligently reconfigure the incident wave, reflecting it towards the intended target without requiring energy for signal processing. Consequently, they have become a promising solution to support the demand for high-throughput satellite communication (SatCom) and enhanced coverage for areas inaccessible to terrestrial networks. This paper presents an asymptotic analysis of an RIS-assisted SatCom system. In this system, an unmanned aerial vehicle equipped with an RIS operates as a mobile reflector between a satellite and users. In particular, a passive beamformer is designed with the aim of asymptotically attaining optimal performance, considering the limitations imposed by practical SatCom systems. Moreover, the closed-form expressions for the ergodic achievable rate and outage probability are derived considering the proposed passive beamforming technique. Furthermore, we extend the system model to a multicast system and asymptotically analyze the optimality of the proposed scheme, leveraging the derived asymptotic results in the unicast system. The results of the simulations confirm that our analyses can precisely and analytically assess the performance of the RIS-assisted SatCom system, confirming the asymptotic optimality of the proposed scheme.

Performance Study of FSO/THz Dual-Hop System Based on Cognitive Radio and Energy Harvesting System

In order to address the problems of low spectrum efficiency in current communication systems and extend the lifetime of energy-constrained relay devices, this paper proposes a novel dual-hop free-space optical (FSO) system that integrates cognitive radio (CR) and energy harvesting (EH). In this system, the source node communicates with two users at the terminal via FSO and terahertz (THz) hard-switching links, as well as a multi-antenna relay for non-orthogonal multiple access (NOMA). There is another link whose relay acts as both the power beacon (PB) in the EH system and the primary network (PN) in the CR system, achieving the double function of auxiliary transmission. In addition, based on the three possible practical working scenarios of the system, three different transmit powers of the relay are distinguished, thus enabling three different working modes of the system. Closed-form expressions are derived for the interruption outage probability per user for these three operating scenarios, considering the Gamma–Gamma distribution for the FSO link, the α−μ distribution for the THz link, and the Rayleigh fading distribution for the radio frequency (RF) link. Finally, the numerical results show that this novel system can be adapted to various real-world scenarios and possesses unique advantages.

Performance analysis of IRS-assist dual-hop wireless communication system

The IRS-assist or smart radio environment technique is a widely developing technology that network providers can use to establish sustained connectivity between end-user terminals and central data units for the next-generation wireless standards. This article introduced a simple and more accurate link-switching technique for dual-hop communication: a single link-switching threshold (SLST) algorithm to provide an uninterrupted linkage between the transceiver terminals. Depending on the severity of the communicating channel under a dual-hop system, links can do auto switches between themselves and furnish continuous connectivity between end-user terminals. Due to a discrete number of phase shifts of the IRS elements, phase and quantization errors are induced in the channel; the proposed system can also optimize the phase and quantization errors. Besides, this work investigates improving the physical layer performance of the dual-hop wireless communication system under the combined effect of phase shift and quantization error with the introduction of the SLST method. For this particular, three performance metrics have been encountered: the outage probability (OP), average bit error rate (ABER), and average capacity (bits/s/Hz). A new, more accurate mathematical framework using the Meijer’G function has been constructed to evaluate worthwhile analytical derivation. Under analytical calculation, we have assumed the primary link experienced with common Rayleigh fading and the IRS-assist link (IAL) experienced with Nakagami-m distribution due to a large number of reflecting elements in the system. The proposed dual-hop system furnishes noteworthy benefits for each performance metric rather than individual links. Moreover, the suitable selection of quantization level and a number of reflecting elements confirm the exhibition of satisfactory outcomes and minimize the channel hardness of the system. Additionally, numerical simulation results from MATLAB, using Monte Carlo simulation, have been added to validate the analytical outcomes for every performance measure.

UAV-assisted dual-hop RF communication: Performance analysis of outage probability and altitude optimization for multi-user systems

