Joint Relay Research Articles

Joint Relay Assignment and Power Allocation for Multiuser Multirelay Networks Over Underwater Wireless Optical Channels

Multiuser multirelay network is a potential scenario to fulfill the transmission requirements of various sources and high-volume traffic for the Internet of Underwater Things. To efficiently complete concurrent transmissions for multiple users, this article investigates the joint relay assignment and power allocation problem for multiuser multirelay networks based on the underwater optical wireless communication (UOWC). Specifically, the multiuser multirelay network for UOWC based on decode-and-forward relaying is modeled, where the absorption, scattering, solar radiation noise, and oceanic turbulence of UOWC are all considered. The joint optimization problem of relay assignment and power allocation is formulated as a mixed-integer programming problem, where the average outage probability is minimized with the constraint of total transmitted power. To solve this joint problem, an alternating optimization method is employed, which alternately optimizes the relay assignment and power allocation subproblems. The relay assignment subproblem is modeled as a weighted bipartite matching problem and solved by an improved Kuhn–Munkres algorithm, whereas the power allocation subproblem is proved to be quasiconvex and solved by an iterative bisection algorithm. The simulation results indicate that the proposed schemes significantly reduce the average outage probability with fast convergence.

Joint relay assignment and energy‐efficiency maximization in energy‐harvesting downlink/uplink clustered nonorthogonal multiple‐access networks

AbstractIn this article, the problem of joint relay assignment and energy‐efficiency maximization (J‐RA‐EE‐MAX) in energy‐harvesting downlink (DL) and uplink (UL) clustered nonorthogonal multiple‐access (NOMA) networks is considered. Specifically, the aim is to perform relay assignment to user clusters in the DL and UL directions, while simultaneously maximizing energy efficiency (EE) over each relay via multiobjective optimization, and satisfying users' quality‐of‐service (QoS) constraints. However, problem J‐RA‐EE‐MAX happens to be nonconvex in each link direction, and thus is computationally prohibitive. Alternatively, a low‐complexity solution procedure is devised to solve problem J‐RA‐EE‐MAX in each link direction by: (i) optimally solving the energy‐efficiency maximizing power allocation (EE‐MAX‐PA) for each (user cluster, relay) combination to construct the relays' preference profile, and (ii) performing relay assignment via Gale's top trading cycles (TTC) matching mechanism. In particular, the optimal solution of the EE‐MAX‐PA problem is obtained by transforming it into a concave maximization problem, while the TTC mechanism is executed in linear time‐complexity so as to obtain a stable relay assignment. Simulation results are presented to validate the proposed solution procedure, which is shown to yield comparable user cluster sum‐rate and relay EE to the J‐RA‐EE‐MAX scheme, and superior to other schemes in both link directions, however, at lower computational complexity, while satisfying users' QoS constraints. Finally, this work sheds light on the importance of decoupling relay assignment in the DL/UL directions, which significantly improves user cluster sum‐rate and relay EE in comparison to coupled relay assignment schemes, and thus serves the requirements of 5G networks and beyond.

Joint relay and jammer selection for secure cooperative networks with a full‐duplex active eavesdropper

In this study, the authors investigate the secure transmission of a cooperative network, in which a source communicates with a destination via multiple cooperative nodes in the presence of a full-duplex active eavesdropper, which can intercept the confidential signals and transmit jamming signals simultaneously. To safeguard the security of legitimate communication, two joint relay and jammer selection schemes are proposed according to the availability of the eavesdropper's channel state information, namely, optimal relay and random jammer selection scheme and optimal relay and optimal jammer selection scheme. The authors first derive the exact closed-form expressions of the secrecy outage probability (SOP) for different selection schemes. Aiming at minimising SOP, they then adopt the deep feedforward neural network to determine the optimal power allocation between the selected relay and jammer. Further, the asymptotic expressions for SOP in the high signal-to-noise ratio regime are derived. Numerical results verify the analysis and demonstrate the performance advantage of the proposed scheme over conventional relay selection scheme with optimal power allocation.

