Presence Of Direct Link Research Articles

Throughput performance optimization of NOMA-assisted cooperative relay system with realistic impairments

Abstract This work aims to investigate the outage and throughput performance of non-orthogonal multiple access assisted cooperative relay system (CRS-NOMA) considering the realistic impairments caused due to in-phase and quadrature-phase imbalance (IQI), channel estimation errors (CEE), and successive interference cancellation (SIC) errors. More specifically, we investigate a model in which two-phase downlink transmission is carried out in two different modes: (i) CRS-NOMA without direct links and (ii) CRS-NOMA with direct links (CRS-DLNOMA). In CRS-NOMA mode, the source broadcasts a composite NOMA signal to destination users with the assistance of a decode-and-forward (DF) relay. In contrast, in CRS-DLNOMA, direct and cooperative links are available for transmission. We derive the analytical expressions of outage probability and throughput for both the NOMA destinations to evaluate the system performance of both CRS-NOMA and CRS-DLNOMA modes of transmission. Furthermore, numerical simulations also study and validate the influence of IQI, CEE, and SIC errors on the outage and throughput performance. The simulation results verify that realistic impairments degrade the system performance, but the presence of direct link has a positive impact on outage and throughput. Additionally, we use the golden search method to optimize the power allocation factor (PAF) and transmission rate to maximize the throughput at the near user while ensuring the throughput constraint at the far user.

Performance analysis for three cases of outage probability in one-way DF full-duplex relaying network with presence of direct link

In this paper, the one-way decode-and-forward (DF) full-duplex relaying network system with presence of direct link is investigated. In the analysis section, we derived the exact, lower, and upper bound for outage probability (OP) with maximal ratio combining (MRC) at the receiver. Furthermore, the system performance's analytical expressions are verified by using the Monte Carlo simulation. In addition, we investigated the effect of the main parameters on the OP of the proposed system. Finally, we can sate that the simulation curves overlap the analytical curves to convince the analysis section. This research can provide a novel recommendation for the communication network.

Exact Coverage Analysis of Intelligent Reflecting Surfaces With Nakagami-MChannels

Intelligent Reflecting Surfaces (IRS) are a promising solution to enhance the coverage of future wireless networks by tuning low-cost passive reflecting elements (referred to as metasurfaces), thereby constructing a favorable wireless propagation environment. Different from prior works, which assume Rayleigh fading channels and do not consider the direct link between a base station and a user, this article develops a framework based on moment generation functions (MGF) to characterize the coverage probability of a user in an IRS-aided wireless systems with generic Nakagami-m fading channels in the presence of direct links. In addition, we demonstrate that the proposed framework is tractable for both finite and asymptotically large values of the metasurfaces. Furthermore, we derive the channel hardening factor as a function of the shape factor of Nakagami-m fading channel and the number of IRS elements. Finally, we derive a closed-form expression to calculate the maximum coverage range of the IRS for given network parameters. Numerical results obtained from Monte-Carlo simulations validate the derived analytical results.

Relay-Selection Refinement Scheme for Secrecy in Untrusted RF-EH Relay Systems With Adaptive Transmission Under Outdated CSI

In this paper, we investigate physical layer security of a dual-hop untrusted multiple amplify-and-forward energy-harvesting relaying network. Two main objectives are considered: saving the relays’ energies and achieving a target secrecy rate. In the presence of direct links between the source and destination nodes, for the first design objective, we propose an adaptive switching scheme between the direct and the relaying modes where the direct link is prioritized. Thus, relay nodes are required only when the direct link fails to achieve the target secrecy rate requirement. For the second design objective, to save energy while forwarding the source information securely, a greedy battery-aware relay-selection scheme is proposed. Furthermore, to reduce the impact of outdated channel state information (CSI) on the system’s secrecy performance, we propose a relay-selection refinement scheme consisting of repeating the relay-selection process prior to the information forwarding phase and we derive a closed-form upper bound of the secrecy outage probability. Our derived upper bound expression shows that our proposed scheme achieves better secrecy performance compared to the direct and conventional two-phase relay transmissions. We also show that, with the relay selection refinement scheme, a significant reduction in the negative impact of outdated CSI is achieved.

