Cooperative Non-orthogonal Multiple Access System Research Articles

Resource allocation in downlink SWIPT-based cooperative NOMA systems

This paper considers a downlink multi-carrier cooperative non-orthogonal multiple access (NOMA) transmission, where no direct link exists between the far user and the base station (BS), and the communication between them only relies on the assist of the near user. Firstly, the BS sends a superimposed signal of the far and the near user to the near user, and then the near user adopts simultaneous wireless information and power transfer (SWIPT) to split the received superimposed signal into two portions for energy harvesting and information decoding respectively. Afterwards, the near user forwards the signal of the far user by utilizing the harvested energy. A minimum data is required to ensure the quality of service (QoS) of the far user. We jointly optimize power allocation, subcarrier allocation, time allocation, the power allocation (PA) coefficient and the power splitting (PS) ratio to maximize the number of data bits received at the near user under the energy causality constraint, the minimum data constraint and the transmission power constraint. The block-coordinate descent method and the Lagrange duality method are used to obtain a suboptimal solution of this optimization problem. In the final simulation results, the superiority of the proposed NOMA scheme is confirmed compared with the benchmark NOMA schemes and the orthogonal multiple access (OMA) scheme.

Performance Analysis of Cooperative NOMA Networks with Imperfect CSI over Nakagami-m Fading Channels.

In this paper, we investigate a downlink cooperative non-orthogonal multiple access (NOMA) network with decode-and-forward relaying, where two scenarios of user relaying with direct link and user relaying without direct link are discussed in detail. More particularly, the performance of cooperative NOMA system under the assumption of imperfect channel state information (ipCSI) is studied over Nakagami-m fading channels. To evaluate the outage performance of the above discussed two scenarios, the closed-form expressions of outage probability for a pair of users are derived carefully. The diversity orders of users are achieved in the high signal-to-noise region. An error floor appears in the outage probability owing to the existence of channel estimation errors under ipCSI conditions. Simulation results verify the validity of our analysis and show that: (1) NOMA is superior to conventional orthogonal multiple access; (2) The best user relaying location for cooperative NOMA networks should be near to the base station; and (3) The outage performance of distant user with direct link significantly outperforms distant user without direct link by comparing the two scenarios.

Ergodic Capacity and Outage Performance Analysis of Uplink Full-Duplex Cooperative NOMA System

We propose an uplink full-duplex (FD) cooperative non-orthogonal multiple access (NOMA) system, where a dedicated Full-duplex (FD) relay is used to help two uplink users transmitting information to the base station. A novel FD relay transmission mode is proposed, in which the FD relay utilizes successive interference cancellation (SIC) and self-interference cancellation to decode two users symbols at the relay receiving antenna, and the relay transmitting antenna transmits superimposed signal to the base station using superposition coding. In this paper, we also consider a more realistic scenario where self-interference cancellation is imperfect. Hence, the residual-self interference exists. Moreover, the ergodic sum rate and the outage probability of the proposed system are investigated and the accurate analytical expressions are derived. Simulation results validate that, compared with the uplink half-duplex (HD) cooperative NOMA system and the uplink HD cooperative orthogonal multiple access (OMA) system, our proposed system obtains better performance of the ergodic sum rate and outage probability in the main signal to noise ratio (SNR) regime.

On the Achievable Max-Min Rates of Cooperative Power-Domain NOMA Systems

The non-orthogonal multiple access (NOMA) schemes, where the users are allowed to share the same time-frequency resource, have recently received a major interest to meet the increasing demands for higher data rates, improved user fairness, and/or lower latency applications in future dense-user wireless networks. Moreover, the achievable user rates of NOMA schemes can be further increased through the integration of these multiple access schemes with other enabling schemes such as full-duplex user cooperative communications. In this paper, the optimization of the achievable max-min rates for full-duplex cooperative NOMA (CNOMA) schemes for the two-user scenario are re-visited and is extended to the three-user scenario where novel expressions for the optimal user cooperation power are derived for both scenarios. The obtained results shed new lights on the optimal transmit power allocation and the optimal user cooperation power allocation in CNOMA systems and the different design trade-offs in such systems.

