Multiuser Two-way Relay Network Research Articles

Performance Evaluation and Optimization of Traffic-Aware Cellular Multiuser Two-Way Relay Networks Over Nakagami-<italic>m</italic> Fading

We consider a cellular multiuser two-way relay network, where a multiantenna base station communicates with one of the several single-antenna mobile stations via a single-antenna relay terminal. We derive the exact and asymptotic overall outage probability (OOP) expressions of the considered system with asymmetric traffic under Nakagami- m fading. Based on the asymptotic analysis, we formulate three optimization problems, namely, optimal power allocation (OPA) with fixed relay location, optimal relay location with fixed power allocation, and jointly OPA and relay location to minimize the OOP. Specifically, we investigate two OPA schemes, called OPA I and OPA II. To this end, OPA I provides the solutions for individual transmit powers at each terminal, whereas under OPA II, relay terminal is assigned with half of the total power and the remaining power is allocated to the end terminals. The numerical and simulation results corroborate our theoretical findings, and reveal that under highly asymmetric traffic, OPA I can achieve a significant OOP performance than OPA II for different relay locations, except at the relay location where both coincide. Under symmetric traffic, OPA I outperforms OPA II, irrespective of relay locations. The OOP performance can be further improved if the power allocation and relay location are jointly optimized, especially under unbalanced conditions.

Joint Impact of Co-Channel Interference and Channel Estimation Error on Outage Performance of Cellular Two-Way Relay Networks

This paper investigates the joint impact of co-channel interference and channel estimation error (CEE) on the performance of cellular multiuser two-way relay network, wherein a multi-antenna base station communicates with one of the several single-antenna mobile stations via a single-antenna relay. Specifically, we derive a tight lower bound closed-form expression for the overall outage probability under Rayleigh fading channels. We also evaluate an asymptotic outage expression in the high signal-to-noise ratio regime. Based on the asymptotic analysis, we study some practical cases of interest under both interference-limited and interference-free scenarios with CEE, from which various insights into the system diversity order are presented. Numerical and simulation results corroborate our theoretical findings.

Cellular Multiuser Two-Way Relay Network With Cochannel Interference and Channel Estimation Error: Performance Analysis and Optimization

In this paper, we study the impacts of cochannel interference (CCI) and channel estimation error (CEE) on the performance of a cellular multiuser two-way relay network. The network comprises of one N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> -antenna base station (BS), M single-antenna users, and one L-antenna relay with N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> number of interferers. In particular, by employing antenna selection at the relay, we propose beamforming and antenna selection transmission schemes at the BS with user scheduling in the presence of CEE and CCI. Then, we derive the tight closed-form lower bound expressions for the overall outage probability under Nakagami-m fading. We also deduce the asymptotic outage expressions in the high signal-to-noise ratio regime, from which the impacts of CEE and CCI on the achievable diversity order are highlighted. Moreover, using the asymptotic expressions, three optimization problems to minimize the outage probability are formulated and solved: first, optimal power allocation (OPA) with fixed relay location. Specifically, we develop two OPA approaches, namely, OPA I and OPA II. OPA I allocates the total power appropriately at each terminal, whereas OPA II allocates the fixed half of the total power at the relay and remaining power to base station and users; second, optimal relay location with fixed power allocation; and third, jointly optimal power allocation and relay location. Our results reveal that the OPA I outperforms OPA II, irrespective of different channel/system parameters, interferers, and relay locations. In addition, we demonstrate that the effect of interferers at the relay can be ignored if we allocate sufficient power to the base station and users. It is also shown that the CEE and CCI can severely degrade the outage performance, and even reduce the diversity order to zero. Further, it is highlighted that the impact of CCI can be alleviated by increasing the number of relay antennas. The numerical and simulation results validate our analytical findings, and also demonstrate that the antenna selection scheme offers comparable outage performance as beamforming scheme.

Performance Analysis of Cooperative Spectrum Sharing With Multiuser Two-Way Relaying Over Fading Channels

In this paper, we analyze the performance of an overlay multiuser two-way relay network with $N$ th-user selection by employing a three-phase cooperative spectrum sharing (3PCSS) scheme. Hereby, a one-end primary user (PU) communicates with the arbitrary (which may be the $N$ th best) selected other-end PUs with the cooperation of a secondary user (SU) in three transmission phases. By splitting up its available power, the SU assists in bidirectional primary transmissions and accesses the spectrum for its own transmission, based on satisfying the primary system outage constraint. Considering the involved $N$ th-order statistics with correlation, we derive the outage probability expressions for both primary and secondary systems over general fading channels. Based on these expressions, we compute the total system throughput of the 3PCSS scheme and compare it with other competitive schemes. We illustrate that the 3PCSS scheme can effectively realize spectrum sharing by harnessing additional cooperative diversity and achieving spectral efficiency at par with the primary two-phase direct transmission system. Numerical and simulation results substantiate our theoretical findings.