Finite Blocklength Non-Orthogonal Cooperative Communication Relying on SWIPT-Enabled Energy Harvesting Relays

A finite blocklength (FBL) twin-user non-orthogonal cooperative downlink system is considered, wherein a base station simultaneously communicates with the users whilst relying on simultaneous wireless information and power transfer (SWIPT) enabled energy harvesting relay. Closed-form analytical expressions are obtained for the end-to-end average block error rate (BLER) assuming potentially dissimilar Nakagami- $m$ fading links. Furthermore, asymptotic expressions are derived for the individual user’s end-to-end average BLER performance and diversity order at high signal to noise ratios (SNRs). Additionally, since, the stronger user’s signal is detected, while assuming the weaker user’s signal to be interference, an analytical bound has been obtained for characterizing the effect of this interference on the BLER performance of both users. Furthermore, a framework is developed for joint relay splitting factor and user power allocation towards end-to-end blocklength minimization under strict reliability constraints by converting the original non-convex optimization problem to a convex geometric program (GP) that can be solved efficiently. Simulation results are presented to validate the analytical expressions derived and illustrate the efficacy of the proposed scheme.

Robust Secure Beamforming for SWIPT-Aided Relay Systems With Full-Duplex Receiver and Imperfect CSI

This paper investigates an energy-constrained relay-assisted secure communication system in the presence of a legitimate source-destination pair and a passive eavesdropper with imperfect channel state information (CSI). The energy-constrained relay exploits a power splitting (PS) protocol to simultaneously harvest energy and decode information from the received signal from the source, and then forwards the confidential signal to the full-duplex (FD) destination while it simultaneously transmits artificial noise (AN) to confuse the passive eavesdropper. This robust beamforming design is developed by maximizing the secrecy rate under the constraints of energy harvesting (EH) requirement at the relay and AN transmit power at the destination. Specifically, we formulate the joint relay beamforming and PS ratio as well as the AN power optimization as a non-convex quadratically-constrained problem. In order to circumvent this non-convexity issue, we decompose the original problem into three subproblems which can be determined by the proposed semidefinite relaxation (SDR) and successive convex optimization methods. Furthermore, we derive closed-form expressions for the optimal solutions of each subproblem. Finally, the original non-convex problem can be solved through a convergence guaranteed iterative algorithm. Simulation results are provided to demonstrate the effectiveness and superior performance of the proposed robust design compared with the benchmark schemes.

A Joint Relay Selection and Power Allocation MAC Protocol for Underwater Acoustic Sensor Network

Different from the mechanism of medium access control (MAC) for the land-based sensor networks, the underwater acoustic sensor network faces problems such as high propagation delay, spatial-temporal uncertainty and high energy consumption. Orthogonal frequency division multiple access (OFDMA) is a channel access mechanism based on OFDM multi-carriers modulation technology. In OFDMA, multiple users can make use of the orthogonality of the subchannels to simultaneously access channel for parallel transmission. For underwater multi-hop network scenario, this paper proposes a MAC protocol of the joint relay selection and power allocation (JRSPA-MAC) based on OFDMA mechanism. The proposed protocol is divided into flowing phases: Firstly, the node determines the hop distance from the sink nodes and sends the request when there are packets to be sent. Then forwarding nodes election algorithm is proposed based on the channel condition and the delay information. On this basis, the sending nodes autonomously use the particle swarm optimization (PSO) algorithm to allocate its subchannels’ power. Secondly, taking advantage of the high underwater acoustic propagation delay, the receiving node reasonably schedule the sending time, makes sure the parallel communication of multiple pairs of nodes in one handshake period. Finally, the destination nodes and the forwarding nodes acknowledge the received packets and completes this data transmission. The simulation results show that compared with the hand-shake based MAC protocols, the JRSPA-MAC reduces the probability of packet loss caused by data conflicts between nodes. In case of low power consumption, the network throughput are effectively improved, and the average end-to-end delay of the network is reduced.