SWIPT-Enabled Cooperative NOMA With mth Best Relay Selection

Non-orthogonal multiple access (NOMA) was recently regarded as a potential technique for next generation wireless networks. Recent works on relay selection for cooperative NOMA systems have mainly addressed the best relay selection to forward its received signals to terminal nodes. Nonetheless, in practical scenarios, the best relay may be unavailable due to non-ideal conditions such as scheduling and overload constraints or possibly due to channel feedback delay. Therefore, there is compelling need to consider a more practical solution, in which the best available relay is selected. In this article, we examine the error rate performance for simultaneous wireless information and power transfer (SWIPT)-enabled NOMA, while considering the selection of the m th best available relay. In particular, we present an exact pairwise error probability (PEP) expression to obtain a bit error rate (BER) upper bound. The asymptotic PEP is investigated to evaluate the achievable diversity order for NOMA users. Finally, simulation results are provided to verify the accuracy of the derived PEP expressions and to give more insights into the system performance.

Performance Analysis and Optimization for Power Beacon-Assisted Wireless Powered Cooperative NOMA Systems

Wireless power transfer (WPT) is an effective way to prolong the lifetime of the energy-constraint networks. In this paper, we investigate a wireless powered cooperative non-orthogonal multiple access (WP-CNOMA) system, consisting of a power beacon (PB), an information transmitter (S), multiple relays (R) and two information receiving devices with near device <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D_{1}$ </tex-math></inline-formula> and far device <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D_{2}$ </tex-math></inline-formula> . We assume both S and R are energy-constraint and there is no direct link between S and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D_{2}$ </tex-math></inline-formula> . With the help of PB, S and R can harvest energy from it to restart the communication for WP-CNOMA network. For such a system, low-complexity but effective relay and antenna selection schemes are applied. To characterize the performance, outage probabilities and average throughput are derived for linear and non-linear energy harvesting (EH) models, respectively. Moreover, to maximize the average throughput, invoking the unimodal feature for average throughput with respect to the EH time, we find the optimal EH time via Golden section search method. Simulation results validate the accuracy of analytical results, and reveal the performance gain for our system over the benchmark schemes. Also, it can be seen that the non-linear EH model shows different outage behaviors from the linear one. On the other hand, considering the practical application and to improve the performance, the optimization for a simple WP-CNOMA system with single-antenna PB and single relay is also investigated, in which we aim to maximize the minimum throughput by jointly optimizing EH time and power allocation. A low-complexity analytical method is developed to find the max-min rate. Numerical results show that through optimization, the system performance can be improved significantly.

Performance analysis of cooperative NOMA with energy harvesting in multi-cell networks

In this paper, an energy harvesting enabled cooperative non-orthogonal multiple access (NOMA) system for a multi-cell network is investigated. Particularly, during the direct transmission phase, base stations send their superposed messages to the near users and far users simultaneously according to a NOMA principle, while the near users act as energy harvesting enabled relays employing a power splitting protocol. During the cooperative phase, the near users transmit their decoded messages to the corresponding far users using harvested energy. Using tools from stochastic geometry, we firstly calculate the signal to interference ratios of the users in each NOMA group including one near user and one far user. Then, the closed-form expressions of the coverage probability, ergodic rate, and energy efficiency are derived respectively. Numerical results validate the derived expressions and show that the energy harvesting enabled cooperative NOMA system in a multi-cell network can improve the coverage probability, ergodic rate, and energy efficiency compared to its counterpart OMA system.

