Two-way Relay Networks Research Articles

Outage Performance Analysis of Two-Way Full-Duplex DF Relaying Networks With Beamforming

In this paper, we investigate the system outage performance of full-duplex (FD) two-way relay networks where all nodes are equipped with FD multiple-input multiple-output (MIMO) capability and two sources exchange information via decode-and-forward (DF) relay protocol in two consecutive time slots. In order to completely separate the desired signal and interference, we consider a finite size of buffer at the relay. We propose zero-forcing based beamforming schemes for two different cases of the buffer-aided relay and non-buffer-aided relay. We derive exact closed-form expression and lower bound of the outage probability for those two-way relay system, respectively. Numerical results show that the proposed buffer-aided relay scheme significantly improves the system performance over the conventional scheme in terms of outage probability.

On the performance of AF full FSO two-way relay networks over exponentiated Weibull fading channels

This work implements free space optical communication (FSO) in an amplify-and-forward (AF) two-way relay network (TWRN) to exploit the advantages of spectral efficiency, full-duplex Gigabit throughput, license-free spectrum, easy installation and to overcome the limitations of free space optical communication due to distance dependent turbulence induced fading. To quantify the performance of the proposed scheme, overall outage probability (OOP) and ergodic sum rate (ESR) expressions are derived over Exponentiated Weibull (EW) fading channels. The simple theoretical results which are validated with the Monte-Carlo simulations show that a system designer can get a quick idea about the FSO TWRN without resorting to complex prototypes or simulations.

Robust Tomlinson-Harashima Precoding for Two-Way Relaying

Most of the non-linear transceivers, which are based on Tomlinson Harashima (TH) precoding and have been proposed in the literature for two-way relay networks, assume perfect channel state information (CSI). In this paper, we propose a novel and robust TH precoding scheme for two-way relay networks with multiple antennas at the transceiver and the relay nodes. We assume imperfect CSI and that the channel uncertainty is bounded by a spherical region. Furthermore, we consider the sum of the mean square error as the objective function, under a limited power constraint for transceiver and relay nodes. Simulations are provided to evaluate the performance and to validate the efficiency of the proposed scheme.

Total transmit power minimization with physical layer security in multiuser peer-to-peer two-way relay networks

In this paper, we investigate a multiuser peer-to-peer (MUP2P) two-way relay network (TWRN) in the presence of an eavesdropper. In this network, only the k∗th pair of users demand secure communication. Also, a multi-antenna eavesdropper aim to wiretap the confidential data of the k∗th pair in both cooperative phases. Our goal is to minimize the total transmit power provided that the secrecy rate for two secure users and the quality of service constraints of each user are met. For simplifying the problem and also omitting the confidential information leakage in the second phase, a null space beamforming (NSBF) scheme is employed. Despite this, the problem is still non-convex. We divided the problem into two subproblems to solve beamforming and users’ power formulated in two iterations, respectively. Three different methods are proposed to solve the first subproblem, semi-definite programming (SDP), second-order cone programming (SOCP), and sequential quadratic programming (SQP). We employ a sequential parametric convex approximation (SPCA) method to convert the users’ power subproblem into inequality form a linear programming (LP) problem to solve it. In the following, we evaluate the computational complexity of these methods. Simulation results confirmed the validity of the proposed methods.

The average achievable rate of multi-antenna two-way relay networks with interference alignment

The large system analysis (LSA) has recently been shown to be a very useful tool for computing the average achievable rate. In this paper, we use LSA to derive the users' average achievable rate of multi-antenna two-way relay networks with interference alignment (IA), and we then derive the rate expressions under both equal power allocation and optimal power allocation. It is shown that the obtained closed-form rate expressions are functions of the average signal-to-noise ratio (SNR) for each data stream. Extensive simulation studies show that the average achievable rate expressions derived through LSA provide accurate estimates of the average achievable rate for two-way relay networks with interference alignment.