Unmanned Aerial Vehicles (UAV) are growingly used as communication relays in this modern era due to their unique capabilities and advantages which includes enhanced connectivity, cost effectiveness, flexibility, risk reduction and real time data surveillance. UAVs offer a versatile and efficient solution for enhancing communication networks, especially in situations where traditional infrastructure is inadequate or unavailable. In this manuscript, we present the dual hop UAV enabled wireless communication system where the data is broadcasted from Source (S) to Destination (D) through a UAV working as decode and forward (DF) relay (R). The data is transmitted from S to R via Radio frequency (RF) link and then the decoded data from R is forward to D with multiple users. Closed form analytical expressions of outage probability is analyzed which is further utilized to do the optimization analysis to find the optimum altitude of the UAV in order to exaggerate its system performance. Numerical results shows that there is need to find out the optimal altitude of the UAV so as to keep down the overall outage probability of the proposed system and to enhance the system quality. We also find the outage probability at higher Signal-to-Noise ratio (SNR) to study the system behavior deeply. It is clearly observable from the numerical analysis that the exact outage probability and asymptotic outage probability are exactly matching at higher SNR values concluding the validity of the proposed system. The derived expressions are authenticated by simulated results via Monte Carlo simulations.

Location-Based Relay Selection in Full-Duplex Random Relay Networks

Full-duplex relay (FDR) has attracted considerable interest in enhancing the performance of relay networks by utilizing resources more efficiently. In this paper, we propose a framework for full-duplex random relay networks (FDRRNs), where relay nodes equipped with full-duplex (FD) capability are randomly distributed within a finite two-dimensional region. We first derive the outage probability of an FDRRN and then identify the potential relay location that minimizes the outage probability. Furthermore, we introduce a low-complexity relay selection algorithm that selects the relay node nearest to the potential relay location. Finally, simulation results show that the proposed relay selection algorithm achieves performance comparable to that of the max-min relay selection algorithm.

Performance Analysis of Reflective Intelligent Surface Assisted Bidirectional Full‐Duplex Communication Systems Over Nakagami‐m$$ m $$ Fading Channels

ABSTRACTIn this paper, a passive reflective intelligent surface (RIS) assisted bidirectional full‐duplex (FD) wireless system in a realistic Nakagami‐ fading with reflective elements and two source nodes is proposed. The source nodes are equipped with dual antennas for FD mode transmission and reception. The primary cause of degradation in the performance of the FD system is self‐interference, which can be reduced by increasing the number of reflective elements on the meta‐surface. Analytical expressions have been derived for the proposed RIS‐aided bidirectional FD wireless systems in terms of outage probability and bit error rate (BER). Numerical results show that doubling the number of reflecting elements considerably improves the outage performance of the proposed system in terms of signal‐to‐noise ratio (SNR) compared with RIS‐assisted half‐duplex (HD) mode. Additionally, different modulation schemes have been employed to evaluate the BER performance of the proposed system. The accuracy of analytical expressions is validated by matching Monte Carlo with analytical simulations.

Optimizing resource allocation for enhanced urban connectivity in LEO-UAV-RIS networks

Sixth-generation (6G) communication advancements target massive connectivity, ultra-reliable low-latency communication (URLLC), and high data rates, essential for IoT applications. Yet, in natural disasters, particularly in dense urban areas, 6G quality of service (QoS) can falter when terrestrial networks—such as base stations—become unavailable, unstable, or strained by high user density and dynamic environments. Additionally, high-rise buildings in smart cities contribute to signal blockages. To ensure reliable, high-quality connectivity, integrating low-Earth Orbit (LEO) satellites, unmanned aerial vehicles (UAVs), and reconfigurable intelligent surfaces (RIS) into a multilayer (ML) network offers a solution: LEO satellites provide broad coverage, UAVs reduce congestion with flexible positioning, and RIS enhances signal quality. Despite these benefits, this integration brings challenges in resource allocation, requiring path loss models that account for both line-of-sight (LOS) and non-line-of-sight (NLOS) links. To address these, a joint optimization problem is formulated focusing on resource distribution fairness. Given its complexity, a framework is proposed to decouple the problem into subproblems using the block coordinate descent (BCD) method. These subproblems include UAV placement optimization, user association, subcarrier allocation via orthogonal frequency division multiple access (OFDMA), power allocation, and RIS phase shift control. OFDMA efficiently manages shared resources and mitigates interference. This iterative approach optimizes each subproblem, ensuring convergence to a locally optimal solution. Additionally, we propose a low-complexity solution for RIS phase shift control, proving its feasibility and efficiency mathematically. The numerical results demonstrate that the proposed scheme achieves up to 43.5% higher sum rates and 80% lower outage probabilities compared to the schemes without RIS. The low complexity solution for RIS optimization achieves performance within 1.8% of the SDP approach in terms of the sum rate. This model significantly improves network performance and reliability, achieving a 16.3% higher sum rate and a 44.4% reduction in outage probability compared to joint optimization of SAT-UAV resources. These findings highlight the robustness and efficiency of the ML network model, making it ideal for next-generation communication systems in high-density urban environments.