Joint subcarrier and power allocation for df-based multiuser two-way relay networks

Two-way decode-and-forward (DF) relay technique is an efficient method to improve system performance in 5G networks. However, traditional orthogonal frequency division multiplexing (OFDM) based two-way relay systems only consider a per-subcarrier relay strategy, which treats each subcarrier as a separate channel, which results in significant sum rate loss, especially in fading environments. In this paper, a joint coding scheme over multiple subcarriers is involved for multi-pair users in two-way relay systems to obtain multiuser diversity. A generalized subcarrier pairing strategy is proposed to permit each user-pair to occupy different subcarriers during the two transmission phases, i.e., the multiple access and broadcast phases. Moreover, a low complexity joint resource allocation scheme is proposed to improve the spectrum efficiency with an additional multi-user diversity gain. Some numerical simulations are finally provided to verify the efficacy of our proposal.

Energy Efficiency for Underlay Cognitive Multiuser Two-Way Relay Networks

In this paper, we investigate the energy efficiency maximization (EEM) problem for underlay cognitive multiuser two-way relay networks, by jointly optimizing the power allocation of secondary users and the beamforming matrix of the cognitive relay. The EEM problem is highly non-convex and thus the global optimum is intractable. We first show that the EEM problem can be equivalently transformed to a difference of convex program. Then an efficient iterative algorithm based on the constrained concave convex procedure is proposed to obtain a local optimum of the EEM problem. Simulation results are presented to demonstrate that our proposed solution performs much better than existing schemes even when the iterative number is very small.

MMSE-Based Filter Designs for Cognitive Multiuser Two-Way Relay Networks

This paper considers cognitive multiuser two-way relay networks (MU-TRNs) where multiple secondary users (SUs) concurrently exchange their data with other SUs with an aid of a two-way relay in licensed primary networks. In this paper, we particularly focus on linear relay filter designs that effectively minimize the mean square error of SUs while providing a certain level of quality of service for the primary network. To this end, we first propose an iterative algorithm that identifies a solution based on an alternating optimization technique. Then, a closed-form relay filter design is presented, which employs a high signal-to-noise ratio approximation to reduce the computational complexity. Through the simulation results, we confirm that the proposed closed-form relay filter design outperforms conventional approaches and provides the performance close to the proposed iterative algorithm with much reduced complexity.

Achievable Degrees of Freedom of Cellular Multiuser Two-Way Relay Networks

In this paper, we study a network information flow problem for cellular multiuser two-way relay networks (MU-TWRNs) where uplink and downlink messages are exchanged through a multiantenna relay node using two-way relaying. Both single- and two-cell scenarios are investigated. In particular, for the two-cell scenario, we consider two schemes with and without base station (BS) cooperation, respectively. By comparing the degrees-of-freedom (DOF) upper bounds obtained via cut-set theory, we derive the feasibility conditions to achieve the optimal DOF for all the schemes. These DOF bounds are shown to be achieved by suitably applying some physical-layer signaling techniques, such as physical-layer network coding, signal alignment, block diagonalization, and the newly proposed successive interference nulling. By comparing one-way relaying in terms of the normalized DOF, we observe that the proposed two-way-relaying-based scheme can double the performance with symmetric antenna configurations; however, the gain is reduced under unbalanced antenna configurations.

Distributed Power Allocation for Multiuser Two-Way Relay Networks Using Stackelberg Game

Recently, two-way relay networks have been regarded as a promising technique that can improve bandwidth utilization. In this paper, the power allocation problem for multiuser two-way relay networks with amplifyand-forward protocol is investigated. In order to describe the self-interestedness of nodes in two-way relay networks, a two-level Stackelberg game is introduced to jointly optimize the benefits of the source pair and the relay nodes, where the relay nodes are modeled as leaders and the source pair is modeled as a follower. To facilitate the power allocation process, a distributed game-theoretic power allocation algorithm is proposed. Then, the existence and optimization of the Stackelberg equilibrium for the proposed power allocation algorithm is proven. The convergence of the presented algorithm is also analyzed by proving that price update is a standard function. Simulation results indicate that the proposed power allocation algorithm can improve energy utilization by jointly optimizing the utilities of both source pair and relay nodes.