Investigation of error performance in OFDM with network coding techniques in multiple relay networks

Abstract The aim of this paper proposes an orthogonal frequency-division multiplexing (OFDM) with network coding to improve the error performance of the system when the messages are transmitted from user to receiver. Two-way relay (TWR) networks are applied to reduce the transmission time slots. The exclusive-OR (XOR) coding is used for network coding in which source nodes exchange their information via TWR nodes. The XOR coded bits provides redundancy to achieve the transmit diversity gain which improves the error performance of the TWR network. OFDM is exploited for TWR to obtain the frequency selective fading nature of wireless channels. The different modulation schemes such as Quadrature Phase Shift Keying (QPSK), 16-Quadrature Amplitude Modulation (QAM) and 64-QAM with OFDM system are simulated and QPSK is selected as it gives the lowest bit error rate (BER). The multiple relaying schemes with different numbers of the information packets are also considered in this paper. Simulation results show that multiple relay schemes provide faster transmission time and better error rate performance. Moreover, different kinds of channel coding schemes such as Convolutional, Reed-Solomon (RS) and turbo codes are applied in OFDM system with network coding to compare and evaluate the BER performance of the proposed system. From the simulation results, network coded OFDM scheme with turbo codes give better BER performance for given Signal-to-Noise Ratio (SNR) in relaying scheme with different numbers of information packets compared to those of convolutional and RS codes. It shows that, the error rate performance and transmission time is reduced 10 percent than the conventional scheme at even at low SNR value.

Performance evaluation of a two-way relay network with energy harvesting and hardware noises

In this paper, we propose a Two-Way Cognitive Relay Network (TWCRN) where the secondary users operate on an underlay mode to access the licensed bands. In the proposed protocols, two secondary sources transmit their data to a relay in the first time slot, and then the relay would forward the received information to both sources in the remaining time. Moreover, the relay is self-powered by harvesting energy from ambient Radio Frequency (RF) signals, using the Time Switching (TS) and the Power Switching (PS) method. This paper concentrates on evaluating the performance of the secondary networks under the impact of hardware impairments and co-channel interference from the primary networks. In particular, based on the secondary transmitters’ constraint power, we derive the closed-form expressions of the outage probability and the throughput over Rayleigh fading channels in two cases: TS and PS. We also investigate the energy efficiency issue and the locally optimal position of the relay to maximize the system throughput, which provides much information to install the relay location. Finally, our derivations are verified by Monte Carlo simulation.

Joint beamforming and time switching designs for energy-constrained cognitive two-way relay networks

In this paper, we investigate a joint beamforming and time switching (TS) design for an energy-constrained cognitive two-way relay (TWR) network. In the network, the energy-constrained secondary user (SU) relay employs TS protocol to harvest energy from the signals sent by the circuit-powered primary user (PU) transmitter, and then exploits the harvested energy to perform information forwarding. Our aim is to maximize the sum rate of SUs under the constraints of the data rate of PU, the energy harvesting and the transmit power of the SU relay. To determine the beamforming matrix and TS ratio, we decouple the original non-convex problem into two subproblems which can be solved by semidefinite relaxation and successive convex optimization methods. Furthermore, we derive closed form expressions of the optimal solutions for each subproblem, which facilitates the design of a suboptimal iterative algorithm to handle the original sum rate maximization problem. Simulation results are presented to illustrate the effectiveness and superior performance of the proposed joint design against other conventional schemes in the literature.

Analysis and Design of Rateless Two-way Relay Networks Based on a Multiply-and-Forward Scheme

In this paper, we propose a rateless, three-stage, two-way multiply-and-forward (MF) relaying system over the Rayleigh flat fading channel, where two source nodes communicate with each other through a relay node and all nodes work on half-duplex and time-division mode. We thoroughly analyze the signals during all three stages in the proposed MF system and derive the closed-form symbol error rate (SER) expressions for an uncoded MF-two-way relay network (MF-TWRN). Furthermore, we provide the equivalent point-to-point fading channel model, which is employed to carry out the asymptotic performance analysis. We finally put forth an optimization model for the MF-TWRN with fountain codes. Simulation results show that our optimized degree distribution can provide outstanding performance for the MF-TWRN compared to those in the literature.

