Chensi Zhang

Performance Analysis of Nonorthogonal Multiple Access for Relaying Networks Over Nakagami-

Nonorthogonal multiple access (NOMA), which can improve the spectrum efficiency and system throughput compared with conventional orthogonal multiple access (OMA), has been regarded as a promising technique for the fifth-generation (5G) mobile communication network. In this paper, we consider a NOMA-based relaying networks over Nakagami-

m

m

Fading Channels

fading channels, where the base station communicates with multiple mobile users simultaneously through the help of an amplify-and-forward (AF) relay. First, we study the system outage behavior, and closed-form expressions for the exact outage probability and simple bounds of the outage probability are obtained, respectively. The analytical results are further evaluated in the high-signal-to-noise-ratio (SNR) regime to explicitly characterize the diversity order of the network. Next, the ergodic sum rate achieved by the network is investigated, and expressions for the lower and upper bounds of the ergodic sum rate are derived. Finally, numerical examples are conducted to confirm the validity of our analysis and show a comparison of NOMA against conventional OMA networks. NOMA is reported to outperform conventional OMA and provide better spectral efficiency and user fairness.

A Unified Approach for Calculating the Outage Performance of Two-Way AF Relaying Over Fading Channels

Amplify-and-forward-based two-way relaying (TWR-AF) promises significant benefits in wireless networks. In this paper, we aim to obtain unified expressions to determine the outage probability of TWR-AF, regardless of different channel fadings. To this end, a visual integral region-based geometric analysis (GA-IR) approach is introduced, which does not depend on the specific functional forms of the channel fading distributions. Applying this method, we have derived the expressions of both individual and system outage probabilities with adjustable accuracy. Importantly, our analysis is then extended to multiuser and multirelay TWR-AF, and a unified expression and a concise lower bound are achieved. It has been shown that all the presented expressions apply to unequal transmitted power values and conventional fading channels, providing valuable and unified insights into practical system pretest evaluation and designs. By performing simulations over several representative fading channels [e.g., Rayleigh, Nakagami-m, Rice (Nakagami-n), and generalized-K (KG)] , the accuracy and generality of the provided expressions are presented. Utilizing these results, the outage performance for TWR-AF can be figured out flexibly and efficiently.

Impact of Channel Estimation Error on Bidirectional MABC-AF Relaying With Asymmetric Traffic Requirements

Channel estimation error results in severe performance deterioration in wireless networks. In this paper, we study the impact of imperfect channel estimation (ICE) on the outage performance of bidirectional relaying where the two sources have asymmetric traffic requirements (ATRs). In particular, we focus on the amplify-and-forward (AF) relay-assisted multiple-access broadcast (MABC) protocol, i.e., MABC-AF, which has received much attention due to its high spectrum efficiency and low complexity. In the single-relay scenario, we derive exact and generalized closed-form expressions for system outage probability, which indicate that the system outage is determined by the unidirectional outage in the case of highly asymmetric traffic patterns. For more insights into our approach, the closed-form asymptotic expressions are also evaluated, manifesting the interesting error floor (EF) phenomenon due to ICE. Using these analytical results, we further develop a robust and practical optimum power-allocation (OPA) algorithm that minimizes the system outage probability under aggregate and individual node power constraints. In the multirelay scenario, by taking into account the traffic knowledge, a novel relay selection criterion is proposed for asymmetric MABC-AF, followed by its impact on the system outage probability in the presence of ICE. Numerical results validate the accuracy of our analytical results and highlight the effect of the proposed OPA algorithm under various traffic requirements and channel estimation errors. Furthermore, the results also show that the proposed relay selection criterion is superior to the classical max–min criterion with ATRs, and the performance improvement becomes remarkable as channel estimation error increases.

