In light of the revolutionary requirements of the sixth generation (6G) and beyond wireless networks, reconfigurable intelligent surface (RIS) and rate-splitting multiple access (RSMA) have emerged as pivotal technologies due to their potential for improving spectral efficiency, user fairness, and interference management. This survey explores the theoretical foundations, architectural frameworks, and design strategies of RIS-assisted RSMA, emphasizing the combined adaptability of RIS’s wireless propagation control and RSMA’s multi-user flexibility for dynamic spectrum management. The article first discusses the fundamental concepts of RSMA and RIS technologies. Then, we investigate various enabling technologies for RIS-RSMA networks, highlighting key advancements in interference mitigation, energy efficiency, and security for future networks. Subsequently, some optimization techniques crucial for enhancing RIS-RSMA network performance are presented. Additionally, we examine advanced machine learning (ML) approaches that enable RIS configurations to dynamically adapt to changing network requirements. Techniques such as deep reinforcement learning support real-time adjustments, creating more scalable and resilient RIS-RSMA architectures. Finally, we discuss open research directions for advancing RIS-assisted RSMA in emerging 6G applications. We also consider the potential of advanced ML techniques, including quantum-based ML and large language models, to handle the complexities of large-scale network optimization. This comprehensive survey addresses critical challenges and current advancements. It offers a roadmap for future research in RIS-assisted RSMA networks, paving the way for robust, intelligent, and adaptive 6G wireless communication systems.

Space-air-ground integrated networks (SAGINs) are revolutionizing wireless communications by integrating space, aerial, and terrestrial networks, providing global connectivity, aiding underserved regions, and enabling 6G applications. This paper proposes a dual reconfigurable intelligent surfaces (RISs)-assisted cooperative hybrid free space optical (FSO)/ radio frequency (RF) communication framework for SAGINs to address the reliable communications challenges. The framework integrates an optical RIS on a low Earth orbit satellite and an RF aerial RIS, enhancing FSO link performance and mitigating RF line-of-sight blockages. The system uses selection combining to dynamically choose the stronger signal between FSO and RF links, ensuring robust communication in varying conditions. We derive analytical expressions for key performance metrics such as outage probability, average symbol error rate, and ergodic capacity, considering pointing errors and atmospheric turbulence effects on the FSO link. Simulation results verify the analytical derivations, highlighting the critical role of RISs in enhancing the system performance.

As terrestrial networks face limitations in delivering global wireless communications, the shift towards non-terrestrial networks, particularly satellite-based systems, becomes increasingly essential for connecting remote regions. However, these networks are susceptible to security risks, such as eavesdropping, making the assurance of physical layer security (PLS) critical to achieving secure global wireless connectivity. Emerging technologies like reconfigurable intelligent surfaces (RISs) and higher frequency bands, such as the THz range, offer promising solutions for enhancing the PLS. This paper investigates the secrecy performance of a RIS-assisted satellite communication system operating in the THz band. We derive expressions for key security metrics, including the secrecy outage probability (SOP), lower bound SOP, and intercept probability (IP). Furthermore, in a practical scenario, we analyze the effect of misalignment and solar scintillation on reliable satellite communication. Simulation results validate the analytical findings, highlighting the importance of utilizing RIS in improving secrecy performance.

The Age of Information (AoI) is a critical metric in next-generation communication networks, quantifying data freshness essential for latency-sensitive applications in 6G systems, such as autonomous driving and industrial IoT. This paper presents an AoI analysis within a downlink full-duplex (FD) cooperative simultaneous wireless information and power transfer (SWIPT) system, employing rate-splitting multiple access (RSMA) for short packet communication to enhance timely data updates. By integrating RSMA with SWIPT and FD capabilities, we propose a robust framework to reduce the AoI. In this regard, closed-form expressions of the average block error rate of the RSMA-enabled FD cooperative SWIPT system are derived and validated via Monte Carlo simulations. The results demonstrate that RSMA outperforms non-orthogonal multiple access (NOMA) and FD cooperative SWIPT NOMA in terms of error performance, while also reducing the inherent system design complexity. Our findings reveal that RSMA is a promising approach for minimizing AoI across various system configurations, offering valuable insights for designing future 6G networks that prioritize low latency, high reliability, and data freshness.

This paper investigates the secrecy performance of a keyhole-aided multi-user communication network in the presence of multiple eavesdroppers. The communication happens through the same keyhole for legitimate users and eavesdroppers. In this context, the secrecy performance is evaluated for a user scheduling technique by obtaining the exact closed-form expression of secrecy outage probability (SOP). Further, a simplified asymptotic SOP expression is derived assuming high signal-to-noise ratio (SNR) scenario for a better understanding of the impact of system parameters. The effect of the keyhole parameters, number of users, number of eavesdroppers, and threshold secrecy rate on the SOP performance are also investigated for the considered system model. In the high-SNR regime, the asymptotic SOP saturates to a constant value and does not depend on the keyhole parameter and the channel parameter of the source-to-keyhole channel.

Integrating aerial reconfigurable intelligent surfaces (ARIS) with unmanned aerial vehicles (UAVs) presents a significant opportunity to enhance the performance of wireless networks. This integration allows ARIS to be mounted on UAVs, providing greater configuration flexibility, establishing reliable air-ground connections, and enabling three-dimensional signal reflections. However, this integration also introduces unique challenges related to physical layer security (PLS). Addressing these security considerations is essential, given the significance of secure and reliable communication. In this paper, we investigate the PLS for ARIS to assist wireless communication systems. Our objective is to select the ARIS that maximizes the secrecy capacity of the proposed system model. Two selection approaches are considered, namely, optimal and sub-optimal ARIS selection, and analytical expressions for the secrecy outage probability and probability of non-zero secrecy capacity over Nakagami-m fading channels are derived. Additionally, we examine the impact of varying the number of UAVs and the locations of the eavesdropper in practical scenarios. Moreover, the collaborative scenario is investigated, where all UAVs cooperate to improve secrecy transmission. Each ARIS reflects identical copies of the transmitted signal on the same time-frequency channel without mutual interference. The optimization problem of UAV locations and RIS phase shifts to maximize the secrecy capacity under specific constraints is formulated and addressed using an improved particle swarm optimization technique. These scenarios highlight the potential of ARIS in achieving secure and efficient wireless communications. Simulation results verify the analytical derivations, highlighting the critical role of selecting the ARIS in enhancing secrecy performance. As revealed by simulations, doubling the number of UAVs leads to a notable improvement in the average secrecy rate by approximately 77.78%. The obtained results highlight the significance of the proposed ARIS-assisted system in enhancing the PLS for wireless communications.