Channel-Aware Adaptive Physical-Layer Network Coding Over Relay-Assisted OFDM-VLC Networks

In this article, we propose and experimentally demonstrate the full-duplex channel-aware adaptive physical-layer network (APNC) coding over orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) to boost the capacity of relay-assisted VLC networks. The network encoding is realized by detecting signals coming from different terminal nodes simultaneously with a single photodiode at the relay node, which then achieves the amplify-and-forward function. At the receiver of each terminal node, the desired signal can be reconstructed by applying the corresponding APNC decoding to the detected interfered signal from the relay node. The use of adaptively loaded OFDM not only addresses the inherent frequency fading issue in VLC systems but also relaxes the synchronization issue at the relay node. The detailed principles of the proposed full-duplex channel-aware APNC scheme are presented, and its performance is experimentally investigated in a two-way-relay (TWR) VLC network. Experimental results show that the proposed scheme exhibits superior BER performance to other existing network scheduling schemes. For a TWR-VLC network with a fixed 600-Mb/s throughput, we achieve around two orders of magnitude reductions in the bit-error-rate (BER) by using the proposed channel-aware APNC scheme compared to other schemes, and BERs of 3.96 × 10−4 and 5.32 × 10−4 are attained for the two transmission links in the TWR network, respectively.

Improving Physical-Layer Security for Full-duplex Radio aided Two-Way Relay Networks

The power allocation optimization problem is investigated for improving the physical-layer security in two-way relaying networks, where a full-duplex relay based half-jamming protocol (HJP-FDR) is considered. Specially, by introducing a power splitter factor, HJP-FDR divides the relay’s power into two parts: one for forwarding the sources'' signals, the other for jamming. An optimization problem for power split factor is first developed, which is proved to be concave and closed-form solution is achieved. Moreover, we formulate a power allocation problem to determine the sources'' power subject to the total power constraint. Applying the achieved closed-form solutions to the above-mentioned problems, a two-stage strategy is proposed to implement the overall power allocation. Simulation results highlight the effectiveness of our proposed algorithm and indicate the necessity of optimal power allocation.

Semi-Blind Joint Timing-Offset and Channel Estimation for Amplify-and-Forward Two-Way Relaying

In this paper, we consider the problem of joint timing-offset and channel estimation for amplify-and-forward (AF) two-way relay networks (TWRNs). This problem is solved for generic pulse-shaping filters, taking into account the filter truncation in practical communication and considering both pilot-based and semi-blind estimation strategies. Beginning with pilot-based estimation, we propose a novel Maximum-likelihood joint timing-offset and channel estimator, as well as an alternative estimator based on the special properties of Zadoff-Chu sequences. The first algorithm offers high accuracy, almost overlapping with the Cramer-Rao bound (CRB), while the second offers very low computational complexity. We then develop a semi-blind estimator based on the expectation maximization (EM) framework, exploiting the underlying Hidden Markov Model to apply Baum-Welch forward-backward recursion. The semi-blind CRB is also obtained as an indicator of the best achievable performance. Using simulations, we show that the semi-blind algorithm yields superior accuracy to pilot-based estimation, as well as improved symbol-error-rates and performs very close to the semi-blind CRB. Additionally, a low-complexity approximate EM algorithm is proposed for the case of rectangular pulses. Finally, we consider the possibility of errors in integer-offset estimation and propose pilot-based and semi-blind generalized likelihood ratio test (GLRT) schemes for correcting such errors.

Interference Suppression and Energy Efficiency Improvement With Massive MIMO and Relay Selection in Cognitive Two-Way Relay Networks

We analyze a relay assisted wireless communication link between two underlay massive multiple-input multiple-output (MIMO) terminals. Specifically, an amplify and forwarding (AF) two-way relay is optimally selected to maximize the sum rate and to keep the interference on the primary user (PU) below an interference threshold. We first obtain asymptotic signal-to-interference-plus-noise ratio (SINR) values for two scenarios: (1) the relays and the two end nodes use transmit power scaling and (2) only the end nodes use transmit power scaling. For these two cases, we derive optimal power allocations subject to the PU interference constraints. With these optimal power allocations, we analyze how relay selection impacts the outage, the sum rate, and the energy efficiency of the network. For the first scenario, the outage can be reduced to zero with appropriate power allocation and the relay selection can be done offline. For the second scenario, outage will depend on the instantaneous channel state between the relays and the PU. Furthermore, for a realistic power consumption model, we analyze the energy efficiency and show that relay selection will increase it.

