Two-way Relay Networks Research Articles

Performance Analysis of NOMA-Based Transmission in Two-Way Relay Network

Performance Analysis of NOMA-Based Transmission in Two-Way Relay Network

Security-reliability analysis and optimization for cognitive two-way relay network with energy harvesting

Security-reliability analysis and optimization for cognitive two-way relay network with energy harvesting

Physical-Layer Network Coding Based on Residual Field of Algebraic Integers in OFDM-VLC Two-Way Relay Networks

Physical-Layer Network Coding Based on Residual Field of Algebraic Integers in OFDM-VLC Two-Way Relay Networks

Joint Power Control and Resource Allocation with Rate Fairness Consideration for SWIPT-Based Cognitive Two-Way Relay Networks.

This paper investigates the power control and resource allocation problem in a simultaneously wireless information and power transfer (SWIPT)-based cognitive two-way relay network, in which two secondary users exchange information through a power splitting (PS) energy harvesting (EH) cognitive relay node underlay in a primary network. To enhance the secondary networks's transmission ability without detriment to the primary network, we formulate an optimization to maximize the minimum transmission rates of the cognitive users by jointly optimizing power allocation at the sources, the time allocation of transmission frames and power splitting at the relay, under the constraint that the transmission power of the cognitive network is set not to exceed the primary user interference threshold to ensure primary work performance. To efficiently solve this problem, a sub-optimal algorithm named the joint power control and resource allocation (JPCRA) scheme is proposed, in which we decouple the non-convex problem into convex problems and use alternative steps in the optimization algorithm to get final solutions. Numerical results reveal that the proposed scheme enhances transmission fairness and outperforms three traditional schemes.

Age of Information in Physical-Layer Network Coding Enabled Two-Way Relay Networks

This paper investigates information freshness of two-way relay networks (TWRNs) operated with physical-layer network coding (PNC). Information freshness is quantified by age of information (AoI), defined as the time elapsed since the generation time of the latest received information update. PNC reduces communication latency of TWRNs by turning superimposed electromagnetic waves into network-coded messages so that end users can send update packets to each other more frequently via the relay. While sending update packets more frequently has the potential to reduce AoI, how to handle packet corruption has not been investigated. Specifically, if an old packet is corrupted in any hop of a TWRN, one needs to decide whether to drop or to retransmit the old packet. Therefore, we study the average AoI with and without automatic repeat request (ARQ) in PNC-enabled TWRNs. Interestingly, our analysis shows that neither the non-ARQ scheme nor the pure ARQ scheme achieves a good average AoI. Hence, we put forth an uplink-lost-then-drop (ULTD) protocol that combines packet drop and ARQ. Experiments on software-defined radios indicate that ULTD significantly outperforms non-ARQ and pure ARQ schemes in terms of average AoI, especially when the two end users have imbalanced channel conditions. We believe the insight of ULTD on TWRNs generally applies to other two-hop networks.

Performance analysis and optimization for cellular two-way relay networks with cooperative NOMA transmission

This paper proposes a cooperative non-orthogonal multiple access (NOMA) based two-way transmission scheme for a two-user cellular system, in which a base station (BS) exchanges information with a directlink user and an indirect link user. With superimposition coding, physical-layer network coding (PLNC) and successive interference cancellation, the BS and the two users are able to exchange messages within three time slots. We derive the sum-rate upper bound of the proposed scheme and investigate the optimal time and power allocations to maximize the sum-rate bound. Numerical results demonstrate that the derived bound is tight and the proposed scheme can improve the sum-rates significantly as compared with existing NOMA-based schemes.

Phase-Aligned Complex Linear Physical-Layer Network Coding in OFDM-VLC Two-Way Relay Network

We propose and experimentally demonstrate a phase-aligned complex linear physical-layer network coding (PA-CLPNC) scheme in orthogonal frequency division multiplexing (OFDM) visible light communication (VLC) two-way relay network (TWRN). The proposed scheme, based on the finite field of the Gaussian integer, achieves PNC of high-order QAMs and reduces the constellation size of the superposed signal at the relay before broadcasting it back to users. We further adopt a phase alignment strategy to enhance the minimum Euclidean distance between classes, into which the constellation points of the superposed signal at the relay are categorized. VLC-TWRN experiments with throughput of 600Mbit/s-1.47Gbit/s, using either uniform 8QAM/16QAM OFDM or bit-and-power loading, show that the proposed PA-CLPNC achieves one to two orders of magnitude reduction in the bit error ratio compared to the conventional amplify-and-forward PNC scheme.