The design of circuits and systems has witnessed a growing interest in leveraging the potential of artificial intelligence in the realm of analog and mixed-signal (AMS) integrated circuits (ICs). This article presents a comprehensive survey on the application of machine learning techniques to the modeling and optimization of AMS ICs and systems. We explore the state-of-the-art research subjects in this domain and identify the advancements made in automating AMS performance modeling and optimization pursuit from a machine learning perspective. We propose to categorize the existing endeavors in the literature from four points of view (i.e., modeling, optimization, structure, and application). By analyzing the existing methods and discussing their advantages as well as limitations, we aim to shed light on the challenges and opportunities in empowering machine learning for tackling more complex AMS IC and system designs.

Integrating aerial reconfigurable intelligent surfaces (ARIS) with unmanned aerial vehicles (UAVs) presents a significant opportunity to enhance the performance of wireless networks. This integration allows ARIS to be mounted on UAVs, providing greater configuration flexibility, establishing reliable air-ground connections, and enabling three-dimensional signal reflections. However, this integration also introduces unique challenges related to physical layer security (PLS). Addressing these security considerations is essential, given the significance of secure and reliable communication. In this paper, we investigate the PLS of UAV-enabled ARIS-assisted wireless communication systems. Our objective is to maximize the secrecy capacity of the proposed system model. We derive analytical expressions for the secrecy outage probability and probability of non-zero secrecy capacity over Nakagami-m fading channels. Additionally, we examine the impact of varying the number of UAVs in practical scenarios. Simulation results verify the analytical derivations, highlighting the critical role of selecting the optimal UAV in enhancing secrecy performance.

The growing demand for improved spectral efficiency is one of the main challenges for the upcoming beyond fifth-generation wireless mobile communications networks. While massive multiple-input multiple-output (MIMO) technology has been demonstrating its potential in achieving higher spectral efficiency, the persistent problem of pilot contamination poses a significant hurdle for these systems. To address this issue, the Rate-Splitting Multiple Access (RSMA) framework has emerged as a potential solution. In this paper, we present a novel approach that leverages reinforcement learning (RL) with the Deep Deterministic Policy Gradient (DDPG) algorithm to maximize the sum spectral efficiency (SUM-SE) in a massive MIMO system implementing the RSMA framework with all users sharing a single pilot. The numerical results indicate that the proposed DDPG-based method is a competitive tool for optimizing the SUM-SE in massive MIMO scenarios employing the RSMA framework.

This paper investigates a practical distributed reconfigurable intelligent surface (RIS)-assisted full-duplex wireless system. For the first time in the literature, the system resources minimization problem is considered by jointly optimizing the RIS phase shifts and their states (ON/OFF) subject to target sum rate constraints. The paper further considers a discrete phase shift model at the RISs. As the formulated problem is mixed-integer and non-convex, it is decoupled into two sub-problems: transmit beamforming and joint RIS phase shifts and RIS elements state optimization. The former problem is mathematically addressed using approximate solutions, while the latter problem is addressed using a novel reinforcement learning (RL) approach. Simulation results illustrate that the proposed RL algorithm is flexible for different target rate constraints. The results further show that the proposed framework efficiently saves a considerable number of reflecting elements by configuring their state.

Visible light communication (VLC) is envisioned as a core component of future wireless communication networks due to, among other reasons, the very large unlicensed bandwidth it offers and the fact that it does not cause any interference to existing radio frequency (RF) communication systems. In order to take advantage of both RF and VLC, most research on their coexistence has focused on hybrid designs where data transmission to any user could originate from either an RF or a VLC access point (AP). However, hybrid RF/VLC systems fail to exploit the distinct transmission characteristics (e.g., susceptibility of VLC transmissions to blockages, limited field-of-view of VLC APs and receivers, more coverage and better reliability of RF systems, etc.) of RF and VLC systems to fully reap the benefits they can offer. Aggregated RF/VLC systems, in which any user can be served simultaneously by both RF and VLC APs, have recently emerged as a more promising and robust design for the coexistence of RF and VLC systems. To this end, this paper, for the first time, investigates AP assignment, subchannel allocation (SA), and transmit power allocation (PA) to optimize the energy efficiency (EE) of aggregated RF/VLC systems while considering the effects of interference and VLC line-of-sight link blockages. A novel and challenging EE optimization problem is formulated for which an efficient joint solution based on alternating optimization is developed. More particularly, an energy-efficient AP assignment algorithm based on matching theory is proposed. Then, a low-complexity SA scheme that allocates subchannels to users based on their channel conditions is developed. Finally, an effective PA algorithm is presented by utilizing the quadratic transform approach and a multi-objective optimization framework. Extensive simulation results reveal that: 1) the proposed joint AP assignment, SA, and PA solution obtains significant EE, sum-rate, and outage performance gains with low complexity, and 2) the aggregated RF/VLC system provides considerable performance improvement compared to hybrid RF/VLC systems.

This paper studies the sum-rate maximization problem of a distributed reconfigurable intelligent surface (RIS)-assisted full-duplex wireless system, where the availability of finite-resolution phase shifts at the RIS is considered. The aim is to optimize the transmit beamformers and RIS phase shifts, subject to the practical discrete phase shift and power constraints. The optimization problem is decoupled into two sub-problems; transmit beamforming and RIS phase shifts optimization. The transmit beamforming problem is mathematically addressed using approximate and closed-form solutions, while the discrete RIS phase shifts are optimized using a reinforcement learning (RL) approach. The existence and absence of a strong direct line-of-sight is investigated to show the effect of the phase shift optimization on the sum-rate. Simulation results illustrate that the proposed RL for the discrete phase shifts optimization provides a near-optimal performance with a small number of bits even for a large number of RIS elements, while improving the sum-rate compared to the random phase shift scenario and reducing the computational complexity compared to the state-of-the-art works.