Parallel Two-Way Relay Channel Estimation in Cloud-Based 5G Radio Access Networks

The cloud radio access network (C-RAN) aims at migrating the traditional base station functionality to a cloud-based centralized base band unit (BBU) pool, thereby providing a promising paradigm for fifth-generation (5G) wireless systems. This results in a novel wireless architecture in which mobile users communicate with the cloud via distributed remote radio heads (RRHs) as relays, through two successive wireless links. The availability of accurate channel state information at the BBU pool is a critical requirement in such systems. This paper addresses the channel estimation problem at the terminals of a C-RAN using a two-way relay network (TWRN) model. To the best of our knowledge, for the first time we introduce a cloud-based channel estimation algorithm implementation leveraging cloud computing capabilities of virtualization and parallelization. By bridging the gap between cloud computing and wireless communication, this work achieves a step towards the open problem of network function virtualization (NFV) in C-RANs. Through a deep serial algorithm analysis, we are able to utilize data decomposition as well as exploratory decomposition in order to achieve significant speedup, which scales well with problem size. We assess the performance gains of our cloud-based algorithm via extensive simulation experiments, and report almost 5 × reduction in computation time as compared to the state-of-the-art.

Secure resource allocation for green two‐way relay networks with mutually untrusted relay and mobile users

The authors investigate secure resource allocation for an orthogonal frequency division multiple access two-way relay network, which has several pairs of mobile users (MUs) and one energy harvesting (EH) relay. In this network, the relay is untrusted and each MU pair may also overhear the other MU pairs' confidential information. In particular, to forward information for each MU, the relay adopts power splitting technique to harvest energy from radio frequency signals emitted by each MU. For this scenario, which has hardly been considered, the joint problem of subcarrier allocation, power control, and power splitting ratio selection is formulated, with the aim to maximise the total secrecy rate of all MUs. A suboptimal algorithm with low complexity and satisfactory performance is proposed to solve the formulated non-convex problem.

Proof of the Equivalency of MSE-Constrained and Rate-Constrained Power Optimization Approaches to Distributed Bi-Directional Beamforming

Considered in this article is an asynchronous single-carrier two-way relay network, where two transceivers aim to communicate through a set of amplify-and-forward (AF) relays. This network is asynchronous in the sense that the signal transmitted by any of the two transceivers arrives the other transceiver through different relaying paths, with possibly different propagation delays, thereby materializing a multipath channel. At sufficiently high data rates, this multi-path end-to-end channel causes inter-symbol-interference (ISI) in the received signals. Considering such a network, this paper presents two contributions. The first contribution is the rigorous characterization of the region of the mean-squared errors (MSEs) of the symbol estimates at the two transceivers under a total power budget, and when linear block post-channel equalization is used at the receiver front-end of the two transceivers. The importance of this MSE region characterization resides in the fact that knowing this region allows for characterization the region of un-coded probabilities of error at the two transceivers. Also, this MSE region characterization paves the way towards presenting the second contribution in this paper. Indeed, in the second contribution, this article relies on this MSE region characterization to rigorously prove that an MSE-constrained total power minimization approach and a rate-constrained total power minimization approach to design transceiver power allocation and distributed beamforming are equivalent, if the MSE thresholds in the former approach and the rate thresholds in the latter approach are properly chosen. The equivalence of these two approaches implies that the un-coded MSE performance of the network can be inferred from the rate-constrained problem, and conversely, the coded rate performance of the network can be inferred from the MSE-constrained total power minimization problem.