Implementation of DNN-Based Physical-Layer Network Coding

Physical-layer network coding (PNC) has been proposed to double the throughput of a wireless two-way relay network (TWRN) and has been adapted so that it can be applied in vehicular ad-hoc networks (VANETs). Deep neural network (DNN)-based physical-layer network coding (PNC) for the TWRN system has been studied in recent years, under the assumption that the channel state information (CSI) is available to the end nodes and the relay node. However, the state-of-the-art DNN-based PNC system was only feasible for theoretical simulations, and no practical system design has been studied. When moving from theory to practical wireless systems, there are two critical issues to tackle: 1) unlike conventional regularly shaped quadrature amplitude modulations (QAMs), the constellation points of a DNN-based PNC system's end nodes are irregular and unpredictable to the relay node; and 2) other than in high-precision simulations, the practical DNN-based PNC system's constellation points are not fixed, that is, there is usually a cluster of points that must be grouped together so that the transceiver's digital-to-analog converter (DAC) and analog-to-digital converter (ADC) can work properly. To solve the above issues, we proposed the DNN-PNC implementation that can be realized using the Universal Software Radio Peripheral (USRP) platform. The computational complexity of our simulated DNN-PNC is one order of magnitude smaller than that given in the literature, while maintaining a good bit error rate (BER). In the implementation phase, we find that the naturally-happened power imbalance between the two end nodes' signals in our implementation can significantly boost the PNC constellation recovery, and hence reduce the system BER. Our experimental results showed that the BER of DNN-PNC can be lower than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10^{-4}$</tex-math></inline-formula> when the signal-to-noise ratio (SNR) is 16 dB for the 2-QAM case and 20 dB for the 4-QAM case, respectively. These results indicate that DNN-based PNC is feasible for VANET applications, where the CSI changes and is difficult to obtain.

Research on non-smooth signal identification algorithm for electronic communication in a two-way relay network

Abstract To better allow accurate identification of non-smooth signals in two-way relay networks. In this paper, an algorithm based on the two-way relay network system model, the RLS algorithm, is proposed. The algorithm introduces the recursive least squares principle algorithm to the basic cost function of nonsmooth signal identification, optimizes the signal identification model by exponential weighting, and uses the time-varying variance matrix and the optimal identification data separation matrix to make real-time estimates of the new cost function for obtaining nonsmooth signals. And a new index, the PI index, is introduced to define whether the non-stationary signal is stable and reliable. Through two sets of comparison tests, we can learn that the RLS algorithm based on the two-way relay network system is better than other algorithms in the identification of non-stationary signals. It shows that the RLS algorithm can better identify and manage the information flow in a two-way relay network, make the non-stationary signal more stable, and further improve the convergence and convergence speed of the non-stationary signal to provide a better information interaction experience for both sides of electronic communication.

Pilot Optimization and Channel Estimation for Two-Way Relaying Network Aided by IRS With Finite Discrete Phase Shifters

In this paper, we investigate the problem of pilot optimization and channel estimation of two-way relaying network (TWRN) aided by an intelligent reflecting surface (IRS) with finite discrete phase shifters. In a TWRN, there exists a challenging problem that the two cascading channels from User1-to-IRS-to-relay and User2-to-IRS-to-relay and two direct channels from User1-to-relay and User2-to-relay interfere with each other. Via smartly designing the initial phase shifts of IRS and pilot pattern, the two cascading channels are separated over only four pilot sequences by using simple arithmetic operations like addition and subtraction. Then, the least-squares estimator is adopted to estimate the two cascading channels and two direct channels. The corresponding sum mean square errors (MSE) of channel estimators are derived. By minimizing Sum-MSE, the optimal phase shift matrix of IRS is proved. Then, two special matrices Hadamard and discrete Fourier transform (DFT) matrix are shown to be two optimal training matrices for IRS. Furthermore, the IRS with discrete finite phase shifters is taken into account. The Hadamard matrix requires only one-bit phase shifters to achieve the optimal Sum-MSE performance for specific numbers of IRS elements. Using theoretical derivation and numerical simulations, we find that 3 or 4-bit phase shifters are sufficient for IRS to achieve a negligible MSE performance loss for any number.