Proactive caching shows great potential to minimize peak download rates by caching popular data, in advance, at the edge. Fast-changing file features, such as fast-changing file popularities and fast-changing file contents (data freshness), represent a challenge for proactive caching if cache content update is much slower, which decreases the efficiency and usability of caching. We present a dynamic caching scheme that updates local user caches and optimizes the use of caching resources. The developed scheme index-code the updates with the delivery messages. The developed scheme is presented for a network with one cache-enabled server, that has a pool of files, communicating with$K$cache-enabled receivers with requests limited to the server’s file pool. The developed scheme assumes partial knowledge of features variation. Asynchronous file delivery is assumed as a result of non-flexible receivers’ request timing. We show that the file delivery messages can be used to proactively and constantly update the receivers’ finite caches by index-coding the update messages with delivery messages at no additional rate-cost. We also show that this mechanism reduces the downloaded traffic and can be used to reduce other QoS metrics.

In this paper, we study the secrecy performance of non-orthogonal multiple access (NOMA) enabled visible light communication (VLC) broadcast channels in the presence of an active eavesdropper (Eve). The considered VLC system consists of multiple separately distributed light-emitting diodes arrays and multiple randomly located users (UEs) in an indoor room. User clustering is conducted to reduce the implementation complexity of successive interference cancellations. Two cooperative precoding strategies based on zero-forcing (ZF) and maximum ratio transmission (MRT) are designed using the effective channel of each cluster. Based on each precoding strategy, a sum secrecy rate maximization problem is developed to obtain the near-optimal power allocation (PA) to strengthen UEs’ confidential transmission and degrade Eve’s reception under minimum secrecy rate requirement, peak amplitude, non-negativity, and power constraints. To tackle the challenging non-convex problem for each precoding strategy, equivalent transformations and arithmetic-geometric mean approximation are conducted to convert the original problem into a series of geometric programming (GP) problems. Based on the reformulated problems, iterative algorithms are proposed to obtain near-optimal solutions by solving the GP problems through successive convex approximations. The convergence and complexity analysis of the proposed algorithms are studied. Simulation results show that the sum security performance of the proposed PA approach outperforms the conventional PA approaches in both ZF-based and MRT-based precoder schemes. The effectiveness of applying NOMA compared with the orthogonal multiple access-based scheme is also validated for the proposed system.

Visible light communication (VLC) has attracted a significant amount of research interest due to its ability to support high data-rates. However, the issue of inter-cell interference (ICI) caused by resource sharing and the line-of-sight (LoS) blockage problem are significant challenges that need to be considered in the design and analysis of VLC systems. This paper investigates the resource allocation problem for the downlink of an orthogonal frequency-division multiple access-based multi-cell VLC system, while considering ICI and LoS blockage. This is carried out under various system configurations employing different transmission modes. Specifically, the joint problem of subchannel allocation (SA) and power allocation (PA) to maximize the sum-rate is formulated as a combinatorial and highly non-convex optimization problem due to the binary and continuous optimization variables. To obtain an efficient solution, the original problem is first separated into the SA problem and the PA problem. Two simple, yet efficient, procedures based on the quality of the channel conditions and matching theory are proposed for the SA problem, respectively. Then, the quadratic transform approach is exploited to develop an algorithm for the PA problem. Simulation results demonstrate the effectiveness of the proposed solutions in terms of their fast convergence and overall performance.

This paper investigates the joint problem of access point (AP) assignment and power allocation (PA) in a three-tier hybrid radio frequency/visible light communication (VLC) heterogeneous network (HetNet). The main goal is to maximize the HetNet’s sum-rate under practical constraints such as APs’ power budgets and users’ quality-of-service (QoS) requirements, while maintaining an acceptable level of illumination in the VLC system. When this design problem is formulated mathematically, it turns out to be a combinatorial decision problem that involves non-linear constraints, and hence is NP hard. To efficiently obtain good quality solutions for the formulated problem, a reformulation into thecollege admission modelis first performed. Then, a distributed and low-complexity algorithm based on matching theory and an efficient heuristic PA scheme are proposed to obtain a good quality suboptimal solution for the joint problem. Simulation results highlight the robustness of the proposed solution and its significant gain in network sum-rate as compared to different benchmark schemes. The effect of various system parameters such as the minimum QoS and maximum illumination requirements on the performance of the proposed solution is studied. Finally, the theoretical analysis of convergence, stability, and complexity of the proposed technique is performed.

This paper investigates the joint problem of access point (AP) assignment and power allocation in a three-tier hybrid radio frequency/visible light communication (VLC) heterogeneous network (HetNet). The target is to maximize the HetNet's sum-rate under practical constraints such as APs' power budgets and users' quality-of-service constraints, while maintaining an acceptable level of illumination in the VLC system. This is an NP-hard problem due to the binary and continuous optimization variables. To obtain an efficient solution, the original problem is first reformulated into the well-known college admissions model. Then, a distributed and low-complexity algorithm based on matching theory and convex optimization is proposed to solve the joint problem. Simulation results highlight the robustness of the proposed algorithm and its significant gain in network sum-rate as compared to the considered benchmarks. Finally, the theoretical analysis on convergence, stability, and complexity of the proposed algorithm is provided.

Energy efficiency (EE) and spectral efficiency (SE) have been identified as key performance indicators in the design of future cellular networks. However, the available radio frequency (RF) spectrum is becoming highly saturated, thus making it difficult for network operators to achieve significant throughput and SE enhancement without increasing their power consumption. To that end, exploiting the abundant unlicensed spectrum in the visible light band to complement RF communication has become an important research direction in the design of wireless systems. Visible light communication (VLC) combines illumination and communication while significantly reducing the power consumption and related carbon footprint of wireless systems. This paper investigates the introduction of a VLC system in a two-tier RF heterogeneous network (HetNet). The EE and SE performance of the resulting three-tier HetNet is investigated, and a novel energy efficient resource allocation scheme is proposed. More specifically, the joint problem of user association and power control to maximize the EE is formulated as a fractional programming problem under the transmit power and quality-of-service requirements constraints. To tackle the nonconvexity of the problem, the original EE problem is first transformed into a parametric subtractive form. Then, the joint problem is separated into a user association and power control sub-problems. An efficient iterative algorithm is proposed to solve these two sub-problems, alternately. The performance of the proposed algorithm in terms of total network throughput, EE, and SE for different user densities is verified using simulation results.

