Cooperative Non-orthogonal Multiple Access System Research Articles

Cooperative Asynchronous Non-Orthogonal Multiple Access With Power Minimization Under QoS Constraints

Recent studies have demonstrated the superiority of non-orthogonal multiple access (NOMA) over orthogonal multiple access (OMA) in cooperative communication networks. In this paper, we propose a novel half-duplex cooperative asynchronous NOMA (C-ANOMA) framework with user relaying, where a timing mismatch is intentionally added in the broadcast signal. We derive the expressions for the throughputs of the strong user (acts as relay) which employs the block-wise successive interference cancellation (SIC) and the weak user which combines the symbol-asynchronous signal with the interference-free signal. We analytically prove that in the C-ANOMA systems with a sufficiently large block length, the strong user attains the same throughput to decode its own message while both users can achieve a higher throughput to decode the weak user’s message compared with those in the cooperative NOMA (C-NOMA) systems. Besides, we obtain the optimal timing mismatch when the block length goes to infinity. Furthermore, to exploit the trade-off between the power consumption of the base station and that of the relay user, we solve a weighted sum power minimization problem under quality of services (QoS) constraints. Numerical results show that the C-ANOMA system can consume less power compared with the C-NOMA system to satisfy the same QoS requirements.

Contract-Based Incentive Mechanism for Cooperative NOMA Systems

In the emerging cooperative non-orthogonal multiple access (NOMA) systems, an important design issue is to incentivize mobile relays (MRs) to participate in the cooperative process and achieve a win–win situation to both the base station (BS) and MRs under asymmetric information, where the BS does not know the channel state information of MRs and NOMA users. As a promising remedy, we apply the adverse selection model from contract theory and derive the optimal feasible contract set for MRs with different wireless characteristics (types). Based on the contracts accepted by all the MRs, we further propose an efficient MR selection scheme for the original combinatorial optimization. The efficiency of the proposed incentive mechanism is verified through the simulation results.

Cooperative NOMA System With Virtual Full Duplex User Relaying

Cooperative non-orthogonal multiple access (NOMA) system employs user relaying to enhance performance. Although full-duplex (FD) user relaying scheme does not incur additional bandwidth for relaying, the imperfect self-interference cancellation degrades the performance significantly. Alternatively, a virtual full-duplex (VFD) user relaying scheme can be used. In this paper, we study a cooperative NOMA system in cognitive radio scenario using VFD relaying scheme, where two half-duplex (HD) secondary users take turn to relay message of a primary user with decode-and-forward (DF) strategy. First, we derive the closed-form and asymptotic expressions of the outage probability and ergodic rate of the users. Then, we demonstrate the outage probability, ergodic rate, and ergodic sum rate performance of the proposed scheme using Monte Carlo simulations. We also provide a comprehensive comparison between the proposed VFD user relaying and existing FD and HD user relaying in orthogonal multiple access and NOMA systems. The simulation results verify the analytical results and demonstrate the advantages of the proposed VFD user relaying over existing schemes.

Secrecy Performance Analysis of Relay Selection in Cooperative NOMA Systems

This paper investigates the security performance of two relay selection schemes for cooperative non-orthogonal multiple access (NOMA) systems, where $K$ randomly distributed relays are employed with either decode-and-forward (DF) or amplify-and-forward (AF) protocols. More particularly, two-stage relay selection (TRS) and optimal relay selection (ORS) schemes are taken into consideration. To characterize the secrecy behaviors of these RS schemes, new closed-form expressions of both exact and asymptotic secrecy outage probability (SOP) are derived. We confirm that the SOP of the TRS scheme is equal to that of the ORS scheme for DF/AF-based NOMA systems. Based on the analytical results, the secrecy diversity orders achieved by the pair of RS schemes for the DF/AF-based NOMA systems are $K$ , which are equal to the number of relays. It is shown that the secrecy diversity orders for the cooperative NOMA systems are determined by the number of the relays. The numerical results are presented to demonstrate that: 1) the secrecy performance of the AF-based NOMA system outperforms that of the DF-based NOMA system, when not all DF relays successfully decode the received information; 2) with the number of relays increasing, the SOP of these RS schemes for the DF-/AF-based NOMA systems becomes lower, and; 3) the TRS/ORS schemes are capable of achieving better secrecy outage behaviors compared with random RS and orthogonal multiple access-based RS schemes.