Outage Probability of &lt;i&gt;N&lt;/i&gt;-th Best User Selection in Multiuser Two-Way Relay Networks over Nakagami-&lt;i&gt;m&lt;/i&gt; Fading

Outage Probability of &lt;i&gt;N&lt;/i&gt;-th Best User Selection in Multiuser Two-Way Relay Networks over Nakagami-&lt;i&gt;m&lt;/i&gt; Fading

Pair-Aware Interference Alignment in Multi-User Two-Way Relay Networks

In this paper, K bidirectionally communicating node pairs with each node having N antennas and one amplify and forward relay having R antennas are considered. Each node wants to transmit d data streams to its communication partner. Taking into account that each node can perform self interference cancellation, a new scheme called Pair-Aware Interference Alignment is proposed. In this scheme, the transmit precoding matrices and the relay processing matrix are chosen in such a way that at any given receiver all the interfering signals except the self interference are within the interference subspace and the useful signal is in a subspace linearly independent of the interference subspace. If the number of variables is larger than or equal to the number of constraints in the system, the system is classified as proper, else as improper. Through simulations it is shown that for a proper system (2Kd≤2N+R-d), interferences can be perfectly aligned and the useful signals can be decoded interference-free at the receivers. An iterative algorithm to achieve the interference alignment solution is proposed. Also for the proper system fulfilling a certain additional condition, which will be derived in this paper, a closed form solution is proposed.

Interference management strategy for multiuser two-way relay networks

We investigate the interference management strategy for multiuser two-way transmission network where multiple pairs of users communicate with their pre-assigned partners, with assistance of an amplify-and-forward relay. The design objective is that the tradeoff between the efficiency and reliability could be achieved as much as possible. By taking advantage of the signal space alignment technique to cancel the multi-user interference, all users can transmit massages to the relay simultaneously over the same channel resource. And a new multiple-input multiple-output (MIMO) precoding matrix is designed to reduce the decoding complexity. Simulation result shows the average sum rate performance of the proposed system, and the achievable degrees of freedom can outperform MU-MIMO scheme and TDMA scheme.

Outage Probability Analysis and Power Allocation for Two-Way Relay Networks with User Selection and Outdated Channel State Information

This paper considers a multiuser two-way relay network (TWRN) consisting of one base station (BS), one amplify-and-forward (AF) relay and K mobile stations (MSs) among which the Nth-best MS is selected to exchange information with the BS. Specifically, we study the impact of outdated channel state information (CSI) on the system over time-varying Rayleigh block-fading channels by deriving a tight closed-form lower bound for the outage probability. We also obtain an asymptotic outage probability expression for high signal-to-noise ratio (SNR). Moreover, by minimizing the asymptotic outage probability, we optimize the power allocation (PA) among the network nodes. Numerical and simulation results collaborate the proposed studies.

Resource Allocation with Subcarrier Pairing in OFDMA Two-Way Relay Networks

This study considers an orthogonal frequency-division multiple-access (OFDMA)-based multi-user two-way relay network where multiple mobile stations (MSs) communicate with a common base station (BS) via multiple relay stations (RSs). We study the joint optimization problem of subcarrier-pairing based relay-power allocation, relay selection, and subcarrier assignment. The problem is formulated as a mixed integer programming problem. By using the dual method, we propose an efficient algorithm to solve the problem in an asymptotically optimal manner. Simulation results show that the proposed method can improve system performance significantly over the conventional methods.

Outage-optimal opportunistic scheduling with analog network coding in multiuser two-way relay networks

This paper investigates the performance of an outage-optimal opportunistic scheduling scheme for a multiuser two-way relay network, wherein an analog network coding-based relay serves multiple pairs of users. Under a Rayleigh flat-fading environment, we derive an exact expression for cumulative distribution function (CDF) of the minimum of the two end-to-end instantaneous signal-to-noise ratios (SNRs) and utilize this to obtain an exact expression for the outage probability of such a greedy scheduling scheme. We then develop a modified scheduler that ensures fairness among user pairs of the considered system. By using a high SNR approximation of derived CDF, we present a simple closed-form expression for outage probability of the overall system and establish that a multiuser diversity of order equal to the number of user pairs is harnessed by the scheme. We also present an efficient power allocation strategy between sources and relay, subject to a total power constraint, that minimizes the outage probability of the overall system. Further, by deriving both upper and lower bound expressions for the average sum-rate of the proposed scheme, we demonstrate that an average sum-rate gain can also be achieved by increasing the number of user pairs in the system. Numerical and simulation results are presented to validate the performance of the proposed scheme.