In visible light communication (VLC) systems, the receiver may incorporate an energy harvesting unit to charge the receiver's battery or power a specific module in the device. This paper proposes a VLC system, which uses the incorporated light-emitting diode (LED) to harvest electrical energy. Most mobile terminals (MTs) utilize a power LED in their torch. During daylight or under a light-beam, MTs exploit this power LED, formerly used as a torch, to harvest energy. When needed, this LED is turned into a lighting point to light up a specific environment. A model of the LED energy harvested output power is given based on the Shockley diode equation. It is demonstrated, through measurements, that the quantity of energy harvested increases with the LED power.

In visible light communications technology, color-shift keying (CSK) is one of the modulation schemes recommended for the implementation of high data rate applications. It exploits light sources generating light with a Lambertian pattern. As a result, the transmitted message reaches the receiver through both line-of-sight and non-line-of-sight paths, and the corresponding channel is modeled using the Rician distribution. This paper analyzes the outage probability, i.e., the probability that the locally measured signal-to-noise ratio of a CSK link falls under a threshold predefined for a specific quality of service. Results are presented for a random white color and the D65 noon daylight.

The study of spectral efficiency (SE) in cellular networks has attracted significant research interest in the wireless community. Heterogeneous networks (HetNets) have emerged as a new paradigm to meet the fast growing demands for higher capacity and coverage in cellular networks. Previous works have focused on investigating either the power and bandwidth allocation problem or the power allocation problem for SE maximization in HetNets. These works were carried out under the assumption that users have already been associated. This paper presents a simple and effective method to optimize the SE of two-tier HetNets. Specifically, the joint optimization of user association and power control is formulated as a mixed integer programming problem. To tackle the non-convexity of the optimization problem, the Lagrange duality theory is applied to decompose the original problem into a user association sub- problem and a power control sub-problem, which are solved alternatively. Simulation results are used to demonstrate the fast convergence rate and the noticeable performance gains of the proposed algorithm as compared with conventional user association schemes under the assumption of equal power allocation.

This paper investigates the physical layer security of inband underlay Device-to-Device (D2D) communication, where there is no direct link between D2D users. In this respect, optimal relay selection is utilized to secure the D2D transmission. The eavesdropper uses either maximal-ratio combining or selection combining to improve the wiretapped signals. The D2D secrecy performance analysis is performed regarding the secrecy out-age probability (SOP) and the probability of non-zero secrecy capacity, and closed-form expressions are provided. To validate the correctness of our analysis, numerical and simulation results are presented. Furthermore, the asymptotic analysis of the SOP is provided, where new insights into the system parameters are revealed.

This paper investigates physical layer security and data transmission in cellular networks with inband underlay Device-to-Device (D2D) communication, where there is no direct links between D2D users. We propose the use of full-duplex (FD) transmission and dual antenna selection at the relay node. Thanks to the FD transmission, the relay node can simultaneously act as a jammer to enhance the cellular network secrecy performance, while improving the D2D communication data transmission. This describes a practical attractive scheme, where spectrum sharing is beneficial for both the D2D and cellular networks in terms of throughput enhancement and security provisioning, respectively. We analyze the secrecy performance of the cellular network and derive closed-form expressions for the secrecy outage probability (SOP) and the probability of non-zero secrecy capacity. We also derive a closed-form expression of the D2D outage probability. Furthermore, asymptotic analysis for SOP is performed. Simulation and numerical results are provided to verify the efficiency of the proposed scheme and to validate the accuracy of the derived expressions.

Visible light communications (VLC) technology is a relatively new emerging telecommunication paradigm. It offers the opportunity to design cost-effective communication systems due to the dual use of the light sources, which are exploited as illumination devices and as communication antennas. However, this technology is mostly deployed in short-range communication applications because of the light diffusion range, which is short by nature. One good response to this dilemma is the implementation of relay-assisted cooperative communication systems. Cooperative VLC systems provide three advantages, which are an increase in the transmission range, an improvement of the detection, hence of the bit error rate (BER), and an improved lighting system. In this paper, we analyze the channel response of a single-relay indoor VLC system based on an amplify-and-forward (AF) strategy. The system takes into account the fact that the relay also receives a reflected message. Results show the influence of the room's reflection index, Lambertian index, the number of scattered rays on the overall channel response and confirms the importance of relay-assisted strategies in improving system's reliability.

Two sequential relay selection strategies are investigated for a cooperative relay-assisted device-to-device (D2D) communication underlaying cellular network over mixed fading channels. The relay selection schemes aim to improve the D2D communication performance and the wireless physical layer security of the cellular network. In this paper, the study is conducted analytically by obtaining an expression for the secrecy outage probability (SOP) of the cellular network. Moreover, in an effort to assess the usefulness of the cooperation between the two networks, an expression for the mutual outage probability (MOP) - which represents the true outage probability across both the D2D and cellular networks - is derived. Also, an asymptotic analysis of the MOP in the high signal-to-noise ratio regime is obtained and the diversity order is evaluated. Monte-Carlo simulations are provided to validate the accuracy of our proposed analytical framework.

This paper considers a cellular system with multihop device-to-device (D2D) communications to extend the coverage of a cell, for user equipments (UEs) experiencing service outage. The D2D system is an inband underlay system. The aim is to satisfy the signal-to-noise-ratio (SNR) requirements on the downlink connections on every pre-allocated resource block, signal-to-interference-plus-noise-ratio (SINR) requirements for every D2D sidelink connection on each of their pre-allocated resource blocks, and a maximum allowable interference at the base station receiver on all uplink frame resource blocks. It is desired to perform joint power control and routing to minimize the expended D2D UE energy in the system while meeting these requirements. An optimization problem is formulated that turns out to be a mixed integer non-linear program. For that, we use a generalized Bender's decomposition approach, which breaks down the problem formulation into a master sub-problem, an auxiliary sub-problem and a feasibility sub-problem. In this paper, we focus on developing an efficient solution method for the relaxed version of the master sub-problem that is responsible for generating lower bounds on the optimal objective function value. A benchmark sub-optimal disjoint scheme for the same problem is also proposed, which performs routing and power control separately. Simulations are conducted to compare the performance of both schemes and results show that the joint scheme is superior when compared with the disjoint scheme in terms of the expended UE energy, the expended base station power and success in satisfying the SINR, SNR, and interference bound requirements.

