Two-way Relay Networks Research Articles

Distributed Beamforming Design for Nonregenerative Two-Way Relay Networks with Simultaneous Wireless Information and Power Transfer

This paper considers the distributed beamforming design for a simultaneous wireless information and power transfer (SWIPT) in two-way relay network, which consists of two sources, K relay nodes and one energy harvesting (EH) node. For such a network, assuming perfect channel state information (CSI) is available, and we study two different beamforming design schemes. As the first scheme, we design the beamformer through minimization of the average mean squared error (MSE) subject to the total transmit power constraint at the relays and the energy harvesting constraint at the EH receiver. Due to the intractable expression of the objective function, an upper bound of MSE is derived via the approximation of the signal-to-noise ratio (SNR). Based on the minimization of this upper bound, this problem can be turned into a convex feasibility semidefinite programming (SDP) and, therefore, can be efficiently solved using interior point method. To reduce the computational complexity, a suboptimal beamforming scheme is proposed in the second scheme, for which the optimization problem could be recast to the form of the Rayleigh–Ritz ratio and a closed-form solution is obtained. Numerical results are provided and analyzed to demonstrate the efficiency of our proposed beamforming schemes.

Power Allocation for Self-Coded Distributed Space-Time Codes in FD Two-Way Relaying Networks

In this paper, a power allocation space-time coding scheme based on the self-coding property of residual loop interference (RLI) for two-way full-duplex (FD) amplify-and-forward (AF) relaying networks is proposed. The relay and direct link cooperate to forward the received signals to destination node, forming the self-coded distributed space-time code, in which the relay performs self-coding using the RLI after implementing some self-interference cancellation techniques. Then, an iterative power allocation algorithm is presented. With the proposed power allocation algorithm, both amplifying factor and transmit powers of terminal nodes are jointly optimized to maximize the smaller signal-to-interference-plus-noise ratio (SINR) of terminal nodes under a total power constraint when estimated instantaneous channel state information is available. We theoretically demonstrate the benefits of the proposed power allocation algorithm compared to equal power allocation. Simulation results show that the proposed scheme achieves significant improvement in performance compared with existing schemes for FD relaying systems and half-duplex relaying systems.

Security-Reliability Tradeoff Analysis for Underlay Cognitive Two-Way Relay Networks

We consider an underlay wiretap cognitive two-way relay network (CTWRN), where two secondary sources exchange their messages via multiple secondary decode-and-forward digital network coding relays in the presence of an eavesdropper by using a three-phase time division broadcast protocol and sharing the licensed spectrum of primary users. To mitigate eavesdropping attacks, an artificial noise (AN)-aided opportunistic relay selection scheme, called generalized max-min (GMM) relay selection is proposed to enhance physical layer security for the wiretap CTWRNs. The performance of the GMM scheme is analyzed, and evaluated by the exact closed-form outage probability and intercept probability. Additionally, we also provide asymptotic approximations for the outage probability and intercept probability at high signal-to-noise ratio. For comparison, we analyze the performance of the conventional max-min (MM) relay selection scheme as well. It is shown that the GMM scheme outperforms the MM scheme in terms of the security-reliability tradeoff (SRT), where the security and reliability are quantified by the intercept probability and outage probability, respectively. Moreover, the SRTs of the MM and GMM schemes can be substantially improved by increasing the number of secondary relays, while the improvement of the GMM scheme is more evident than that of the MM scheme.

Perfect Self-Interference Cancellation Based on Mode-Switching for Differential Channel-Unaware Two-Way Relay Networks

We consider channel-unaware two-way relay networks in which two single-antenna nodes exchange information through multiple single-antenna half-duplex amplify-and-forward relays. For these networks, we develop a novel self-interference (SI) cancellation scheme that does not invoke channel information neither at the relays nor at the nodes. Unlike its previous counterparts, the scheme proposed herein enables perfect SI cancellation even when the relays have a single antenna each. This was not possible previously without employing an even number of antennas at each relay. In the first phase of the proposed scheme, the network is operated in a one-way relaying mode, whereas in the second phase it is operated in a two-way relaying mode. In the latter mode, the vectors received by the nodes at the end of the first phase are used to completely eliminate SI, without estimating the channel as in existing methodologies. To analyze the effectiveness of the proposed scheme, we derive upper bounds on the pairwise error probability for the Alamouti and $SP(2)$ codes. Using these bounds, we investigate the dependence of the system performance on the data rate. In addition, we show that for all data rates, the proposed scheme outperforms previously proposed ones. Theoretical results are confirmed by simulations.