Performance of SWIPT-Enabled FD TWR Network With Hardware Impairments and Imperfect CSI

Cooperative communication with energy harvesting capability increases network coverage and provides a green and sustainable solution for future wireless technologies. In this article, we analyze the performance of a two-way relay network, where bidirectional information exchange between two nodes takes place via simultaneous wireless information and power transfer-enabled relay node, operating in full-duplex (FD) mode over generalized Nakagami- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula> fading channels. Different nonlinear power amplifier (NLPA) models, such as solid-state power amplifier, traveling wave tube amplifier, and soft envelope limiter, are considered at the relay node. The effect of practical constraints, such as imperfect channel state information (CSI) and transceiver hardware impairments (HIs), is considered. Further, we consider the impact of residual self-interference (RSI) due to imperfect successive interference cancellation in the FD mode. For performance analysis, the closed-form expressions of outage probability (OP), asymptotic OP, system throughput, energy efficiency, ergodic capacity, and asymptotic ergodic capacity are derived. The diversity order of the considered network is evaluated. Further, the impact of imperfect CSI, NLPA, HIs, RSI, and other parameters are highlighted on the system performance. The performance of FD and half-duplex mode is also compared. Monte Carlo simulations confirm the accuracy of the derived closed-form expressions.

Integrated Satellite Multiple Two-Way Relay Networks: Secrecy Performance Under Multiple Eves and Vehicles With Non-Ideal Hardware

In this paper, we study the secrecy performance for an integrated satellite multiple two-way terrestrial relay network, where the non-ideal hardware, multiple vehicle eavesdroppers and multiple legitimate vehicle users are applied in the considered networks. To derive better secrecy performance, opportunistic terrestrial selection scheme is considered for the networks. Besides, the colluding eavesdropping scheme is also considered, where all the vehicle eaves work together to overhear the information. On these foundations, the closed-form expression for the secrecy outage probability is gotten. To obtain the further insights of system parameters and channel parameters in high signal-to-noise ratio regime, the asymptotic investigations for the secrecy outage probability is also investigated. At last, Monte Carlo results are given to show the efficiency and correctness of the analytical results.

DNN-Based Physical-Layer Network Coding for Visible Light Communications

The key difference between visible light communication (VLC) and radio frequency (RF) communication is the former’s line-of-sight (LOS) transmission nature, and hence a relay node has to be adopted for VLC to extend its coverage. Physical-layer network coding (PNC) has the advantage of doubling the throughput of a two-way relay network (TWRN), where two end nodes exchange information via the help of a relay, compared with the conventional store-and-forward routing strategy. Although PNC has been studied for VLC in the literature, the state-of-the-art schemes are highly inefficient, requiring tight phase synchronization between the two end nodes, and hence difficult to realize. This paper proposes the application of a deep neural network (DNN) to a PNC VLC system, named DP-VLC, that enables misaligned phases and can deal with the light channel gains and noises in a satisfactory manner without introducing additional computation complexities. We implement DP-VLC using the universal software radio peripheral (USRP) software radio platform and a self-developed VLC optical front-end using commercial off-the-shelf (COTS) light-emitting diodes (LEDs) and photo-diodes (PDs). We find that irregular constellations generated by DP-PNC can be transmitted and recovered in a 1.5 m VLC link effectively. Experimental results show that our DP-PNC prototype performs better than conventional PNC VLC system when the signal-interference-to-noise ratio (SINR) of received optical signals is larger than 13.63 dB and can achieve a throughput of up to 77.38 Mbps in a 20 MHz channel under PNC scheme when the SINR is 22.86 dB. More importantly, we find that DP-VLC performs even better than fixed-constellation PNC system in the saturated SINR regime (e.g., 20–25 dB) where non-linear effects may happen compared with moderate SINR regimes (e.g., 10–20 dB), showing its adaptability to unpredictable impairments in optical links. Our first attempt at realizing DNN-based optical PNC in a TWRN has paved the way for future PNC-enhanced VLC systems.