Performance Analysis of Cooperative NOMA Systems With Adaptive Mode Selection and Subchannel Allocation

In this paper, we study the mode selection and subchannel allocation for the downlink of a multi-user multi-subchannel cooperative non-orthogonal multiple access (C-NOMA) system. Specifically, each non-orthogonal multiple access (NOMA) pair can work in two different modes, i.e., non-cooperative NOMA (NC-NOMA) and C-NOMA, where the mode selection is based on the users’ channel conditions. We propose an adaptive mode selection, subchannel allocation and user pairing (AMS-SAUP) scheme and derive the achievable diversity order by analyzing the upper bound for the outage probability. Simulation results reveal that the AMS-SAUP scheme achieves the same diversity order as the exhaustive search scheme.

Secrecy Outage Analysis for Cooperative NOMA Systems With Relay Selection Schemes

This paper considers the secrecy outage performance of a multiple-relay assisted non-orthogonal multiple access (NOMA) network over Nakagami-m fading channels. Two time slots are utilized to transmit signals from the base station to destination. At the first time slot, the base station broadcasts the superposition signal of the two users to all decode-and-forward relays by message mapping strategy. Subsequently, the selected relay transmits superposition signal to the two users via power-domain NOMA technology. Three relay selection schemes, i.e., optimal single relay selection (OSRS) scheme, two-step single relay selection (TSRS) scheme, and optimal dual relay selection (ODRS) scheme are proposed and the secrecy outage performance is analyzed. As a benchmark, we also examine the secrecy outage performance of the NOMA systems with traditional multiple relay forwarding (TMRF) scheme in which all the relay that successfully decode signals from the source forward signals to the NOMA users with equal power. Considering the correlation between the secrecy capacity of two users and different secrecy requirement for two NOMA users, the analytical expressions for the security outage probability (SOP) of the proposed OSRS, TSRS, and ODRS schemes along with the TMRF scheme are derived and validated via simulations. To get more insights, we also derive the analytical expressions for the asymptotic SOP for all the schemes with fixed and dynamic power allocations. Furthermore, the secrecy diversity order (SDO) and secrecy array gain of cooperative NOMA systems are obtained. The results demonstrate that our proposed schemes can significantly enhance the secrecy performance compared to the TMRF scheme and that all the schemes with fixed power allocation obtain zero SDO and the OSRS scheme with dynamic power allocation obtains the same SDO as TMRF.

Joint relay‐and‐antenna selection for cooperative non‐orthogonal multiple access

Non-orthogonal multiple access (NOMA) has been widely recognised as a promising technology due to its potential to achieve high spectral efficiency and massive connectivity. The authors study a multiple-relay multiple-input and multiple-output cooperative NOMA system. In particular, a joint relay-and-antenna selection (JRAS) scheme with coordinated direct and relay transmission is proposed, which makes it possible to maximise the instantaneous rate of far user with high quality ofservice (QoS) priority while guaranteeing the requirement of near user with low QoS priority. Then, closed-form expressions for outage probability are derived and asymptotic expressions for outage probability in the high signal-to-noise ratio regime are also obtained in order to provide further insights. Analysis results demonstrate that the proposed JRAS scheme significantly enhances the outage performance for cooperative NOMA, and simulation results are provided to verify the accuracy of theoretical analysis.

Performance analysis of nonorthogonal multiple access‐based underlay cognitive relay network

SummaryThis paper considers cooperative non‐orthogonal multiple access (NOMA) scheme in an underlay cognitive radio (CR) network. A single‐cell downlink cooperative NOMA system has been considered for the secondary network, consisting of a base station (BS) and two secondary users, ie, a far user and a near user. The BS employs NOMA signaling to send messages for the two secondary users where the near user is enabled to act as a half‐duplex decode‐and‐forward (DF) relay for the far user. We derive exact expressions for the outage probability experienced by both the users and the outage probability of the secondary system assuming the links to experience independent, nonidentically distributed Rayleigh fading. Further, we analyze the ergodic rates of both the users and the ergodic sum rate of the secondary network. The maximum transmit power constraint of the secondary nodes and the tolerable interference power constraint at the primary receiver are considered for the analysis. Further, the interference caused by the primary transmitter (PT) on the secondary network is also considered for the analysis. The performance of the proposed CR NOMA network has been observed to be significantly better than a CR network that uses conventional orthogonal multiple access (OMA) scheme. The analytical results are validated by extensive simulation studies.