Joint Beamforming and Relay Selection in AF Two-Way Relay Networks With Energy Transfer

In this paper, we investigate a joint beamforming and relay selection design problem in amplify-and-forward two-way relay networks with energy transfer. In the relay network, multiple relays are responsible for not only information exchange of two sources, but also energy transfer to two sources. Two sources use the power-splitting scheme to harvest energy. The relay selection constraint, included in the aforementioned problem, causes it to be nonconvex. We prove that the relay selection constraint can be replaced by a convex constraint and a concave constraint. Furthermore, we propose to use a sequential parametric convex approximation-based algorithm to solve the problem. Simulation results demonstrate the effectiveness of our proposed scheme.

Optimum Design of Energy Harvesting Relay for Two-Way Decode-and-Forward Relay Networks Under Max–Min and Max-Sum Criterions

We study the optimum design of an energy harvesting relay for two-way decode-and-forward (DF) relay networks. In the networks, the relay harvests energy as well as decodes information using the received signal from two sources with power splitting relaying (PSR) and time switching relaying (TSR) strategies. Since the two-way relay network has two opposite traffic flows, it can be considered as a special case of multi-user systems. Applying the max–min criterion for fairness and max-sum criterion for maximum resource utilization, we optimize the operations of the energy harvesting relay. Specifically, considering the transmission rate constraints of individual hops, we derive optimum power splitting coefficients and optimum time switching coefficients, respectively, for PSR-based and TSR-based networks under both criterions, and analytically calculate the resulting maximum transmission rates. Numerical results confirm that our analyses exactly match with exhaustive search simulations. The obtained optimum coefficients are given in closed-form, therefore, they can be easily adopted in simple two-way relay networks with limited computational power such as sensor/Internet of Things (IoT) networks and can help two-way DF relay networks.

The influence of hardware impairment on the system performance of two-way relaying network

<span>In this research, the system performance of the half-duplex two-way relay network under hardware impairment condition is presented and investigated. In this system model, the time switching protocol and amplify-and-forward (AF)-based are considered. First, of all, the analytical expressions of the outage probability, and achievable throughput with the exact closed form and asymptotic form were proposed and derived. Furthermore, the research results are derived and convinced by Monte-Carlo calculation. The numerical results demonstrated and convinced the analytical and the simulation results are agreed for all possible system parameters.</span>

Interference-Limited Mixed MUD-RF/FSO Two-Way Cooperative Networks Over Double Generalized Gamma Turbulence Channels

In this letter, the performance of multiuser-radio frequency/free space optics (RF/FSO) two-way relay network in the presence of interference is investigated. The FSO link accounts for pointing errors and both types of detection techniques, i.e., intensity modulation/direct detection as well as coherent demodulation, which is modeled as double generalized gamma (D-GG) turbulence channel. On the other hand, the multiple users on the RF link are assumed to undergo Nakagami-m fading. Multiple co-channel interferers (CCIs) which corrupt the signal at relay node are modeled using Nakagami-m distribution. Specifically, the exact closed-form expressions for the outage probability (OP) of the overall system are derived. Moreover, the closed form expression for the achievable sum-rate (ASR) of the considered system is presented. In order to simplify the results, the asymptotic approximations of the OP and ASR are derived in terms of elementary functions. The results presented in the letter are validated by the Monte Carlo simulations.

Asymmetric Adaptive Modulation With Symbol-Based Network Coding for Heterogeneous Two-Way Relay Network

In this paper, we propose an asymmetric adaptive modulation scheme for a heterogeneous two-way relay network (TWRN). In this scheme, due to the heterogeneity of the TWRN, different modulation schemes with different bit rates are assigned to the source nodes. Due to the mismatch between the bit rates of the two opposite traffic flows, the conventional XOR-based network coding is not applicable to this scheme. To cope with this heterogeneity at the relay node, we propose a network coding scheme in which the two data streams are combined at the level of symbol. We optimize the pair of modulation schemes and bit rates such that the instantaneous transmission rate is maximized under the constraint that a quality-of-service requirement in terms of the end-to-end BER is satisfied. To find the solution of this multi-dimensional problem, we propose a simple procedure with very low computational complexity by reducing the problem dimension to one. We analyze the system performance in terms of the average end-to-end BER, the average transmission rate, and the outage probability over Nakagami fading channel. Our analysis shows that this scheme not only achieves high spectral efficiency but also mitigates error propagation from the relay to the transceivers.