This paper studies the physical layer security (PLS) of underlay multihop device-to-device (D2D) relaying. New closed-form expressions on the lower bound of the outage probability, the secrecy outage probability (SOP), and the probability of non-zero secrecy capacity are derived. Hence, new insights into the secrecy performance of the underlay multihop D2D relaying are revealed. The obtained analysis is validated through Mont-Carlo simulations.

This paper investigates the secrecy performance of a multiple-input multiple-output (MIMO) system with transmit antenna selection (TAS) and maximal-ratio combining (MRC) over generalized α-μ fading. In the underlying wireless communication system, a single antenna is selected to transmit confidential messages to a multi-antenna receiver in the presence of a passive multi-antenna eavesdropper. In our analysis, new closed-form expressions are obtained for the secrecy outage probability (SOP) and the probability of strictly positive secrecy capacity (SPSC), and are subsequently corroborated through Monte Carlo simulations to assess their validity. Moreover, asymptotic studies of the secrecy outage probability are derived and explicitly show the effects of various system parameters on the secrecy diversity gain.

This paper investigates the secrecy performance of multiple-input multiple-output systems under generalized α - μ fading conditions. To this end, we focus on two distinct scenarios: 1) the transmitter has knowledge of the channel state information (CSI) of the eavesdropper channel and 2) the transmitter is not aware of the CSI of the wiretap link. By considering transmit antenna selection in the underlying system, we develop closedform analytical expressions for the lower bound of the secrecy outage probability (SOP) and the probability of the strictly positive secrecy capacity. Furthermore, two novel approaches are proposed to derive an analytical expression of the average secrecy capacity (ASC). First, the ASC is expressed as a function of the average capacity of the desired link and an interaction term regarded as the ASC loss. Second, the ASC is expressed in terms of the average capacities of the desired and eavesdropper links, plus an interaction term that can be regarded as some sort of ASC gain due to the statistical independence between the desired and eavesdropper links. In addition, asymptotic studies of the SOP and the ASC at high signal-to-noise ratio are carried out, which precisely reveal the secrecy diversity and array gains.

In this paper, multihop device-to-device (D2D) communications for cell coverage extension are studied. An in-band underlay D2D mode is considered, and the aim is to satisfy the signal-to-noise-ratio requirements for pre-allocated resource blocks (RBs) on the downlink connections, the signal-to-interference-plus-noise-ratio requirements on pre-allocated RBs for every D2D sidelink connection, and a maximum allowable interference at the base station receiver on all uplink RBs. Power control and routing are performed to minimize the expended user equipment energy in the system while meeting these requirements. An optimization problem is formulated that turns out to be a mixed-integer nonlinear program, which is solved using the generalized Benders decomposition (GBD). The GBD breaks down the formulation into a master sub-problem, an auxiliary sub-problem, and a feasibility sub-problem. In this paper, we focus on finding efficient solution methods for the relaxed version of the master sub-problem that is responsible for generating lower bounds on the optimal objective function. Also, an efficient solution technique for the feasibility sub-problem is proposed. Furthermore, a benchmark disjoint scheme for the same problem is proposed, which performs routing and power control separately. The simulations are conducted to compare the performance of both schemes, which show the superiority of joint routing and power control scheme.

In this paper, we investigate the joint effect of optimizing power allocation and backhaul links flow assignment within the backhaul network on the energy efficiency (EE) of a millimeter wave backhaul heterogeneous network (HetNet). We consider enforcing a strict throughput demand on all user equipment(UEs), called joint EE, power, and flow control (JEEPF), versus allowing an acceptable range of demands for each, called joint EE, power, flow control, and throughput (JEEPFT). This little change causes a drastic difference in the formulations of both problems. JEEPF is convex while JEEPFT is quasiconvex, for which we propose a bisection method based approach. Our simulation results show the superiority of JEEPFT over JEEPF and other simple benchmark schemes.

This paper considers a cellular system with multihop device-to-device (D2D) communications to extend the coverage of a cell, for user equipment (UEs) experiencing service outage. The D2D system we consider is an inband underlay system. The target is to perform power control and routing to minimize the expended UE energy while satisfying the signal-to-noise-ratio (SNR) requirements on the downlink connections on every pre-allocated resource block (RB), signal-to-interference-plus- noise-ratio (SINR) requirements for every D2D sidelink connection on every pre-allocated RB, and a maximum allowable interference at the base- station (BS) receiver on all uplink frame RBs. An optimization problem is formulated that turns out to be a mixed integer non-linear program. We propose the generalized Benders decomposition which breaks down the formulation to a master sub-problem, an auxiliary sub-problem and a feasibility sub-problem. In this paper, we focus on finding efficient solution methods for the auxiliary sub-problem that is necessary for generating under-estimators to one of two types of Benders' cuts. These cut-of infeasible power allocations in a relaxed version of the master problem, and tighten the lower bounds on the optimal objective function value. Simulations are conducted to show how multihop D2D routing improves the cell coverage, by reducing the probability of service outage. The simulation results also show the effect of the requirements of SNR, SINR and interference at the BS on the expended UE energy.

One of the restrictions of Visible Light Communications (VLC) technology is related to its transmission range, which is short by nature. This short distance is imposed by the lighting range which barely reaches a couple of hundred meters. This has forced VLC to be applied in specific applications where the transmitting and receiving nodes are close enough to enable communication, in both outdoor and indoor environments. As a result, VLC has not been exploited as access system technology. In this paper, we propose an access communication system using VLC technology. Laser light (LL) sources, reputed for their long lighting range, are exploited for the outdoor access system while in indoor, light emitting diodes (LEDs) are used. The combination of indoor and outdoor is based on an amplify-and-forward (AF) strategy to reduce implementation cost. The overall channel frequency response is presented for varying transmitting parameters such as the outdoor attenuation coefficient, γ, the outdoor transmission distance, L, and the indoor reflection coefficient, ρ.