Secrecy Outage Performance Analysis for Cooperative NOMA Over Nakagami-$m$ Channel

In this paper, we investigate the secrecy outage performance of a typical cooperative downlink non-orthogonal multiple access (NOMA) system over Nakagami-m fading channel, in which the base station transmits a superimposed signal to two users via a relay. First, the secrecy outage behavior of the considered system over Nakagami-$m$ fading channel under three wiretapping cases, e.g., one eavesdropper (Eve), non-colluding and colluding eavesdroppers, are studied, and both analytical and asymptotic expressions for the secrecy outage probability are derived. Next, by considering the availability of Eves' channel state information, we adopt the two-stage relay selection (RS) strategy to improve the system's secrecy outage performance. Finally, simulation results are provided to corroborate the accuracy of our derived expressions. The results show that: 1) there exists secrecy performance floor for cooperative NOMA system, and it was determined by the weak user's secrecy requirement and the channel conditions of the Eves; 2) the two-stage RS scheme can increase the secrecy outage performance significantly under three wiretapping cases; 3) the secrecy performance of cooperative NOMA network is superior to that of orthogonal multiple access network on the condition of low and medium signal-to-noise ratio regions.

Energy Efficiency Maximization Design for Full-Duplex Cooperative NOMA Systems With SWIPT

This paper investigates non-orthogonal multiple access (NOMA) system, where the near NOMA user is able to communicate with a base station (BS) directly while the far NOMA user needs to resort to a full-duplex (FD) relay. Wireless power transfer is applied to our system where the FD relay integrated with power splitting architecture can be powered wirelessly by the ambient radio signals simultaneously. Moreover, we assume that self-interference cancellation at the relay and inter-user interference cancellation at the near NOMA user are imperfect. For the purpose of maximizing the energy efficiency of the NOMA system, the power splitting ratio and transmit beamforming vectors are researched for the BS and relay with the guarantee of QoS requirements for two users. The formulated problem appears to be nonlinear fractional programming, which is highly nonconvex and the global optimality is hard to obtain. By introducing Charnes-Cooper's transformation and inner approximation method, we develop an iterative algorithm to gain the near optimal solution. The simulation results show that the proposed joint optimizing algorithm outperforms the existing scheme in terms of energy efficiency as well as the effect of SI cancellation on performance gain.

Performance of Max-Min User Relay Cooperative NOMA Systems with Imperfect CSI

Performance of Max-Min User Relay Cooperative NOMA Systems with Imperfect CSI

Analysis and Optimization for Weighted Sum Rate in Energy Harvesting Cooperative NOMA Systems

We consider a cooperative non-orthogonal multiple access system with radio frequency energy harvesting, in which a user with good channel harvests energy from its received signal and serves as a decode-and-forward relay for enhancing the performance of a user with poor channel. We here aim at maximizing the weighted sum rate of the system by optimizing the power allocation coefficient used at the source and the power splitting coefficient used at the user with good channel. By exploiting the specific structure of the considered problem, we propose a low-complexity one-dimensional search algorithm which can provide optimal solution to the problem. As a benchmark comparison, we derive analytic expressions and simple high signal-to-noise ratio (SNR) approximations of the ergodic rates achieved at two users and their weighted sum with fixed values of the power allocation and the power splitting coefficients, from which the scaling of the weighted sum in the high SNR region is revealed. Finally, we provided numerical results to demonstrate the validity of the optimized scheme. Index Terms-Cooperative NOMA, RF-energy harvesting, weighted sum rate analysis and optimization.