Joint Relay Selection, Full-Duplex and Device-to-Device Transmission in Wireless Powered NOMA Networks

This paper investigates non-orthogonal multiple access (NOMA), cooperative relaying, and energy harvesting to support device-to-device (D2D) transmission. In particular, we deploy multiple relay nodes and a cell-center D2D device which can operate in full-duplex (FD) or half-duplex (HD) mode to communicate with a cell-edge D2D device. In this context, there are two possible signal transmission paths from the base station (BS) to the far D2D user either through multiple decode-and-forward (DF) relay nodes or through a near D2D user. Consequently, we propose three schemes to support D2D-NOMA systems, namely non-energy harvesting relaying (Non-EHR), energy harvesting relaying (EHR) and quantize-map-forward relaying (QMFR) schemes. For each of the proposed schemes, closed-form expressions of the outage probabilities of both D2D users are derived. Extensive Monte-Carlo simulation results are provided to validate the derived analytical expressions. The study results show that the proposed schemes can improve the outage performance compared to conventional orthogonal multiple access (OMA) schemes. Moreover, it is shown that the Non-EHR scheme achieves the best outage performance among the three considered schemes.

Oblique projection: An interference mitigation for MIMO multihop full‐duplex relay

AbstractFull‐duplex (FD) assisted multiple‐input–multiple‐output relaying increases the spectral efficiency through efficient spectrum utilization. The major concerns for the FD assisted relay system are the cancellation of inter‐relay interference (IRI) and relay self‐interference (RSI). Previous works were idealized only on the basis of either RSI or IRI; hence, all other realistic models to overcome the RSI or IRI were unappreciated. This paper aims to reduce the interference‐to‐signal ratio. We propose the joint relay's receive beamforming based on oblique projection to mitigate the RSI and IRI in multihop environments. Applying first orthogonal projectors to the relay's received signal, the relay's received signals are divided into interference subspace and signal subspace. Then, the desired signal is recovered from signal subspace by using oblique projection that cancels out the RSI and IRI. The geometric interpretation of oblique projection is analyzed to understand its properties. The major merit of this method is that it can be adopted even when the RSI and IRI channels are unknown. In comparison to the existing interference cancellation methods, the signal‐to‐interference ratio of our proposed method does not deteriorate even when the transmit power is high. Additionally, we demonstrate that the oblique projection output is equivalent to an orthogonal projection using geometrical interpretation, simulation, and theoretical derivation.

Joint Relay Selection and Power Allocation for Underwater Cooperative Optical Wireless Networks

Cooperative transmission is a promising technology to expand communication range and improve system performance for underwater optical wireless communications (UOWCs). To adequately exploit the benefits of cooperative UOWCs, the relay selection and power allocation problems require to be explored. This paper investigates the relay selection and power allocation issues for the cooperative UOWC in the presence of solar radiation noise. Specifically, the cooperative UOWC based on amplify-and-forward (AF) relaying is modeled, where the effects of absorption, scattering, and solar radiation noise are all considered. We design two optimization schemes that minimize the energy consumption and maximize the overall signal-to-noise ratio (SNR), respectively. Both schemes are proved to be strictly quasi-convex and solved by Levenberg-Marquardt (LM) algorithm. The effectiveness of the proposed schemes is evaluated in line-of-sight (LOS) links and non-line-of-sight (NLOS) links, and the main factor affecting the relay selection and power allocation are derived for both LOS links and NLOS links.

A Robust Stackelberg Game Approach for Joint Relay Selection and Optimal Power Allocation for Cooperative Device-to-Device Communication Under Channel Uncertainties

Device-to-device (D2D) communication enables direct communication among users in close proximity, bypassing the core network. With relay assistance, D2D users can have the leverage of transmitting data directly over long distances with increased throughput. However, due to dynamic nature of wireless networks, there may be uncertainty in the channel parameters known to a user. In this paper, the authors address the joint problem of relay selection and optimal power allocation in single-source multi-relay D2D networks when the perfect channel state information for relay channels is unknown. The uncertainty has been modeled as a bounded difference between actual and nominal values. An incentive-based robust Stackelberg game is proposed in which the relay devices determine the price of power allocated to them by the source device. A closed form expression for the optimal power was obtained. Further, the Stackelberg equilibrium was derived and its existence and uniqueness was demonstrated. The performance of the proposed game with the nominal game was compared. Simulation results confirm the effectiveness of the robust game-theoretic solution in an incomplete information environment.