Application of block Markov superposition transmission to physical-layer network coding for maritime communications

Physical-layer network coding (PNC) is a promising technology that can improve the throughput of two-way relay networks. This paper focus on designing physical-layer network coding schemes for maritime communication systems. Unlike terrestrial environment, radio propagation at sea is seriously affected by wave movement, ship shaking and other complicated environmental factors. In order to improve the transmission reliability, we apply block Markov superposition transmission (BMST) to PNC schemes. Simulation results show that the proposed schemes outperform the existing convolutional coded PNC schemes over Rician channels, suggesting that the proposed schemes are more appropriate for marine applications.

Relay selection based on energy harvesting in two-way full-duplex relay networks

In this paper, the time switching (TS) relay protocol and the self-energy recycling (SER) relay protocol are integrated into a two-way full-duplex (FD) relay network. For the proposed TS FD relay protocol, the joint optimization of the TS factor and the relay selection (RS) problem is proposed, where the relay nodes switch between energy harvesting (EH) and FD information relaying in different phases. To remove the self-interference (SI) operation at the relay nodes, a novel FD transmission protocol suitable for the two-way relay system is proposed, in which the relay nodes can recycle the SI as part of energy. Then two relay selection schemes are proposed to improve the outage probability and the sum rate of the two-way relay networks. Besides, the proposed RS schemes and two FD relay protocols can improve the performance metrics and solve the power constraint of the relay node. Simulation results show that the proposed two RS schemes for two-way FD relay networks can improve the output performance than traditional schemes.

Beamforming for Cooperative Secure Transmission in Cognitive Two-Way Relay Networks

In this paper, we investigate beamforming design for cooperative secure transmission in cognitive two-way relay networks, where the cognitive transmitter (CT) with multiple antennas helps to forward the signals of two primary transmitters (PTs) and tries to protect the PTs from wiretapping by a single-antenna eavesdropper. With the objective of maximizing the secrecy sum rate (SSR) for PTs, we jointly design the beamforming matrix for the PTs’ signals, the beamforming vector for the cognitive receiver (CR)’s signal, and the artificial noise (AN)’s beamforming matrix, under the quality of service constraint at the CR and the transmit power constraint at the CT. We propose the monotonic optimization-based algorithm to obtain the global optimal solution to the SSR maximization problem, which is a double-layer iterative algorithm and has very high complexity. To balance the complexity and the performance, we also propose a sequential parametric convex approximation-based single-layer iterative algorithm and a zero-forcing-based closed-form algorithm, the latter of which has the lowest complexity. Furthermore, we derive the asymptotic achievable secrecy sum rate when the CT transmit power goes to infinity. We present the numerical results to verify the effectiveness of our proposed algorithms.

On the performance of cognitive two-way relay networks with multi-user scheduling and shadowing

This work investigates the performance of opportunistic multi-user scheduling for next generation cognitive two-way relay networks, where the communication between two secondary source terminals is affected by both fast fading and shadowing. In this approach, all users send pilot signals to the secondary relay terminal and the latter selects the best users based on the shadowing information. To quantify the performance of the proposed scheme, user and system outage probabilities, overall error probability and ergodic sum rate expressions are derived. Moreover, asymptotic analysis is performed to obtain the overall diversity gain and the effects of key parameters including fading/shadowing severity and the number of users in the system. The analytical results, which are validated through the simulations, show that the proposed scheme can be a preferable user scheduling approach for next generation wireless systems.

Performance of Random Access Markov Modelling for Two-Way Buffer-Aided Relaying Networks with Wireless Assisted Links

In this paper, we consider two-way relaying networks (TWRNs) in which information of two users randomly exchange via buffer-aided relay. We assume that all nodes work on half-duplex decode-and-forward for relaying of data packets. The relay is equipped with two finite-size buffers and both the users know the exact number of data packets. The data packets are scheduling with different channel access probabilities to avoid collision at relay. The protocols are developed on the basis of selective transmission scheme with states of Markov chain at relay for buffer state information (BSI) and Hybrid buffer state information/channel state information (BSI/CSI). On the basis of these protocols, we investigate closed-from expressions of maximum achievable sum throughput and end-to-end throughput for TWRNs. The numerical results are obtained with the help of CVX software package. Our proposed scheme significantly outperforms the existing two-way buffer-aided relaying scheme in literature.