Decode-and-Forward Relaying for Cooperative NOMA Systems With Direct Links

This paper investigates a cooperative non-orthogonal multiple access system, in which a base station communicates with two far users with the aid of a decode-and-forward (DF) relay. Three cooperative relaying schemes, namely, the fixed relaying (FR), the selective DF with coordinated direct and relay transmission (SDF-CDRT), and the incremental-selective DF (ISDF) relaying are proposed to enhance the outage performance for the two far users by utilizing both the direct and relay links. Taking into account the received signal-to-noise ratio (SNR) events at the relay, the SDF-CDRT scheme adaptively forms an orthogonal transmission branch with respect to the direct link or keeps silent to reduce error propagation. Besides considering the relay detection results, the ISDF scheme further exploits the limited feedback of the received SNR events from two users, so that error propagation can be avoided and unnecessary relaying can be reduced. Analytical expressions for the outage probabilities and average throughputs of the paired users are derived in closed-form for the three cooperative relaying schemes. Asymptotic expressions for the outage probabilities are derived in the high SNR region. It is shown that the FR and SDF-CDRT schemes achieve a diversity order of one for both users, while the ISDF scheme achieves a diversity order of two for both users. The superior system performance achieved by the proposed schemes over those of the existing methods is verified by Monte Carlo simulations.

On the Impact of User Scheduling on Diversity and Fairness in Cooperative NOMA

In this correspondence paper, we investigate the problem of user scheduling in a cooperative non-orthogonal multiple access (NOMA) system consisting of a base station, a weak user, and $K$ intermediate users. During each transmission, an intermediate user is scheduled to receive its own message and forward the message destined for the weak user. For this type of cooperative NOMA system, a novel scheduling scheme is proposed to achieve full diversity and scheduling fairness simultaneously. With the consideration that all channels experience independent but non-identically distributed Rayleigh fading, outage probabilities of the weak user and the scheduled intermediate user are derived in closed-form expressions. It is theoretically shown that the proposed scheme provides full diversity for both the weak user and the scheduled intermediate user. Furthermore, theoretical results also demonstrate that the proposed scheme schedules each intermediate user with the same probability 1/ $K$ , demonstrating that scheduling fairness is also guaranteed.

Non-Orthogonal Multiple Access Assisted Device-to-Device Communication: Best Helping Base Station Approach

<span>It can be studied in this paper that a cooperative non-orthogonal multiple access (NOMA) helps device-to-device (D2D) communication system through base station (BS). In particular, we investigate BS selection scheme as a best channel condition for dedicated devices where a different data transmission demand on each device is resolved. The analysis on amplifying-and forward (AF) relay is proposed to evaluate system performance of the conventional cooperative NOMA scheme. Under the realistic assumption of perfect channel estimation, the achievable outage probability of both devices is investigated, and several impacts on system performance are presented. The mathematical formula in closed form related to probability has also been found. By implementing Monte-Carlo simulation, the simulation results confirm the accuracy of the derived analytical results. Also, the proposed D2D cooperative NOMA system introduces expected performance on reasonable selected parameters in the moderate signal to noise ratio (SNR) regime.</span>

Performance analysis of a novel uplink cooperative NOMA system with full‐duplex relaying

In this study, the authors examine the performance of an uplink non-orthogonal multiple access (NOMA) system with cooperative full-duplex relaying (CFR-NOMA), where the user closer to the base station (BS) is considered as a full-duplex relay to aid the transmission from the other user to the BS. The closer user first decodes the signals transmitted from the far user and then forwards them using superposition coding to the BS on top of transmitting its own information signals to the BS. First, they analyse the performance of the CFR-NOMA system in terms of the outage probability and average sum rate. Then, they analytically obtain the optimal power allocation coefficients that minimise the outage probability. They also study the optimal power allocation to maximise the average sum rate under specified constraints. Their examination demonstrates that the CFR-NOMA system can significantly outperform the uplink conventional orthogonal multiple access and uplink NOMA with half-duplex relaying system in terms of achieving a lower outage probability or a higher average sum rate.