Successive User Relaying in Cooperative NOMA System

User relaying in cooperative non-orthogonal multiple access (C-NOMA) system is able to enhance reliability. However, for user operates in half-duplex (HD) mode, additional bandwidth is needed for relaying operation. This reduces spectral efficiency of C-NOMA system. This letter proposes a successive user relaying (SR) to improve the spectral efficiency of C-NOMA system. In the proposed SR scheme, a pair of HD users decode the received signal and relay the decoded signal successively to improve the spectral efficiency. A novel algorithm is proposed to optimize power split factor in order to improve fairness of outage performance between users. The performance of the proposed SR scheme is superior to existing HD relaying scheme in C-NOMA system and existing SR in cooperative orthogonal multiple access system in terms of outage probability and ergodic sum rate. The closed-form expressions of outage probability and asymptotic ergodic sum rate of proposed SR are also derived.

Energy-Efficient Power Allocation for Cooperative NOMA Systems With IBFD-Enabled Two-Way Cognitive Transmission

To compensate for the transmission efficiency in a cooperative non-orthogonal multiple access (NOMA) system, we enable in-band full-duplex (IBFD) and two-way cognitive transmission at the mobile relay, which assists the NOMA user in weak channel condition while transmitting and receiving its own data by means of spectral sharing. To strike a balance between spectral efficiency and energy conservation, we consider the power allocation problem to maximize the energy efficiency of the hybrid system while maintaining the bit rate requirements of NOMA users. To overcome the non-convexity of the original problem, we apply the successive inner approximation technique and propose an efficient iterative scheme to obtain a Karush–Kuhn–Tucker point in polynomial time. The gain of IBFD-enabled two-way cognitive transmission and the efficiency of the proposed power allocation scheme are validated via simulation results.

Robust Transmit Beamforming for SWIPT-Enabled Cooperative NOMA With Channel Uncertainties

In this paper, we study the robust beamforming design for a simultaneous wireless information and power transfer (SWIPT) enabled system, with cooperative non-orthogonal multiple access (NOMA) protocol applied. A novel cooperative NOMA scheme is proposed, where the strong user with better channel conditions adopts power splitting (PS) scheme and acts as an energy-harvesting relay to transmit information to the weak user. The presence of channel uncertainties is considered and incorporated in our formulations to improve the design robustness and communication reliability. Specifically, only imperfect channel state information is assumed to be available at the base station (BS), due to the reason that the BS is far away from both users and suffers serious feedback delay. To comprehensively address the channel uncertainties, two major design criteria are adopted, which are the outage-based constraint design and the worst-case-based optimization. Then, our aim is to maximize the strong user’s data rate, by optimally designing the robust transmit beamforming and PS ratio, while guaranteeing the correct decoding of the weak user. With two different channel uncertainty models respectively incorporated, the proposed formulations yield to challenging nonconvex optimization problems. For the outage-based constrained optimization, we first conservatively approximate the probabilistic constraints with the Bernstein-type inequalities, which are then globally solved by 2-D exhaustive search. To further reduce the complexity, an efficient low-complexity algorithm is then proposed with the aid of successive convex approximation (SCA). For the worst-case-based scenario, we first apply the semidefinite relaxation method to relax the quadratic terms and prove the rank-one optimality. Then the nonconvex max-min optimization problem is readily transformed into convex approximations based on ${S}$ -procedure and SCA. Simulation results show that for both channel uncertainty models, the proposed algorithms can converge within a few iterations, and the proposed SWIPT-enabled robust cooperative NOMA system achieves better system performance than the existing protocols.