Harmonic-Mean-Based Dual-Antenna Selection with Distributed Concatenated Alamouti Codes in Two-Way Relaying Networks

Harmonic-Mean-Based Dual-Antenna Selection with Distributed Concatenated Alamouti Codes in Two-Way Relaying Networks

Robust Optimization for Energy Efficiency in MIMO Two-Way Relay Networks With SWIPT

Energy efficiency (EE) is a key issue in energy-constrained wireless networks because of the energy scarcity. In this paper, we present the robust energy-efficient optimization design for multiple-input–multiple-output two-way relay networks with simultaneous wireless information and power transfer, where the available channel state information is considered to be imperfect. We model the channel errors by using the norm bounded model, and aim to maximize the worst-case EE by jointly designing the precoders of the sources and relay, and the power splitting ratio of the relay under the worst-case transmit power constraints at the sources and relay. To solve the robust EE optimization problem, we propose an iterative optimization algorithm based on the weighted minimum mean-square-error method, where the $\mathcal {S}$ -procedure and sign-definiteness lemma are employed to eliminate the channel errors. To balance the performance and complexity, we propose a channel diagonalization algorithm based on the generalized singular value decomposition, which has lower complexity. The effectiveness of the proposed algorithms is shown by numerical results.

Impact of Co-Channel Interference on Two-Way Relaying Networks with Maximal Ratio Transmission

Amplify-and-forward (AF) two-way relay networks (TWRNs) have become popular to provide spectrally efficient communication when range extension or energy efficiency is needed by utilizing a simple relay. However, their performance can be significantly degraded in practice due to co-channel interference (CCI) which is increasing due to growing number of wireless devices and recent cognitive and non-orthogonal multiple access techniques. With the motivation of improving the performance of AF-TWRNs, the use of maximal ratio transmission (MRT) is investigated to achieve high reliability while requiring low receiver complexity for the relay. First, the signal-to-interference-plus-noise ratio (SINR) expression is formulated and upper bounded. Then, tight lower bound expressions of outage probability (OP), sum symbol error rate (SSER), and upper bound ergodic sum rate (ESR) for each source and for the overall system are obtained. Besides, array and diversity gains are provided after deriving the asymptotic expressions of OP and SSER at high signal-to-noise ratio (SNR). Furthermore, the impact of channel estimation errors on the performance is also included. Finally, Monte Carlo simulation results which corroborate our theoretical findings are illustrated.

Joint beamforming optimisation for NOMA‐based wireless powered multi‐pair two‐way AF and DF relaying networks

Joint beamforming optimisation for a wireless-powered multi-pair two-way relay network (MP-TWRN) is studied. Different from the existing works, the authors investigate relay beamforming and power-domain non-orthogonal multiple access for the wireless powered MP-TWRN. The authors' goal is to optimise the energy-transfer beamforming matrix and the relaying beamforming matrix in the sense of maximising the minimum of the achievable rates among all the users with the power constraint at the relay. Further, they explore the above issue under two relaying schemes: amplify-and-forward and decode-and-forward. The formulated problems are highly non-convex, and thus, it is challenging to solve. To address these, they propose two iterative algorithms to solve those two problems based on the difference of convex programming transformation and the successive convex approximation. Simulations demonstrate that the proposed schemes outperform the orthogonal multiple access schemes.

Research on the security of multiple pairs of massive MIMO system in two-way relay networks

Research on the security of multiple pairs of massive MIMO system in two-way relay networks

Modeling and Analysis of Two-Way Relay Networks: A Joint Mechanism Using NOMA and Network Coding

As a promising technology, non-orthogonal multiple access (NOMA) enhances spectral efficiency and system capacity by allocating the same resource to multiple users. Network coding (NC) has the advantages of compressing data and high spectral efficiency, and it plays a crucial role in two-way relay networks. However, conventional two-way relay networks suffer from throughput limitations due to the use of OMA scheme. In this paper, we utilize a hybrid concept to design a two-way relaying system (namely Hybrid-TWRS) which combines NOMA and NC. Furthermore, we investigate the size-mismatch problem caused by asymmetric channel in the NOMA scheme and propose a bit-match scheme and a symbol-match scheme based on the Hybrid-TWRS. Theoretical derivation and numerical results demonstrate that the proposed method distinctly outperforms both (i) traditional two-way relaying system with OMA in the uplink and NC in the downlink (namely NC-TWRS); and (ii) NOMA-based two-way relaying system (namely NOMA-TWRS).