In this paper, an incremental selective decode-and-forward (ISDF) relay strategy is proposed for power line communication (PLC) systems to improve the spectral efficiency. Traditional decode-and-forward (DF) relaying employs two time slots by using half-duplex relays which significantly reduces the spectral efficiency. The ISDF strategy utilizes the relay only if the direct link quality fails to attain a certain information rate, thereby improving the spectral efficiency. The path gain is assumed to be log-normally distributed with very high distance dependent signal attenuation. Furthermore, the additive noise is modeled as a Bernoulli-Gaussian process to incorporate the effects of impulsive noise contents. Closed-form expressions for the outage probability and the fraction of times the relay is in use, and an approximate closed-form expression for the average bit error rate (BER) are derived for the binary phase-shift keying signaling scheme. We observe that the fraction of times the relay is in use can be significantly reduced compared to the raditional DF strategy. It is also observed that at high transmit power, the spectral efficiency increases while the average BER decreases with increase in the required rate.

In this paper, spatial modulation (SM) is used in a cooperative decode-and-forward (DF) cognitive radio system in order to enhance the overall spectral efficiency. In particular, a multi- antenna secondary transmitter communicates with a single antenna secondary receiver with the help of DF secondary relays in the presence of multiple primary users (PUs). To study the secondary system performance, we derive a closed-form expression for the average pairwise error probability (PEP) over Rayleigh fading channels assuming limited feedback from the PUs. A tight upper bounded average bit error rate is obtained using the PEP expression. Moreover, simple approximate expressions are obtained to get insights on the system diversity and estimation errors effects. Numerical results, which match simulations, show the effectiveness of SM in improving the overall secondary performance in the presence of channel estimation errors.

In this paper, we propose a novel joint routing and medium access control (MAC) protocol with traffic differentiation, based on quality of service (QoS) for wireless sensor networks (WSNs). This is referred to as joint routing and MAC (JRM) protocol. By leveraging the classical layered approach and combining routing and MAC layer functions, the proposed JRM protocol achieves a solution for energy efficiency in WSNs. JRM also ensures low latency for prioritized traffic. There are three major advantages of the proposed protocol. Firstly, the instantaneous network information (e.g., estimated time to destination, and node's unavailability to forward additional packet) is piggy-backed with the control packets acknowledgement and clear- to-send of the MAC frame. Based on the updated network knowledge and the objective of the required performance metrics (e.g., energy and latency), the next hop neighbor is chosen dynamically with reduced control overhead. Secondly, the JRM protocol introduces an approach for finding the constrained shortest path for forwarding packets, which results in load balancing in WSNs. Finally, routers (nodes) in JRM require very little forwarding and routing table information, which is compatible with the resource constraints in WSNs. The efficiency of the proposed protocol is shown through simulation results.

In this paper, relay selection is considered to enhance security of a cooperative system with multiple threshold-selection decode-and-forward (DF) relays. Threshold-selection DF relays are the relays in which a predefined signal-to-noise ratio is set for the condition of successful decoding of the source message. We focus on the practical and general scenario where the channels suffer from independent non-identical Rayleigh fading and where the direct links between the source and destination and source and eavesdropper are available. Based on channel state information knowledge, three relay selection strategies, namely traditional, improved traditional, and optimal, are studied. In particular, the secrecy outage probability of all three strategies are obtained in closed-form. It is found that the diversity of secrecy outage probability of all strategies can improve with increasing the number of relays. It is also observed that the secrecy outage probability is limited by either the source to relay or relay to destination channel quality.

In this paper, the secrecy of a cooperative system consisting of a single source, relay, destination and eavesdropper is analyzed. The threshold-selection decode-and-forward relay is considered, where the relay can correctly decode and forward only if it satisfies a threshold signal-to-noise ratio (SNR). Both destination and eavesdropper take advantage of the direct and relayed transmissions through maximal ratio diversity combining. The secrecy outage probability (SOP) and ergodic secrecy rate are derived in closed-form for different channel state information (CSI) availability. It was observed that when the required rate is low, having CSI knowledge is more advantageous than in the case of higher rate. An increase in the required threshold SNR at the relay can increase the SOP if the relayed link SNR is relatively higher than the direct link SNR. It was also shown that SOP cannot be improved beyond a certain value when keeping either dual-hop link average SNR fixed and increasing the other link SNR, whereas the ergodic secrecy rate can be increased by keeping the source to destination average SNR fixed.

In this paper, spatial modulation (SM) is used in multiple-input multiple-output (MIMO) spectrum sharing systems in order to enhance the overall spectral efficiency. In particular, a multi-antenna secondary transmitter, employing SM as a modulator, communicates with a multi-antenna secondary receiver in the presence of multiple primary users. To study the effect of estimation errors on the secondary system performance, we derive a closed-form expression for the average pairwise error probability (PEP) over Rayleigh fading channels assuming limited feedback from the PUs. A tight upper bounded average bit error rate is obtained using the PEP expression. Moreover, simple approximate expressions are obtained to get insights on the system diversity and estimation errors' effects. Numerical results, which match with simulations, show the efficacy of SM in improving the overall secondary performance in the presence of estimation errors.

This paper studies the use of spatial modulation (SM) in multiple-input multiple-output (MIMO) cognitive radio networks considering the primary receiver interference constraint and the maximum transmit power of the secondary transmitter. In particular, we investigate the effect of estimation errors on the secondary system performance, where a closed-form expression is derived for the average pairwise error probability (PEP) in Rayleigh fading environments. Based on this PEP expression, a tight upper bounded average bit error probability is obtained using the union bound formula. In addition, an asymptotic analysis is conducted and simple approximate expressions are derived to get useful insights on the system diversity and estimation errors' effects. Numerical results, which are validated through simulations, show that the SM is robust against estimation errors.

This paper puts in Cascade the power line communications (PLC) channel and the visible light communications (VLC) channel, in order to use the PLC channel as backbone for the VLC channel. This combination is suitable for applications in which hybrid PLC-VLC systems are needed. We investigate the behavior of the cascaded channels for a full link transmission. Quadrature phase shift keying combined with orthogonal frequency division multiplexing (QPSK-OFDM) is used over the PLC channel and color shift keying (CSK) is deployed over the VLC channel to convey the information. Cascaded channel variances are analyzed. Complete simulated bit error rate (BER) is analyzed and presented for multiple scenarii that could occur in the two channels.

In this paper, a joint resource allocation problem in amplify and forward (AF) OFDM-based two-way multiple-relay cognitive radio network is considered, where two transceiver nodes exchange information via a relay node. The full transmission happens in two phases: multiple access (MA) phase and broadcast (BC) phase. Considering individual power and interference constraints, the power allocation, subcarrier pairing and relay selection are jointly optimized in order to maximize the sum-rate. The dual decomposition technique is applied to obtain the optimal solution. Additionally, an efficient suboptimal algorithm which drastically reduces the computational complexity of the optimal solution with a small performance degradation is proposed. Finally, simulation results are shown to demonstrate the performance gain of the proposed algorithms.