Joint optimal power allocation and relay selection scheme in energy harvesting asymmetric two-way relaying system

Energy harvesting, which is an emerging technology to prolong the lifetime of energy-constrained wireless networks, has received significant attention. In this study, the authors consider an amplify-and-forward-based asymmetric two-way relaying system, where two source nodes exchange information through an energy-constrained relay node. The relay node scavenges energy from the received signals and uses the harvested energy to forward the received signals to the two terminal nodes. The signal power splitting ratio, depicting the trade-off between the harvesting energy and the forward signals’ power, is a crucial factor for the system outage probability. Therefore, an optimal power allocation (OPA) scheme, which jointly takes into consideration the optimisation of signal power splitting ratio and the transmission power allocation, is proposed to minimise the outage probability. In addition, a close-form solution for the OPA scheme is obtained. Finally, a joint OPA and relay selection schemes are presented to further improve the system performance. Simulation results highlight the superiority of the authors proposed scheme.

Energy Efficiency and Spectral Efficiency Tradeoff for Asymmetric Two-Way AF Relaying With Statistical CSI

Both energy efficiency (EE) and spectral efficiency (SE) are two important design criteria that can hardly be achieved simultaneously. In this paper, we investigate the EE-SE tradeoff for a two-way relaying system with amplify-and-forward (AF) strategy. Applying a unified EE-SE tradeoff metric, a power-allocation problem is formulated as the joint EE and SE maximization problem with a tradeoff factor. In particular, only statistical channel-state information (CSI) is required, and the asymmetric traffic requirements are taken into consideration in our design. Through the exploitation of the theorem of nonlinear fractional programming, a unique closed-form optimum solution is obtained, and some useful insights are also achieved. It has been proved that the provided optimal solution is Pareto optimal for EE-SE optimization. Utilizing these results, one can flexibly make the tradeoff between EE and SE by simply setting the tradeoff factor. Simulation results highlight the effect of the proposed power-allocation scheme.

Performance Evaluation for Asymmetric Two-Way AF Relaying in Rician Fading

In this letter, the performance of asymmetric two-way amplify-and-forward (AF) relaying network in Rician fading environments with different K-factors is investigated, using a novel method which features slight integral region adjustment. A tight lower bound of the system outage probability has been derived, which is in good agreement with the exact outage probability in high SNR. Based on this tight bound, the diversity and coding gains are achieved providing valuable insights into practical system designs. The deducing method, characterized by high efficiency and low-complexity, can easily be extended to evaluate the system performance over other fading channels. Simulation results are finally presented to examine our results.

Optimal and Suboptimal Joint Relay and Antenna Selection for Two-Way Amplify-and-Forward Relaying

This paper investigates joint relay and antenna selection schemes for two-way amplify-and-forward (AF) relaying system, where a multiantenna source exchanges information with a single-antenna source by exploiting a group of single-antenna relays. In view of performance and complexity tradeoffs, two optimal and one suboptimal joint selection schemes are proposed and compared. In contrast to the optimal selection schemes, the suboptimal counterpart is characterized by its low overhead and implementation complexity. The performance of these selection strategies has been investigated by deriving the outage probability approximations in closed form. Moreover, the achievable diversity orders and the corresponding coding gains are derived through high signal-to-noise ratio (SNR) approximations of the outage probability. Our theoretical and simulation results show that the performance gap between the optimal and suboptimal schemes disappears for specific scenarios, which provides useful design guidelines for practical two-way relaying system.

Performance analysis and power allocation for a two-way amplify-and-forward relay with channel estimation errors

This study deals with the performance analysis and power allocation of a two-way amplify-and-forward relay system with channel estimation errors. Exact closed-form expressions for outage probability and average symbol error rate (SER) are first presented. To provide more insights, their closed-form asymptotic expressions are then obtained. It is shown that the presence of channel estimation error causes outage probability and average SER maintain a fixed level even when a noiseless channel is adopted. These results are applied further to study the optimal power allocation problem for each node. From the perspective of service quality, rather than minimise the outage probability as much as possible, the goal is to use the minimum energy consumed to satisfy the traffic requirements and thereby conserve the energy resource. Furthermore, based on the optimal solutions to power allocation, the optimal relay location is investigated, which indicates that the system has higher energy efficiency with the relay located around the middle point of the two end nodes for any asymmetric traffic requirement. The simulation results verify that the derived outage probability and average SER expressions are accurate and highlight the effect of power allocation under various traffic requirements and channel estimation errors.