Secure Beamforming for Cooperative Wireless-Powered Networks With Partial CSI

In this paper, we investigate the physical layer security (PLS) of cooperative wireless-powered networks where a source transmits confidential information to a destination with the aid of wireless-powered intermediate nodes equipped with multiple antennas in the presence of a passive eavesdropper. We consider two generalized joint relay and jammer selection (GJRJS) frameworks based on the power splitting (PS) and time switching (TS) techniques, respectively. Specifically, the intermediate nodes which cannot decode the source signal successfully are selected to act as friendly jammers to transmit artificial noise, and the remaining nodes are exploited as relays to simultaneously forward the source signal through cooperative beamforming. We further propose two cooperative secure beamforming (CSB) schemes for the PS-based GJRJS (PS-GJRJS) and TS-based GJRJS (TS-GJRJS) frameworks, respectively. To be specific, we investigate the optimization of the beamforming vector of our selected relays for maximizing the secrecy rate of the source–destination transmission. A closed-form solution is derived under the assumption of available instantaneous channel state information (CSI) of the main link and statistical CSI of the wiretap link. In addition, we also illustrate that the pure relay selection (PRS) scheme is a special case of our GJRJS framework at high signal-to-noise ratios (SNRs). The numerical results show that the proposed CSB scheme achieves a higher secrecy rate than the traditional maximal ratio transmission (MRT) method for both the PS-GJRJS and TS-GJRJS frameworks. Additionally, the GJRJS framework outperforms the PRS as well as the joint best relay and jammer selection (JBRJS) methods in terms of secrecy rate.

Joint Relay Selection and Destination Assisted Cooperative Jamming with Power Allocation for Secure DF Relay Network

To enhance the physical-layer security of a decode-and-forward (DF) cooperative relay network in the presence of a potential eavesdropper, a joint relay selection and destination assisted cooperative jamming (CJ) scheme is proposed. In phase I, the source transmits its confidential signal, and concurrently, it cooperates with the friendly jammers and the destination to send jamming signals to confuse the eavesdropper without affecting the forwarding relay which is preselected. In phase II, the forwarding relay retransmits the decoded signal, and simultaneously, the particular relay cooperates with the friendly jammers to send jamming signal to confuse the eavesdropper without affecting the destination. The achievable secrecy rate maximization (SRM) problem is investigated by optimally allocating the power between the data signal and jamming signals. Although SRM problem is non-convex, the power allocation problem can be solved by a bisection method together with a one-dimensional search. Optimal relay selection and suboptimal relay selection schemes are also proposed to further improve the secrecy rate. Simulation results are presented to show that the joint scheme can attain higher achievable secrecy rates than some existing works.

Time-Switching EH-Based Joint Relay Selection and Resource Allocation Algorithms for Multi-User Multi-Carrier AF Relay Networks

In this paper, an energy efficiency maximization (EEM) optimization problem for the multi-user multi-carrier energy-constrained amplify-and-forward (AF) multi-relay network is formulated under the total source transmit power budget and energy-causality constraints. We consider that each relay node is solely powered by the source nodes, employing energy harvesting time-switching (EHTS) protocol to harvest the energy through the ambient radio-frequency (RF) signal transmitted from the source nodes under the simultaneous wireless information and power transfer (SWIPT) paradigm. First, we propose a subcarrier and energy causality-based multi-relay selection policy. Second, we jointly optimize the parameters that control the energy efficiency (EE) of the system namely multi-relay selection, subcarrier pairing, user allocation, power allocation, and RF EHTS time block, that renders the problem to be a mixed integer non-linear programming problem (MINLP) which remains NP-hard to solve. Hence, we remodel the problem to a tractable quasi-concave form by applying a string of convex transformations. Later, we propose an iterative EEM algorithm to optimize the multi-parameter problem. Further, a suboptimal and best relay selection algorithm is studied by trading-off between complexity and performance. The effectiveness of the proposed algorithms is demonstrated by simulation results.