Energy-Efficient Full-Duplex Cooperative Nonorthogonal Multiple Access

The full-duplex (FD) cooperative nonorthogonal multiple access (NOMA) achieves a superior throughput over the conventional half-duplex (HD) cooperative NOMA, wherein the strong users (SUs) with good channel conditions can act as an FD relay node for the weak users with poor channel conditions. However, the energy efficiency (EE) of a cooperative NOMA may be degraded due to the additional power consumption incurred at the SUs. We are, therefore, motivated to investigate the EE maximization problem of an FD cooperative NOMA system. More importantly, we investigate the “signal-to-inference-noise ratio gap reversal” problem of cooperative NOMA systems, which imposes successive interference cancellation (SIC) performance degradation at the SUs. This problem has not been documented in the existing cooperative NOMA literature. A low-complexity algorithm is proposed for maximizing the system's EE while guaranteeing a successful SIC operation. Our numerical results show that the proposed algorithm achieves both a higher EE and throughput over the existing HD cooperative NOMA and nonadaptive FD cooperative NOMA. More importantly, the proposed scheme guarantees a successful SIC operation at the SUs.

Energy Efficient Resource Allocation for Downlink Cooperative Non-Orthogonal Multiple Access Systems

Energy Efficient Resource Allocation for Downlink Cooperative Non-Orthogonal Multiple Access Systems

Amplify-and-Forward Virtual Full-Duplex Relaying-Based Cooperative NOMA

This letter studies the performance of a virtual full-duplex relaying technique in cooperative non-orthogonal multiple access (NOMA)-based systems. In particular, two half-duplex amplify-and-forward relays imitate the operation of full-duplex relaying in cooperative NOMA systems using the successive relaying (SR) technique, in order to recover the loss of spectral efficiency due to the half-duplex constraint at the relays. The closed-form outage probability of the proposed scheme is derived. The simulation results show that the proposed SR-based cooperative NOMA system with an opportunistic power split factor achieves a lower outage probability and a higher ergodic rate than the existing SR-based orthogonal multiple access system and ideal full duplex relaying-based NOMA systems.

Cooperative Non-Orthogonal Multiple Access in Multiple-Input-Multiple-Output Channels

Cooperative non-orthogonal multiple access (NOMA) systems inherit advantages of the NOMA protocol and the cooperative relay. In this paper, we propose cooperative NOMA systems in multiple-input-multiple-output channels. The whole transmission is divided into two phases. In the first phase, the base station broadcasts signals using the NOMA protocol to a central user and a cell-edge user. In the second phase, the central user helps the base station cooperatively relay signals intended for the cell-edge user. Our objective is to maximize achievable rate from the base station to the cell-edge user under transmit power constraints and achievable rate constraint from the base station to the central user. The difficulty of this problem is the joint beamforming of the base station and the central user in the second phase. We propose a constrained convex-concave procedure (CCCP)-based algorithm. To reduce computational complexity, we also propose a closed-form search-based suboptimal algorithm. Simulation results demonstrate that our proposed cooperative NOMA system with CCCP-based algorithm outperforms the conventional NOMA scheme. When achievable rate constraint to the central user is low, our proposed cooperative NOMA system with the closed-form search-based suboptimal algorithm outperforms the NOMA scheme.