An Error Propagation Free Decode and Forward Scheme for Channel-Unaware Two-Way Relay Networks

This paper considers channel-unaware two-way relay networks (TWRNs) with a single-antenna decode-and-forward (DF) relay. These networks typically rely on differential signalling which results in error propagation and error floors. To overcome this drawback, we develop a novel differential signalling scheme, which, in contrast with existing ones, does not cause detection errors to propagate from one block to the next. We obtain a union bound on the block error rate (BLER), which shows that the proposed differential method exhibits superior performance to existing schemes, eliminates error floors and results in a BLER that decays as the inverse of the signal-to-noise ratio. Furthermore, the proposed scheme removes the unitarity restriction on the transmitted symbols in conventional differential signalling. Therefore, non-unitary symbols can be used in the differential signalling which provides more degrees of design freedom and better performance. Our numerical results confirm our analytical findings.

On the secrecy rate balancing problem of two‐way multiple‐input multiple‐output relaying networks using rank‐1 beamforming strategy

AbstractThe physical layer security of a two‐way relay network (TWRN) in the presence of an eavesdropper is considered, where two multi‐antenna nodes try to exchange messages via a multi‐antenna two‐way relay node. It is assumed that the transmission occurs in two hops. Throughout the first hop, the messages are concurrently transmitted at the same time from two transmitting nodes to the relay, while an eavesdropper tries to overhear the signals. Throughout the second hop, to prevent eavesdropping, the relay broadcasts a combination of the received signals in the null‐space of the eavesdropper's channel gain vector. The objective is to explore the proper power allocation policy at transmitting ends as well as an effective beamforming strategy at the relay to address the secrecy rate balancing (SRB) problem. The radio frequency multiple‐input multiple‐output (RF‐MIMO) architecture design is proposed in which the relay can merely process one data stream; hence, the relay beamforming matrix is of rank‐one. The underlying problem is non‐convex. To tackle this issue, the original problem is divided into two sub‐problems, where using semi‐definite relaxation (SDR) and sequential parametric cone approximation (SPCA) techniques, are converted into convex forms, where suboptimal solutions are derived. Numerical results demonstrate effectiveness of the proposed method compared to the existing works addressed in the literature and also show that our proposed solution has better performance even for the secrecy energy efficiency (SEE) metric.

Outage minimisation for two‐way piece‐wise and forward (PF) relaying with SWIPT based energy harvesting

Simultaneous wireless information power transfer with power splitting is a new technology for energy harvesting that splits the received signal into two streams with adjustable power levels for information decoding and energy saving. Traditionally, decode and forward (DF) and amplify and forward (AF) schemes are used for energy harvesting in three-step two-way relay networks (TWRNs). In this paper, piece-wise and forward (PF) scheme are applied with power splitting for energy harvesting in TWRNs. In PF, the received signal from each source at relay is compared to an adaptive threshold, if the signal strength is above the threshold, the relay functions similar to DF scheme, otherwise, the relay transmits a particular combination of its received signals from the sources. Next, the outage performance of PF protocol in a three-step TWRN is studied and optimal lossless energy saving (OLES) method are presented to minimise the outage probability by computing the optimum power splitting ratio. The simulation and analytical results indicate that the outage probability of PF can be significantly improved using OLES technique in comparison with other related relaying schemes. The results show that the outage probability of PF-OLES is 13, 3, and 5 dB $5\nobreakspace \text{dB}$ lower than AF-OLES, DF-OLES, and DAPS [1], respectively.