Joint Beamforming Design and Power Minimization for Friendly Jamming Relaying Hybrid RF/VLC Systems

In this paper, we study physical layer security (PLS) aspects of a hybrid radio frequency (RF)/visible light communication (VLC) system with multiple decode-and-forward relaying nodes equipped with friendly jamming capabilities. We first propose a novel joint relay–jammer selection algorithm to enhance the secrecy performance of the network. Next, we obtain beamforming vectors and the minimal power values, which satisfy the required secrecy threshold. Our simulation results show that the proposed hybrid RF/VLC scheme obtains higher secrecy capacity compared to the standalone VLC and RF technologies. Finally, we proved the proposed technique to be an eavesdropping-resilient PLS solution.

Joint relay selection and energy‐efficient power allocation strategies in energy‐harvesting cooperative NOMA networks

AbstractIn this paper, the problem of joint relay selection and energy‐efficient power allocation (J‐RS‐EE‐PA) in energy‐harvesting cooperative nonorthogonal multiple‐access networks is considered. In particular, a base station communicates with multiple users via a selected energy‐harvesting relay, such that energy efficiency (EE) is maximized, subject to quality‐of‐service constraints. Four different EE power allocation strategies are considered, namely, global energy efficiency, minimum energy efficiency, product energy efficiency, and sum energy efficiency. For all the aforementioned strategies, the J‐RS‐EE‐PA problem happens to be nonconvex and, thus, is computationally prohibitive. In turn, the J‐RS‐EE‐PA problem is decoupled into two subproblems: (1) relay selection and (2) energy‐efficient power allocation. Different relay selection strategies are explored, and then, computationally efficient algorithms are devised to optimally solve the different EE power allocation strategies for the selected relay. In addition, a low‐complexity solution procedure is proposed to solve the J‐RS‐EE‐PA problem. Simulation results are presented to evaluate the resulting EE of the different relay selection and power allocation strategies in comparison to the formulated J‐RS‐EE‐PA problem. More importantly, the proposed solution procedure is shown to be superior to the different explored relay selection strategies as well as yielding the global optimal solution of the J‐RS‐EE‐PA problem for the global energy efficiency, minimum energy efficiency, product energy efficiency, and sum energy efficiency strategies, while satisfying quality‐of‐service requirements.

Joint relay selection and max‐min energy‐efficient power allocation in downlink multicell NOMA networks: A matching‐theoretic approach

AbstractIn this paper, the problem of joint relay selection and max‐min energy‐efficient power allocation (J‐RS‐MMEE‐PA) in downlink multicell nonorthogonal multiple access (NOMA) networks is studied. In particular, the goal is to perform relay selection for each cellular user within each cell, so as to achieve max‐min energy efficiency while satisfying quality‐of‐service (QoS) constraints. However, the formulated J‐RS‐MMEE‐PA problem happens to be nonconvex (ie, computationally intensive). In turn, a solution procedure for max‐min energy‐efficient power allocation is devised to determine the energy efficiency of each user per potential relay while meeting the target minimum rate per user. After that, the relay selection problem is modeled as a student‐project allocation with preferences over projects matching problem. A polynomial‐time complexity stable matching (SM) algorithm is proposed, which takes into account the maximum number of users that can select a relay as well as the number of users that can be associated with a base station. However, the proposed SM algorithm can only guarantee an SM size that is at least half the size of the maximum “optimal” cardinality SM. Thus, the improved SM (I‐SM) algorithm with polynomial‐time complexity is devised, so as to guarantee an SM size that is at least two‐thirds of that of the optimal matching. Lastly, simulation results are presented to compare the proposed matching algorithms to the J‐RS‐MMEE‐PA scheme, where it has been shown that the I‐SM algorithm is superior to its SM counterpart and yields comparable energy efficiency per cellular user to the J‐RS‐MMEE‐PA scheme while satisfying QoS constraints.