In this paper, we consider a cognitive radio network (CRN) with multiple primary users (PUs) and multiple SUs and propose a cooperative spectrum sensing and transmission for some selected SUs in order to avoid interference with PUs. The distributed collaboration strategy is done through a coalitional game and we present a distributed algorithm for the coalition formation. In the proposed scheme, the SUs that sense a particular channel with non-identical sensing times, form a coalition in which a coalition head is selected to determine whether or not the channel is idle. If a channel is not active, then the SU chosen as coalition head can transmit its packets through a cooperative scheme in the data transmission slot. Our simulation results show that the proposed scheme can significantly improve the average throughput compared to its non-cooperative counterpart. Moreover, it is shown that the proposed scheme is energy efficient in comparison to a scheme with fixed and identical sensing times.

In this paper, the performance of a multichannel cognitive radio network (CRN) where each secondary user (SU) can assemble a constant number of primary channels is investigated. The effects of imperfect spectrum sensing (with false alarms and mis-detections) are taken into account and it is assumed that spectrum handover is enabled in the secondary network. A buffering mechanism is introduced to save blocked SU requests and interrupted SU services. An analytical model based on Continuous-time Markov chains (CTMC) is developed, and the system performance is evaluated in terms of throughput, blocking probability, and forced termination probability. Numerical results show that the buffer is able to significantly reduce the SU blocking probability while increasing its throughput.

This paper investigates the performance of a wideband cognitive radio network where each cognitive user can assemble multiple primary channels. Two channel assembling schemes are considered: a constant channel assembling (CCA) and a variable channel assembling (VCA). In the variable channel assembling scheme, cognitive users assemble their channels on the basis of the number of detected residual channels that are unoccupied by primary users or cognitive users. The effects of imperfect spectrum sensing (with false alarms and misdetections) are taken into account and it is assumed that spectrum handover is implemented in the secondary network. These channel assembling schemes are analyzed by using Continuous-time Markov chains (CTMC), and the system performance is evaluated in terms of throughput, blocking probability, and forced termination probability. Numerical results show that channel assembling achieves lower forced termination probability, but does not increase achieved system throughput and leads to higher blocking probability. They also show that VCA outperforms CCA in terms of throughput and forced termination probability.

A reliability-based iterative majority-logic decoding algorithm for regular low-density parity-check (LDPC) codes was recently proposed by Huang et al. In this paper we present an improved version of that algorithm by introducing a different reliability measure for each check-sum of the parity-check matrix, and taking it into account in the computation of the extrinsic information that is used to update the reliability measure of each received bit in each iteration. Some simulations results are given, which show that the new algorithm, while requiring very little additional computational complexity, not only achieves a considerable error performance gain over the standard one, but also, importantly, outperforms the iterative decoding based on belief propagation (IDBP), especially for short and medium block length finite-geometry (FG) LDPC codes.

This paper considers a cognitive radio network where the cognitive user has a constant bandwidth which is K times that of a primary user. The primary system consists of constant M primary channels and the cognitive system consists of N = [M/K] channels, where [x] is the largest integer not greater than x. The effects of imperfect spectrum sensing (with false alarms and misdetections) are analyzed using a Markov chain. Explicit expressions for state dependent transition rates are derived for the case M = 6 and K = 2, and the system performance is evaluated in terms of throughput, blocking probability, and forced termination probability.

This paper presents a hybrid decoding algorithm for low-density parity-check (LDPC) codes based on the interior point method with barrier function for linear programming (LP) decoding introduced by Wadayama . First, an efficient implementation of Wadayama's algorithm is presented. The main idea behind the modification is to approximate the barrier function for the fundamental polytope defining the code so that it contains only one linear constraint for each of the parity-check constraints. A two-stage hybrid decoding which combines the interior point decoding and a low-complexity decoding algorithm for LDPC codes is then proposed. Simulation results show that the approximations introduced in the proposed algorithms do not result in any performance degradation, while considerably reducing the decoding complexity and latency.

A new channel model for binary additive noise communication channel with memory, called weighted queue-based channel (WQBC), is introduced. The proposed WQBC generalizes the conventional queue-based channel (QBC) such that each queue cell has a different contribution to the noise process, i.e. the queue cells are selected with different probabilities. Suitably selecting the modeling function, the generalization introduced by the WQBC does not increase the number of modelling parameters required compared to the QBC. The statistical and information-theoretical properties of the new model are derived. The WQBC and the QBC are compared in terms of capacity and the accuracy in modeling a family of hard decision frequency-shift keying demodulated correlated Rayleigh and Rician fading channels. It is observed that the WQBC requires a much smaller Markovian memory than the QBC to achieve the same capacity, and provides a very good approximation of the fading channels as the QBC for a wide range of channel conditions.

This paper presents an improvement of the reliability-based iterative majority-logic decoding algorithms for regular low-density parity-check (LDPC) codes proposed by Huang et al. We improve the computation of the extrinsic information that is used to update the reliability measure of each received bit in each iteration with some kind of reliability measures of the check-sums that are orthogonal on the considered bit. The improved algorithm achieves a significant gain over the standard one with only a modest increase in computational complexity.

In this paper, a hybrid decoding algorithm for finite-geometry low-density parity-check (FG-LDPC) codes is proposed. The algorithm is based on the interior point method with barrier function introduced by Wadayama. First, an efficient implementation of Wadayama's algorithm is presented. The main idea behind the modification is to approximate the barrier function for the fundamental polytope defining the code so that it contains only one linear constraint for each of the parity-check constraints. A two-stage hybrid decoding which combines the interior point decoding (IPD) and a low-complexity decoding algorithm for FG-LDPC codes is then proposed. In the first stage, the interior point decoding is used to generate a search point. If the first stage decoding fails, the decoding is continued by the low-complexity algorithm that is initialized by the result of the IPD. Compared with a conventional iterative message-passing (IMP) decoder, the proposed hybrid algorithm achieves better bit-error rate (BER) and frame-error rate (FER) for small block lengths at medium to high signal-to-noise ratio (SNR).