Noncoherent Distributed Beamforming in Decentralized Two-Way Relay Networks

Many noncoherent distributed strategies for cooperative sensor networks that do not require channel knowledge at any antenna to overcome the overhead involved in channel estimation are lately suggested; however, these strategies suffer from low system performance in terms of bit error rate (BER) and a comparably high decoding complexity. Differential beamforming strategies have recently been proposed to overcome these problems; however, they are implemented using the four-phase protocol. Thus, we propose a new strategy based on the three-phase protocol to increase the symbol rate. By doing this, a significant improvement can be achieved in the overall system performance. Hence, in this article, a new bidirectional differential beamforming strategy is suggested: 1) to be applied on the three-phase protocol instead of the four-phase protocol; 2) to be applicable for a decentralized wireless sensor network using single-antenna sensors distributed randomly between the communicating base stations; 3) to enjoy low decoding complexity; and 4) to improve the network performance in terms of BER by maximizing the received signal-to-noise ratio at the receiving base station without requiring channel knowledge at any antenna in the whole network. From our simulation results, the proposed strategy shows a substantially improved BER performance compared with the current state-of-the-art ones.

Security and Reliability Analysis of a Two-Way Half-Duplex Wireless Relaying Network Using Partial Relay Selection and Hybrid TPSR Energy Harvesting at Relay Nodes

In recent years, physical layer security has been considered as an effective method to enhance the information security beside the cryptographic techniques that are used in upper layers. In this paper, we provide the security analysis for a two-way relay network, where the two sources can only communicate through the intermediate relay nodes. In particular, we consider the scenario that there is an eavesdropper in the vicinity of one source node. Both reliability and security aspects are taken into consideration in our work. To enhance the reliability of communication, the intermediate relays are supplied with the energy harvested from the sources' radio frequency (RF) signals using hybrid time-switching and power splitting (TPSR) protocol. Also, we apply the relay selection technique to select the best relay for the information exchange between two sources. Regarding security, the secrecy of information is improved with the help of friendly jammers nearby the eavesdropper. We provide the in-dept reliability and security analysis in terms of the closed-form expressions of the outage probability (OP) at the source nodes, the intercept probability (IP) at the eavesdropper, the secrecy outage probability (SOP), and the average secrecy capacity (ASC) of the system. Finally, the Monte Carlo simulations are also conducted to verify the correctness of our analysis and the effectiveness of the proposed scheme. Numerical results confirms that with the appropriate and feasible choices of involved parameters, both outage OP and IP can be kept at small values to guarantee the reliable and secure communication of the system.

Two-Way Relaying Non-Orthogonal Multiple Access With Imperfect Successive Interference Cancellation in Power Line Communications

In this article, we study the application of cooperative non-orthogonal multiple access (NOMA) in power line communication (PLC) where two users exchange information using a two-way relay (TWR). Under the assumption of perfect and imperfect successive interference cancellation (SIC), we analyze key performance metrics for PLC log-normal channels impaired by impulsive noise. In particular, for delay-limited (DL) and delay-tolerant (DT) modes, approximate closed-form expressions are derived for the user outage probability and user ergodic rate. The analytic results are verified via Monte Carlo simulations. Compared with the conventional orthogonal multiple access (OMA) scheme, the simulation results show that our proposed NOMA TWR enhances the user outage probability and the user ergodic rate performance in the low signal-to-noise ratio (SNR) region. However, this gain diminishes as the imperfect SIC error increases and is non-existent beyond a certain level of imperfect SIC. Furthermore, the system performance is maximized by optimizing the power allocation coefficients where the system throughput and ergodic sum-rate of the NOMA TWR network in DL and DT operation, respectively, are shown to approach a ceiling in the presence of imperfect SIC. The energy efficiency (EE) of the proposed system is also analyzed for DL and DT modes.

Cooperative non-orthogonal multiple access for two-way relay networks

In this paper, we propose a cooperative non-orthogonal multiple access (NOMA) based relaying scheme for a two-way relay network (TWRN) where one user communicates with the base station (BS) directly while the other indirect-link user exchanges message with the BS via the help of a decode-and-forward (DF) relay station. We analyze the sum-rate performance of the proposed scheme, and derive a closed-form expression for the ergodic capacity. Furthermore, we analyze the asymptotic performance in the high signal-to-noise ratio (SNR) region. Simulation results show that the proposed DF based cooperative NOMA TWR transmission scheme achieves a much higher sum-rate than its AF counterpart and conventional time-division multiple access (TDMA) based scheme.