User power allocation and relay beamforming design for secrecy sum rate maximization in two-way relay networks

In this paper, we investigate the physical-layer security (PLS) for multiple peer-to-peer communications in a two-way relaying networks involving 2 L single-antenna users, an N R -antenna relay, and an N E -antenna eavesdropper. Only a pair of peer-to-peer users demand security to exchange messages because the eavesdropper intends to wiretap them in both transmission phases. Our goal is to maximize the achievable secrecy sum rate (SSR) of specific peer-to-peer users, provided that restrictive constraints are satisfied. We endorse a null space beamforming technique at the relay to simplify the optimization problem and exclude private information leakage. The main problem is still very complicated due to coupled variables of the user transmit power with the relay beamforming matrix. Therefore, we convert the main problem into two subproblems, one in terms of the relay beamforming matrix and the other in terms of the user transmit power. To find a locally optimal solution for the relay beamforming, we turn the main problem into a difference of convex (DC) programming problem and for user power allocation, we drive the main problem into a constrained nonlinear optimization problem. Finally, simulations are performed to evaluate the performance benefits of the proposed method.

Process-and-Forward two-way relay using multiple space-time line codes

In this paper, we consider a two-way relay (TWR) network that two source nodes (SNs) with multiple antennas exchange data to each other via a relay node (RN) with multiple antennas, when full channel state information (CSI) is available at RN and no CSIs are available at SNs. In case that both SNs transmit space-time block coded multiple data streams to the RN, a new precoding method is proposed for the RN by combing block-wise processing and multiple space-time line codes (STLCs) requiring no CSI at the receiver except the channel norm. An optimal precoding matrix is derived that maximized the minimum achievable rate using a gradient-based iterative algorithm. Moreover, to reduce the design complexity, we derive the closed-form precoding matrix minimizing the total mean square error (MSE), and its theoretical performance is analyzed in terms of the per-stream MSE and detection signal-to-interference-plus-noise ratio. Through numerical simulations, the theoretical MSE is verified by comparing with an experimental MSE, and it is demonstrated that the proposed precoding method is more beneficial than existing retransmission methods for the RN when the SN transfers both single and multiple data stream(s).

Joint Trajectory and Resource Optimization for UAV-Aided Two-Way Relay Networks

Unmanned aerial vehicle (UAV)-aided two-way relay networks with multiple terrestrial user pairs is investigated, where a fix-wing UAV is served as a two-way relay to assist information transmission. We aim to maximize the total rate of the two-way relay networks while the quality of service (QoS) being guaranteed. A novel relay strategy, namely <i>time-slots pairing</i>, is proposed by exploiting physical-layer network coding (PNC) protocol. Specifically, the UAV receives and buffers the signals from the scheduled user pair when the UAV approaches one of them, then forwards the modulated signal when the UAV is close to the other. As a result, a non-convex total rate maximization problem is formulated by joint trajectory design and resource optimization. To solve this problem, we first decompose it into three sub-problems, and then a three-step iterative algorithm that can effectively handle the non-convex problem with no worse than a polynomial complexity is developed leveraging the block coordinate descent (BCD) technique and successive convex approximation (SCA) method. Simulation results finally illustrate that: 1) Compared with amplify-and-forward (AF) or decode-and-forward (DF) protocols, the proposed design based on PNC protocol can significantly improve the total rate of the two-way relay networks; 2) the time-slots pairing relay strategy always achieves a significant enhancement on performance than the traditional non-time-slots pairing relay strategy; 3) the time-slots pairing relay strategy is more favorable for the mobile PNC relay networks since it can well resist the degradation of the entire networks caused by users&#x2019; increased service quality requirements.

Phase Optimization for Massive IRS-Aided Two-Way Relay Network

In this paper, with the help of an intelligent reflecting surface (IRS), the source (S) and destination (D) exchange information through the two-way decode-and-forward relay (TW-DFR). We mainly focus on the phase optimization of IRS to improve the system rate performance. Firstly, a maximizing receive power sum (Max-RPS) method is proposed via eigenvalue decomposition (EVD) with an appreciable rate enhancement, which is called Max-RPS-EVD. To further achieve a higher rate, a method of maximizing minimum rate (Max-Min-R) is proposed with high complexity. To reduce its complexity, a low-complexity method of maximizing the sum rate (Max-SR) via general power iterative (GPI) is proposed, which is called Max-SR-GPI. Simulation results show that the proposed three methods outperform the case of random phase method, especially the proposed Max-SR-GPI method is the best one achieving at least 20\% rate gain over random phase. Additionally, it is also proved the optimal rate can be achieved when TW-DFR and IRS are located in the middle of S and D.