In this paper, a cognitive radio (CR) network composed of K secondary users who cooperatively sense a channel using the k-out-of-K fusion rule to determine the presence of the primary user is studied. The sensing-throughput tradeoff problem is investigated in a realistic environment where both the sensing channels and reporting channels are characterized by fading channels. It is observed that taking into consideration the probability of reporting error in the CR network increases the sensing time and reduces the maximum average throughput of the secondary users. To mitigate the effect of the probability of reporting error, a transmit diversity based cooperative spectrum sensing method using randomized space-time block coding (RSTBC) is proposed. Simulations results show that the spatial diversity gain induced by RSTBC significantly decreases the sensing time and improves the throughput of the secondary users.

In this paper, an efficient implementation of the interior point algorithm recently proposed by Wadayama for linear programming (LP) decoding of low-density parity-check (LDPC) codes is presented. The main idea behind the modification is to approximate the barrier function for the fundamental polytope defining the code so that it contains only one linear constraint for each of the parity-check constraints. Simulation results show that the approximations introduced do not result in any performance degradation, while considerably reducing the decoding complexity and latency.

This paper presents a low-complexity iterative joint detection and decoding algorithm for finite geometry-low density parity check (FG-LDPC) coded multiple-input multiple-output (MIMO) systems, in which the MIMO channel detector and the FG-LDPC decoder iteratively exchange soft information. The key to the simplicity of the algorithm is the use of a bit flipping-based decoder for FG-LDPC code. An important issue addressed here is the generation of soft information from the binary outputs of the bit-flipping decoder to be fed back to the MIMO detector. Simulation results show that the proposed joint detection and decoding algorithm achieves a substantial reduction in decoding error probability compared to a cascaded detector and a decoder. We also compare the performance with belief-propagation (BP) based detector-decoders which are significantly more complex. The new algorithm provides a practical approach to joint detection and decoding of popular FG-LDPC codes in a MIMO system, a task which is computationally unmanageable with a BP algorithm.

In this letter, a low complexity decoding algorithm for binary linear block codes is applied to low-density paritycheck (LDPC) codes and improvements are described, namely an extension to soft-decision decoding and a loop detection mechanism. For soft decoding, only one real-valued addition per code symbol is needed, while the remaining operations are only binary as in the hard decision case. The decoding performance is considerably increased by the loop detection. Simulation results are used to compare the performance with other known decoding strategies for LDPC codes, with the result that the presented algorithms offer excellent performances at smaller complexity.

In this letter, an improved bit-flipping decoding algorithm for high-rate finite-geometry low-density parity-check (FG-LDPC) codes is proposed. Both improvement in performance and reduction in decoding delay are observed by flipping multiple bits in each iteration. Our studies show that the proposed algorithm achieves an appealing tradeoff between performance and complexity for FG-LDPC codes.

In this paper, a modified bit-flipping decoding algorithm for low-density parity-check (LDPC) codes is proposed. Both improvement in performance and reduction in decoding delay are observed by flipping multiple bits in each iteration. Our studies show that the proposed algorithm achieves an appealing tradeoff between performance and complexity for many constructions of LDPC codes.

In this paper, the construction of new families of error-correcting codes adapted from Boutros and Lentmaier's generalized low-density (GLD) codes, which we called generalized irregular low-density (GILD) codes, is investigated. By introducing irregular base matrices, significant improvements on Boutros and Lentmaier's results are achieved. Two cases are considered; in the first case, the same constituent code is used, and in the second case, different constituent codes are used. It is proved by an ensemble performance argument that these codes exist and are asymptotically good in the sense of the minimum-distance criterion. It is also proved that the performance of GILD codes approaches the binary symmetric channel capacity limit. Iterative decoding of GILD codes for communication over an additive white Gaussian noise channel is also studied. The high flexibility in selecting the parameters of GILD codes and their better performance and higher rate make them more attractive than GLD codes, and hence, suitable for small and medium block lengths forward-error-correcting schemes. Comparisons of simulation results between some GILD codes and some good low-density parity-check codes show very close performances

In this paper, a low complexity algorithm for binary linear block codes is applied to low-density parity-check (LDPC) codes and improvements are described, namely an extension to soft-decision decoding and a loop detection mechanism. For soft decoding, only one real-valued addition per code symbol is needed, while the remaining operations are only binary as in the hard decision case. The decoding performance is considerably increased by the loop detection. Simulation results are used to compare the performance with other known decoding strategies for LDPC codes, with the result that the presented algorithms offer excellent performance at smaller complexity.

In this paper, reduced-complexity trellis-based soft-input soft-output (SISO) decoding of linear block codes is considered. A new low-weight subtrellis based SISO decoding algorithm for linear block code to achieve near optimal error performance with a significant reduction in decoding complexity is presented. The proposed scheme is suitable for iterative decoding of linear block codes when used as component codes in compound or concatenated coding schemes. It has the following important features. An initial candidate codeword is first generated by a simple decoding method that guarantees a successful decoding. A low-weight subtrellis diagram centered around the candidate codeword is constructed. The maximum a posteriori probability (MAP) algorithm is then applied to the subtrellis. The generated extrinsic information is then used as a priori information to improve the generation of a candidate codeword for the next stage of iteration. Simulation results in the case of the newly introduced generalized irregular low-density (GILD) codes indicate that the proposed algorithm achieves practically optimal performance with a significant reduction in decoding complexity.

A class of pseudo-random compound error-correcting codes, called generalized low density (GLD) parity-check codes, has been proposed recently. As a generalization of Gallager's low-density parity check (LPDC) codes, GLD codes are also asymptotically good in the sense of minimum distance criterion and can be effectively decoded based on iterative soft-input soft-output (SISO) decoding of individual constituent codes. The code performance and decoding complexity of GLD codes are heavily dependent on the employed SISO decoding algorithm. In this paper, we present an efficient SISO decoding algorithm for GLD codes. The proposed algorithm utilizes Kaneko's decoding algorithm for soft-input hard-output decoding. These hard outputs are converted to soft-decision using reliability calculations. The algorithm proposed here presents a major advantage over existing decoding algorithms for GLD codes by being applicable even when long and powerful component codes are used. The complexity of the proposed algorithm is less than that of trellis-based decoding algorithms with a performance degradation of only 0.3 dB.