Static Security Assessment: A Case Study of the Saudi National Grid

The static security assessment of the Saudi National Network helps to identify the weaknesses and potential threats and provides a comprehensive understanding of the network’s ability to address such incidents due to probable outages. This paper presents the static security assessment of the Saudi national grid with reference to the transmission line overloads and the bus voltage deviations in the system following the initiation of the specified contingencies. Critical contingencies are identified, and these are ranked on the basis of the limit violations of the line overloads and the bus voltage deviations obtained through the Newton-Raphson load flow analysis. The system loading conditions considered correspond to that of a typical day of the Hajj Pilgrimage period, during which millions of pilgrims from various parts of the country and also the world visit the Holy Mosque at Makkah. The preliminary investigations reveal that there are some transmission lines and certain substations requiring special attention to enhance the system security.

Impact of Meteorological Conditions on Overhead Transmission Line Outages in Lithuania

This study investigates the impact of meteorological conditions on unplanned outages of overhead transmission lines (OHTL) in Lithuania’s 0.4–35 kV power grid from January 2013 to March 2023. Data from the Lithuanian electricity distribution network operator and the Lithuanian Hydrometeorological Service were integrated to attribute outage events with weather conditions. A Bayesian change point analysis identified thresholds for these meteorological factors, indicating points at which the probability of outages increases sharply. The analysis reveals that wind gust speeds, particularly those exceeding 21 m/s, are significant predictors of increased outage rates. Precipitation also plays a critical role, with a 15-fold increase in the relative number of outages observed when 3 h accumulated rainfall exceeds 32 mm, and a more than 50-fold increase for 12 h snowfall exceeding 22 mm. This study underscores the substantial contribution of lightning discharges to the number of outages. In forested areas, the influence of meteorological conditions is more significant. Furthermore, the research emphasizes that combined meteorological factors, such as strong winds accompanied by rain or snow, significantly increase the risk of outages, particularly in these forested regions. These findings emphasize the need for enhanced infrastructure resilience and targeted preventive measures to mitigate the impact of extreme weather events on Lithuania’s power grid.

Modeling Analysis for Downlink RIS-UAV-Assisted NOMA over Air-to-Ground Line-of-Sight Rician Channels

This paper proposes a drone-assisted NOMA communication system equipped with a reconfigurable intelligent surface (RIS). Given the Line-of-Sight nature of the Air-to-Ground link, a more realistic Rician fading environment is chosen for the study of system performance. The user’s outage performance and secrecy outage probability of the RIS-UAV-assisted NOMA downlink communication under the Rician channels are investigated. Jointly considering the Line-of-Sight and Non-Line-of-Sight links, the closed-form expressions of each user’s outage probability are derived by approximating the composite channels as Rician distributions to characterize the channel coefficients of the system’s links. Considering the physical layer security in the presence of the eavesdropper, the secrecy outage probability of two users is further studied. The relationship between the system outage performance and the Rician factor of the channel, the number of RIS elements, and other factors are analyzed. The results of this study show that compared with Rayleigh fading, the Rician fading is more practical with the actual Air-to-Ground links; the user’s outage probability and the secrecy outage probability are lower over the Rician channels. The number of RIS elements and the power allocation factor by the base station for the users are inversely proportional to the user’s outage probability, and RIS element number, path loss index, and distance factor also have a greater impact on the outage probability. Compared with OMA, NOMA has a certain enhancement to the system performance.

Joint Sensing and Communications in Unmanned-Aerial-Vehicle-Assisted Systems

The application of joint sensing and communications (JSACs) technology in air–ground networks, which include unmanned aerial vehicles (UAVs), offers unique opportunities for improving both sensing and communication performances. However, this type of network is also sensitive to the peculiar characteristics of the aerial communications environment, which include shadowing and scattering caused by man-made structures. This paper investigates an aerial JSAC network and proposes a UAV-selection strategy that is shown to improve the communication performance. We first derive analytical expressions for the received signal-to-interference ratio for both communication and sensing functions. These expressions are then used to analyze the outage and coverage probability of the communication part, as well as the ergodic radar estimation information rate and the detection probability of the sensing part. Moreover, a performance trade-off is investigated under the assumption of a total bandwidth constraint. Various numerical evaluated results have been presented complemented by equivalent simulated ones. These results reveal the applicability of the proposed analysis, as well as the impact of shadowing and multipath fading severity, and interference on the system’s performance.