Performance Analysis of Cooperative NOMA Schemes in Spatially Random Relaying Networks

In this paper, we propose a general framework to investigate cooperative non-orthogonal multiple access (NOMA) using two-stage relay selection (TSRS) in spatially random relaying networks. More specifically, we consider both amplify-and-forward and decode-and-forward protocols and compare the performance between them. From practical consideration, we adopt a stochastic geometry-based model and assume that the spatial topology of relays is modeled by using homogeneous Poisson point process (PPP). Based on such a setting, an effective coverage area of the relays modeled by using homogeneous PPP in cooperative NOMA systems is developed and performance comparison between TSRS and the conventional max–min RS scheme is also presented. According to the locations of the NOMA users, we develop the complete strategies for calculating the effective coverage area of the relays. Furthermore, in the high signal-to-noise ratio regime, asymptotic expressions are provided to show that the outage probability tends to a constant which is only related to the density of homogeneous PPP and the effective coverage area of the relays. For a given outage probability, we reveal the relationship between the shortest and longest radii of the effective district of the relays. Finally, Monte Carlo simulations are provided to verify the accuracy of the analytical results.

Two-Stage Superposed Transmission for Cooperative NOMA Systems

In this paper, a two-stage superposed transmission scheme for cooperative nonorthogonal multiple access (NOMA) systems is proposed. During the first $N$ time slots, the source simultaneously transmits the superposition coded symbols to the relay and destination, both of which, instead of decoding, keep the receptions in reserve. At the last time slot, the relay decodes and forwards a new superposition coded symbol with corresponding power allocation factors to the destination. The destination jointly decodes the received signals during the total $N + 1$ time slots by employing maximum ratio combining. Assuming Rayleigh fading channels, the ergodic sum rate (SR), outage probability, and outage capacity of the system are investigated considering the high transmit signal-to-noise ratio cases. An approximate expression for the ergodic SR is also derived at the expense of a negligible performance loss. By means of numerical results, it is shown that the transmission rate and ergodic SR of the proposed scheme overwhelm that of the time-division multiple access and conventional NOMA schemes.

Hybrid Half-Duplex/Full-Duplex Cooperative Non-Orthogonal Multiple Access With Transmit Power Adaptation

Power allocation is an important issue in order to optimize the performance of non-orthogonal multiple access (NOMA) systems. However, the power allocation problem for cooperative NOMA systems has not been well investigated. In this paper, we investigate the power allocation problems for half-duplex cooperative NOMA (HD-CNOMA) and full-duplex cooperative NOMA (FD-CNOMA) systems, respectively. From the fairness standpoint, the optimization problem for each system is formulated to maximize the minimum achievable user rate in a NOMA user pair. Even though both of the formulated problems are neither concave nor quasi-concave, the optimal closed-form solutions of both cases are still obtained with the proposed two-step method. First, we transform the initial problem into a quasi-concave problem by treating the relay transmit power, namely $P_{R}$ , as a constant, and then solve the obtained quasi-concave problem. Second, we convert the original problem into a univariate problem of $P_{R}$ based on the results of the first step, and eventually obtain the optimal power allocation. In addition, a hybrid half/full-duplex cooperative NOMA scheme, which dynamically switches between the HD-CNOMA and FD-CNOMA mode, is proposed. After that, a relay selection scheme is also investigated to extend the hybrid scheme into general networks with multiple users. Numerical results demonstrate that the proposed hybrid relaying scheme can achieve a significant performance improvement with respect to the conventional NOMA, HD-CNOMA, and FD-CNOMA scheme.

Performance Analysis of a Downlink Cooperative NOMA Network Over Nakagami-m Fading Channels

This paper assesses the performances of a downlink cooperative non-orthogonal multiple access (C-NOMA) network over Nakagami-m fading channels. The analytical outage probability expression for each user is derived and verified by Monte Carlo simulations. Based upon the outage probability, the maximal throughputs are obtained and analyzed under different channel conditions. Furthermore, the effects of channel fading on the probability and throughput are investigated. The results indicate that the performance of the C-NOMA system is excellent over the source-relay link with less fading. This analysis can be used for designing the C-NOMA systems in the future fifth-generation wireless communication.