Joint Relay and Eavesdropper Selection Strategy Against Multiple Eavesdroppers over Nakagami-$m$ Fading Channels in Cooperative Decode-and-Forward Relay Networks

Physical layer security (PLS) in the presence of multiple eavesdroppers over Nakagami-m fading channels for cooperative decode-and-forward (DF) relay network that consists of one source, one destination, and multiple relays is investigated. Different from the recent PLS system model that only considers one eavesdropper during eavesdropping attack, this paper extends the one-eavesdropper case to a multiple-eavesdropper scenario and investigates joint relay and eavesdropper selection (JRES) strategy against eavesdropping attack over Nakagami-m fading channels. In the proposed strategy, the best relay is selected via the maximum relay forward channel capacity. In addition, since eavesdroppers are noncooperative, the worst case is considered. Namely, the wiretap channel between the best relay and the chosen eavesdropper has the maximum capacity. Traditional direct transmission and opportunistic relay selection (ORS) scheme in the presence of one eavesdropper over Rayleigh channel are regarded as benchmarks. Moreover, a security-reliability tradeoff (SRT) performance is analyzed, where the reliability performance is expressed by outage probability (OP), while the security performance is measured by intercept probability (IP). Closed-form expressions of OP and IP are derived. The numerical results show that the proposed JRES scheme outperforms the traditional direct transmission and the ORS scheme in the presence of one eavesdropper over Rayleigh channel. The SRT performance is enhanced obviously with the increasing of relay numbers and Nakagami channel fading factor m for a given number of eavesdroppers, which extends the PLS and SRT performance analysis to a more general case in a cooperative DF relay network.

Channel-Aware Energy-Efficient Two-Hop Cooperative Routing Protocol for Underwater Acoustic Sensor Networks

This paper sought to investigate the performance of multi-hop underwater acoustic sensor networks (UW-ASNs) when they deploy cooperative routing algorithms combining the cooperative communication and routing methods. Taking into account energy efficiency, the studied schemes are discriminated by different policies of selecting next hop nodes as well as relay nodes of one-hop cooperative communications such that the transmission energy of routing paths is minimized. In order to take full advantage of broadcast nature in wireless communication, we propose to use a node exploited as a joint relay for two-hop cooperative communication. In addition, the unreliable communication of acoustic channels is addressed by the incorporated channel-aware mechanism which updates the links by exploiting packet receptions. On the bases of communication, two cooperative routing protocols are developed. The simulation results show that the network employing the proposed schemes achieves improved performance in terms of energy efficiency, throughput, and end-to-end delay as compared to the related works.

Analysis of Joint Relay Selection and Resource Allocation Scheme for Relay-Aided D2D Communication Networks

Device-to-device (D2D) communications, as one of the promising technology for the future network, allow two devices in proximity to communicate directly with each other to alleviate the data traffic load for the base station (BS). In this paper, we analysis the joint relay selection and resource allocation scheme for relay-aided D2D communication networks. The objective is to maximize the total transmission data rates of the system while guaranteeing the minimum quality of service (QoS) requirements for both cellular users (CUs) and D2D users (DUs). We first propose a social-aware relay selection algorithm with low computational complexity to obtain the suitable relay nodes (RNs) for D2D links. Then the power control schemes are considered when a D2D link works in the D2D direct or relay mode to maximize the system transmission data rates. On basis of this, we propose a greedy-based mode selection and channel allocation algorithm, which can not only allocate the appropriate channel resource for D2D links, but also select the optimal communication mode for them. Numerical results demonstrate that the proposed scheme is capable of substantially improving the performance of the system compared with the other algorithms.

Joint relay selection and power allocation protocol for two-way relaying system

The Joint Relay Selection and Power Allocation (JRSPA) for Amplify-and-Forward (AF) relaying (JRSPA-AF) combines Power Allocation (PA) and Relay Selection (RS) very well in Two-Way Relaying (TWR) system. In order to explore the system performance of JRSPA-AF protocol, the outage probability and Diversity-Multiplexing Trade-off (DMT) of JRSPA-AF protocol are analysed. One upper bound, two lower bounds and approximate expressions of outage probability along with the DMT are derived. Based on this, the idea of JRSPA is introduced into Decode-and-Forward (DF) relaying TWR system, and the JRSPA-DF protocol is proposed. Moreover, the exact expressions of outage probability and DMT for JRSPA-DF are derived. Simulations are developed to validate the analytical results, the results show that the approximation outage probability of JRSPA-AF can be used well to evaluate system outage probability over the entire transmission SNR region, and the analysis results of outage probability of JRSPA-DF are consistent with its simulation results. In addition, the simulation results indicate that JRSPA-DF can achieve lower outage probability than JRSPA-AF and other protocols.