Secrecy performance of D2D assisted cooperative uplink NOMA system with optimal power allocation strategy

This paper investigates the physical-layer security (PLS) aspects of the uplink cooperative non-orthogonal multiple access (NOMA) system in the context of two user scenario. More specifically, this work employs device-to-device (D2D) pair users (i.e., D1, and D2) to further improve the spectral efficiency (SE) of the proposed cooperative uplink NOMA (CU-NOMA) system. One D2D user acts as a decode-and-forward relay, improving the performance of both the cell-edge user (CU) and another D2D user i.e., (D2). We analyze the secrecy performance of CU and D2 under perfect and imperfect successive interference cancellation (SIC), and optimizes power allocation (PA) to boost the performance. The proposed CU-NOMA system is evaluated in terms of its performance metrics like ergodic secrecy capacity (ESC), ergodic secrecy sum capacity (ESSC), non-zero secrecy capacity (NSC), effective secrecy throughput (EST), and secrecy outage probability (SOP) under the presence of an external eavesdropper (Eav). In addition, this work derives the closed-form analytical expressions of ESC, NSC, EST, and SOP metrics for both CU and D2 under perfect SIC (pSIC) and imperfect (ipSIC) cases in order to characterize the secrecy performance of the proposed secure CU-NOMA network. Further, the outcomes of the simulations are shown as evidence for both the validation of the mathematical analysis and the performance of the method being suggested. The simulation result analysis of this work infers that the secrecy performance of CU-NOMA system with optimal PA exhibits superior performance than that of the fixed PA scheme.

An Innovative Method for Improving the Performance of UAV‐SM‐NOMA for 6G Networks Over Generalized Fading Channels

ABSTRACTThe emergence of 6G wireless communications has brought numerous challenges and opportunities for meeting the growing demands of enhanced system capacity, higher data rates, lower latency, improved security, and stringent quality of service (QoS) requirements. One promising solution to tackle these challenges involves utilizing unmanned aerial vehicles (UAVs) in conjunction with nonorthogonal multiple access (NOMA) and spatial modulation (SM). However, optimizing the performance of UAV‐based SM‐NOMA systems for both perfect and imperfect channel state information (CSI) over generalized fading channels (α–μ, k–μ and η–μ) while simultaneously considering multiple objectives is a complex task. To address these challenges, this paper proposes integrating the monarch butterfly optimization algorithm (MBOA) into UAV‐SM‐NOMA systems for 6G networks. Inspired by the foraging behavior of monarch butterflies, the MBOA aims to enhance UAV‐based SM‐NOMA systems' performance by optimizing resource allocation, increasing system sum rate, reducing outage probability (OP) and bit error rate (BER), improving spectrum efficiency (SE), and ensuring desirable signal‐to‐interference‐plus‐noise ratio (SINR) levels. Key findings include that the MBOA effectively improves performance metrics across generalized fading channels compared to existing UAV‐assisted NOMA and orthogonal multiple access (OMA) models. Simulation results confirm that the proposed UAV‐assisted SM‐NOMA model with MBOA significantly outperforms conventional approaches, demonstrating superior capacity, reliability, and efficiency in 6G network scenarios.

Analysis of error rates and capacity outage of wireless links under jamming and multipath signal fading

AbstractThe author addresses the question, under which conditions an active jammer will be successful in corrupting a wireless communication link, when both the desired link and the jamming link are subject to multipath signal fading. To this end, a simple closed‐form analytical expression is derived for the resulting block‐error probability (BLEP) under Nakagami‐ fading with in general unequal fading parameters for the desired and the jamming link. As a by‐product, a novel expression for the corresponding probability of a capacity outage is obtained. An asymptotic analysis and numerical evaluation of the BLEP expression show that in the single‐antenna case the jammer will indeed be successful in corrupting the wireless link, unless there is a rather high signal‐to‐noise ratio (SNR) advantage for the desired link. This even holds true, when signal fading for the jamming link is significantly more severe than for the desired link. Finally, the case of multiple antennas is considered and the benefits of array processing and diversity reception at the desired receiver are explored. It is found that receive array processing can significantly improve the resulting BLEP, provided that the desired link is characterized by favorable fading conditions. On the other hand, diversity reception can significantly improve the BLEP, if the desired link has favorable SNR conditions.