Distributed Secure Switch-and-Stay Combining Over Correlated Fading Channels

In this paper, we study decode-and-forward relaying networks in the presence of direct links, where they are used by the eavesdropper to overhear the confidential message from the source and relay. The secure data transmission can go through from either the direct or the relaying branch, and we focus on the practical communication scenarios, where the main and eavesdropper channels are correlated. Although traditional opportunistic selection techniques can choose one better branch to ensure the secure performance, it needs to continuously know the channel state information (CSI) of both branches and may result in a high branch switching rate. To overcome these limitations, we propose a distributed secure switch-and-stay combining (DSSSC) protocol, where only one between direct and relaying branches is activated to assist the secure data transmission, and the switching occurs when the branch cannot support the secure communication any longer. The DSSSC protocol uses either the instantaneous or the statistics of the eavesdropping CSI. For both cases, we quantify the impact of correlated fading on secure communication by deriving an analytical expression for the secrecy outage probability (SOP) as well as an asymptotic expression for the high main-to-eavesdropper ratio region. From the asymptotic SOP, we can conclude that the DSSSC can achieve the optimal secure performance of opportunistic selection with less implementation complexity, and the channel correlation can further enhance the transmission security.

Distributed Multi-Stream Beamforming in MIMO Multi-Relay Interference Networks

In this paper, multi-stream transmission in interference networks aided by multiple amplify-and-forward (AF) relays in the presence of direct links is considered. The objective is to minimize the sum power of transmitters and relays by beamforming optimization under the stream signal-to-interference-plus-noise-ratio (SINR) constraints. For transmit beamforming optimization, the problem is a well-known non-convex quadratically constrained quadratic program (QCQP) that is NP-hard to solve. After semi-definite relaxation (SDR), the problem can be optimally solved via alternating direction method of multipliers (ADMM) algorithm for distributed implementation. Analytical and extensive numerical analyses demonstrate that the proposed ADMM solution converges to the optimal centralized solution. The convergence rate, computational complexity, and message exchange load of the proposed algorithm outperforms the existing solutions. Furthermore, by SINR approximation at the relay side, distributed joint transmit and relay beamforming optimization is also proposed that further improves the total power saving at the cost of increased complexity.

Simultaneous Wireless Information and Power Transfer Strategies in Relaying Network With Direct Link to Maximize Throughput

In this paper, we investigate the optimal throughput performance for simultaneous wireless information and power transfer relaying network in the presence of direct link between the source and the destination. By considering the direct link, system performance can be improved when the channel condition is good from source to destination. The cooperative network consists of a source, an amplify-and-forward relay, and a destination, where the relay harvests energy from the source and then uses the harvested energy to forward information to the destination. At the destination node, the signals from the direct link and relay are combined by the maximum ratio combining (MRC) method. The time switching (TS) relaying protocol is applied to enable energy harvesting and information processing simultaneously. At the same time, we made progress on analyzing the throughput of information transmission on the basis of the MRC method with power switching (PS) relaying protocol. Additionally, adaptive-MRC protocol is proposed based on the TS and PS relaying protocols, which can choose better mode to achieve the communication. The numerical analyses provide practical insights into the effect of various system parameters, such as energy harvesting time splitting ratio, source transmission rate, distance between nodes, and noise power at the destination and the relay.

Physical layer security in cognitive radio network with energy harvesting relay and jamming in the presence of direct link

In this paper, we examine the physical (PHY) layer security in terms of secrecy outage of an energy harvesting (EH) relay based underlay cognitive radio network (CRN) in the presence of an eavesdropper. A dual hop communication network consisting of an energy harvesting relay that harvests energy from radio transmission of secondary user (SU) network using a time switching relay (TSR) scheme is studied. The relay works in decode and forward (DF) mode and forwards the confidential message to the destination, if it is successfully decoded at the relay. The advantage of the direct link between source to destination is exploited, if the relay link is in outage. Furthermore, simultaneous transmission of message signal and jamming signal to the legitimate nodes and illegitimate node respectively degrades the eavesdropping link capacity. We evaluate the secrecy outage probability (SOP) of the network under study. It is found that exploiting the direct link at the destination increases the secrecy in transmission as compared to the case of only relay based transmission without direct link. The impact of interference threshold, energy harvesting time, power allocation factor, target data rate, target secrecy rate and direct link on SOP of this proposed network is investigated.