Beamforming Design and Performance Analysis of Full-Duplex Cooperative NOMA Systems

We consider downlink non-orthogonal multiple access transmission where an access point communicates with a set of near and far users via a full-duplex multiple antenna relay. To deal with the inter-user interference at the near user and self-interference at the relay, we propose the optimum and suboptimal beamforming schemes. In addition, we consider two different user selection criteria, namely: 1) random near user and random far user (RNRF) selection and 2) nearest near user and nearest far user (NNNF) selection, and we derive the outage probabilities of the near and far users. Our findings reveal that as compared to half-duplex operation, full-duplex relaying can reduce the outage probability of the near users up to 63% in the case of NNNF user selection. With suboptimal beamforming schemes, the NNNF user selection shows a superior performance as compared to the RNRF user selection for all choices of transmit power, while with the optimum beamforming, the performance of the RNRF user selection converges to the NNNF user selection at high transmit power. The simulation results are provided to confirm the accuracy of the developed analytical results and facilitate a better performance comparison.

Relay selection for cooperative NOMA system over correlated fading channel

The effect of correlated fading channel on outage probability for decode-and-forward (DF) relaying non-orthogonal multiple access (NOMA) system is studied in this paper, where a source transmits messages to dual destinations with the help of multiple relays. To obtain the best system outage performance, an optimal three-stage relay selection algorithm is adopted. Both the analytical expressions and the asymptotic expressions for large transmission power are derived. According to the theoretical and simulation results, we find that the large channel correlation coefficient can be helpful to the system performance, while it has no effect on the diversity order. Moreover, the three-stage scheme yields a significant gain over the max–min selection scheme. Furthermore, the suboptimal max–min selection scheme is deficient in terms of outage probability.

Online Adaptive Transmission Strategy for Buffer-Aided Cooperative NOMA Systems

This paper investigates a buffer-aided cooperative NOMA system, where a source is connected to multiple destinations through a buffer-aided NOMA relaying. We aim to maximize a long-term average network utility by jointly optimizing working mode selection, admission control, and power allocation in each time slot, subject to queue stability constraints and both the peak and average power constraints at the relay. To solve this problem, by leveraging the recently-developed Lyapunov optimization framework, we design an Online Adaptive Transmission (OAT) strategy, which converts the original long-term optimization problem into a series of online admission control and power allocation subproblems in each time slot, without loss of optimality. By considering the fact that the power allocation problem is non-convex in nature and thus cannot be solved with a standard method, we transform it into an equivalent convex optimization based on the specific feature of the formulation, and then propose a Fast Decision Criteria (FDC) based algorithm to derive the optimal solution with high computational efficiency. Extensive simulation results are provided to evaluate the performance of the proposed OAT strategy with FDC based power allocation for the buffer-aided cooperative NOMA system, and demonstrate that the proposed strategy can significantly outperform multiple benchmark schemes.

Performance of User Relaying Cooperative NOMA System Underlay Cognitive Radio Networks with Partial CSI

Performance of User Relaying Cooperative NOMA System Underlay Cognitive Radio Networks with Partial CSI

Energy Efficiency–Delay Tradeoff for a Cooperative NOMA System

The tradeoff between the energy efficiency (EE) and delay problem in the cooperative relaying system is studied by using non-orthogonal multiple access (NOMA) in this letter. To obtain an efficiency tradeoff between EE and delay, a stochastic-based EE optimization problem is formulated by considering the system queue stability. Then, the fractional programming and control parameter-based Lyapunov optimization method is proposed to solve the formulated problem. Furthermore, we derive the analytical bounds of EE and delay based on the control parameter. Finally, simulation results verify that the proposed cooperative NOMA system performs better than the traditional orthogonal multiple access cooperative system.

Outage Probability for Cooperative NOMA Systems With Imperfect SIC in Cognitive Radio Networks

In this letter, a cooperative non-orthogonal multiple access system with imperfect successive interference cancellation is investigated in an underlay cognitive radio network. Considering that the channel coefficients between the primary source and the secondary receiving nodes follow Rayleigh distribution, we derive an exact closed form of the exact outage probability for each secondary destination. Also, asymptotic expressions for the outage probability are derived: 1) when the interference constraint goes to infinity and 2) when the transmit powers at the secondary source and relay go to infinity. The simulation results verify our analytical results.