A Joint Resource Allocation Scheme for Multiuser Two-Way Relay Networks

In this letter, we study the problem of resource allocation in amplify-and-forward (AF) based multiuser two-way relay network that is operated under orthogonal frequency division multiple access (OFDMA) modulation. We formulate an end-to-end throughput maximization problem subject to limited power constraint at individual user and relay. The optimization targets to find the best sub-carrier allocation to each user, sub-carrier pairing at the relay, as well as the power allocation at all nodes, which turns out to be a mixed integer programming problem. We then derive an asymptotically optimal solution through Lagrange dual decomposition approach and further design a suboptimal algorithm to trade the performance for computational complexity. Finally, simulation results are provided to demonstrate the performance gain of the proposed algorithms.

Multi-User Two-Way Relay Networks with Distributed Beamforming

We consider a two-way relay network consisting of multiple pairs of single-antenna users and multiple distributed single-antenna relays. The two communication peers in each pair of users transmit simultaneously to the relays in the first time slot, and the relays rebroadcast the received sum signal weighted by a complex gain, in the second time slot. For multi-user systems, the signal arriving at the users contains not only self interference from the back-propagation of user signals, but also inter-pair interferences from other pairs of users. In this paper, we use zero-forcing (ZF) to cancel the inter-user interference, assuming that channel-state information for all relay-peer connections are known at every relay, but no data exchange occurs between relays. We also derive two closed-form expressions for zero-forcing beamforming weights, corresponding to two different relay power constraints, which can be implemented in a distributed manner. The first approach uses standard ZF to null out every inter-pair interference and the second approach sets the total inter-pair interference to zero. We also derive a closed-form upper bound of the achievable sum-rate and show that both methods achieve the same multiplexing gain when the number of relays N is sufficient for perfect zero-forcing, namely 2K2 + K, where K is the number of user pairs. For the case of insufficient number of relays, we also propose two solutions for beamforming weights, i.e., based on diagonal loading and use of the pseudo-inverse, and compare their advantages and weaknesses.

Diversity order of multiuser two-way relay networks with beamforming over Nakagami-m fading channels

The diversity performance of multiuser two-way relay networks is investigated over Nakagami-m fading channels. We consider an amplify-and-forward relay network with beamforming at the base station and all the mobile stations, and derive closed-form lower and upper bounds on the outage probabilities of two unidirectional links, respectively. Moreover, the exact expression for the diversity order and the asymptotic expressions for the outage probabilities of two unidirectional links are given too. Finally, simulation results are presented to verify our formulations.

Power allocation for F/TDMA multiuser two-way relay networks

We consider a multiuser two-way relay network where multiple pairs of users exchange information with the assistance of a relay node, using orthogonal channels per pair. For a variety of two-way relaying mechanisms, such as decode- and- forward (DF), amplify-and-forward (AF) and compress-and-forward (CF), we investigate the problem of optimally allocating relay's power among the user pairs it assists such that an arbitrary weighted sum rate of all users is maximized, and solve the problem as one or a set of convex problems for each relaying scheme. Numerical results are presented to demonstrate the performance of the optimum relay power allocation as well as the comparison among different two-way relaying schemes.

Multiuser two-way relaying: detection and interference management strategies

We consider a multiuser two-way relay network where multiple pairs of users communicate with their preassigned partners, using a common intermediate relay node, in a two-phase communication scenario employing code division multiple access (CDMA). By taking advantage of the bidirectional communication structure, we first propose that each pair of partners share a common spreading signature and design a jointly demodulate-and-XOR forward (JD-XOR-F) relaying scheme, where all users transmit to the relay simultaneously followed by the relay broadcasting an estimate of the XORed symbol for each user pair. We derive the decision rules and the corresponding bit error rates (BERs) at the relay and at the users' receivers. We then investigate the joint power control and receiver optimization problem for each phase for this multiuser two-way relay network with JD-XOR-F relaying. We solve each optimization problem by constructing the iterative power control and receiver updates that converge to the corresponding unique optimum. Simulation results are presented to demonstrate the performance of the proposed multiuser two-way JD-XOR-F relaying scheme in conjunction with the joint power control and receiver optimization algorithms. Specifically, we observe significant power savings and user capacity improvement with the proposed communication scheme as compared to the designs with a "one-way" communication perspective.