Outage Probability Analysis and Power Splitter Designs for SWIPT Relaying Systems With Direct Link

This letter investigates the outage performance for simultaneous wireless information and power transfer (SWIPT) relaying systems in the presence of direct link between the source and the destination. For the SWIPT, we employ a power splitter (PS) at the relay, which splits the received signal into the information transmission and the energy harvesting parts. First, we provide an analysis of the outage probability as a closed-form based on a high signal-to-noise ratio approximation. From the analysis, it is recognized that the diversity order of the SWIPT relaying systems equals that of the non-SWIPT cases. The closed-form outage expression also enables us to obtain a simple expression for the PS factor which minimizes the outage probability. Simulation results demonstrate the accuracy of the derived analysis and the efficiency of the proposed PS scheme.

Secrecy Analysis of Multiuser Untrusted Amplify-and-Forward Relay Networks

This paper investigates the secure communications of multiuser untrusted amplify-and-forward relay networks in the presence of direct links. In the considered system, one user is selected among multiple ones for secure transmission to the destination node with the help of an untrusted relay node. To reduce the information leakage to the untrusted relay, the paper considers two selection criteria to select the user based on the direct and the relaying links, respectively. The impact of direct and the relaying link on the system secrecy performance is studied by deriving the close-form ergodic secrecy rate (ESR) as well as the asymptotic expression. From the asymptotic expression, it can be found that the asymptotic ESR increases linearly with the logarithm of the average channel gain ratio of the direct link to the relaying link.

Transceiver Optimization for Full-Duplex Massive MIMO AF Relaying With Direct Link

This paper focuses on the end-to-end signal-to-noise ratio (SNR) maximization for full-duplex massive multiple-input multiple-output (MIMO) amplify-and-forward (AF) relay systems in the presence of direct link. First, we rigorously prove the asymptotic optimality of the maximum-ratio combining/maximum-ratio transmission (MRC/MRT) relaying strategy by taking into account the massive MIMO setup. Then, concerning the equivalent optimization problem with respect to source beamformer, we advocate a two-tier iterative algorithm relying on bi-section search, which guarantees a globally optimal solution. As a byproduct of this approach, we show that the optimal source beamformer has an interesting generalized channel matching structure associated with both source-relay and source-destination links. In addition to the optimal design, we devise a high SNR approximation-based suboptimal scheme, which admits a closed-form solution. Simulation results verify the advantage of our full-duplex relaying designs, and also demonstrate a negligible performance gap between the proposed optimal and suboptimal methods.

A New Relay and Jammer Selection Schemes for Secure One-Way Cooperative Networks

This paper presents different relay and jammer selection schemes for one-way cooperative networks to increase the security against malicious eavesdroppers. We consider a single source-destination cooperative network with multiple intermediate nodes and one or more eavesdroppers. The selection in the proposed schemes is made with the presence of direct links and the assumption that the broadcast phase is unsecured. The proposed schemes select three intermediate nodes. The first selected node operates in the conventional relay mode and assists the source to deliver its data to the corresponding destination via a Decode-and-Forward strategy. The second and third selected nodes are used in different communication phases as jammers to create intentional interference at the eavesdroppers' nodes. Moreover, a hybrid scheme which switches between jamming and non-jamming modes is introduced in this paper. The proposed schemes are analyzed in terms of ergodic secrecy capacity and secrecy outage probability. Extensive analysis and a set of simulation results are presented to demonstrate the effectiveness of the different schemes presented in this work. The obtained results show that the proposed schemes with jamming outperform the conventional non-jamming schemes and the hybrid switching scheme further improves the secrecy capacity. The impact of changing both the eavesdroppers and the relays location on ergodic secrecy capacity and secrecy outage probability is also discussed. Finally, the impact of the presence of multiple eavesdroppers is studied in this paper.