Downlink Non-orthogonal Multiple Access System Research Articles

Nonorthogonal multiple access systems optimization to ensure maximum fairness to users

AbstractIn this contribution, the optimization of power proportion allocated for each user in the downlink (DL) nonorthogonal multiple access (NOMA) systems have been developed. Successive interference cancellation technique recovers users' signal with high difference between channel gains, to find the lowest optimum power proportion required to guarantee an equal data rate for all active users. Following the same approach, the optimum power proportion for each user and also the minimum total power to achieve the same rate for all users (maximum fairness) were obtained as a design goal. Moreover, the same design methodology was developed seeking to maximize the NOMA system energy efficiency (EE). It was possible to find the maximum EE point and the respective power distribution among the users for a certain circuitry power consumption. For all NOMA users, in which the optimal operation point for EE maximization was parameterized, it was possible to find the total power, power ratio, and the equal rate values. As a result, one can find the trade‐off point between EE and sum rate for each system scenario, as well as the best resource efficiency operation point. By considering the same rate for all users, the system attains maximum fairness among the users.

Threshold Based Signal Detection and the Average Symbol Error Probability for Downlink NOMA Systems With M-ary QAM

Non-orthogonal multiple access (NOMA) is a technique which allows multiple users to share the same radio access resources. However, there exist interferences among these users in the NOMA system. These interferences need to be taken into account in the signal detection and error performance analysis. In the downlink NOMA system, the far user detects its signal by treating the near user's signal as interference, while the near user employs successive interference cancellation (SIC) to detect its signal. Based on the conventional signal detection, it is not straightforward to derive the error performance for both the far user and near user. By analyzing the constellation relationships between the far user's signal and superposition coded (SC) signal, and the near user's signal and SC signal, in this article, a simple threshold based detection algorithm is proposed to detect both the far user's signal and near user's signal. Furthermore, by aid of the constellation relationships, in this article, simple expressions of the average symbol error probability (ASEP) with squared M -ary quadrature amplitude modulation for both the far user and near user are easily derived in Rayleigh fading channels. Simulation results validate the derived theoretical ASEP results.

Performance Assessment for Energy Efficient NOMA over Nakagami Fading Channel

Nowadays, the Non-Orthogonal Multiple Access (NOMA) has become the most candidate as the multiple access technique for the Fifth Generation (5G). Although, the massive increase in traffic usage is necessary, but it must not come on the account of Energy Efficiency (EE). Therefore, Green Radio (GR) which one of its main points is energy efficiency (EE) gained a lot of attention in last few years in both academic and industrial fields. In this paper, the Energy-Efficiency in the NOMA downlink system under the effect of the Nakagami-m fading channel is studied. Moreover, the Nakagami-m fading channel can tackle several fading cases according to the parameter m. The system has been solved via an iterative algorithm to investigate the required power level for maximum energy efficiency. Different channel models will be investigated such as: one-sided Gaussian faded channel, Rayleigh faded channel, Raician (Nakagami-n) faded channel, and LOS.

Performance Analysis and Fairness Maximization in NOMA Systems With Improper Gaussian Signaling Under Imperfect Successive Interference Cancellation

In this paper, we investigate a downlink non-orthogonal multiple access (NOMA) system consisting of a base station (BS) and two users, where each user decodes its own signal through successive interference cancellation (SIC). When the signal of one user, called the weak user (WU) who has a bad channel, is being decoded, the signal of the other user, called the strong user (SU) who has a good channel, acts as interference. In an interference channel, we can increase the achievable rate by applying improper Gaussian signaling (IGS). Therefore, we apply IGS to SU, aiming to mitigate the interference. We also consider imperfect SIC at SU, which is a more general assumption for a NOMA system. For the considered NOMA system model, we derive the outage probability of each user in a closed form, provide its asymptotic expression, and obtain its diversity order. We also derive the ergodic rate of each of the two users in a closed form. We propose two algorithms to maximize the fairness of the NOMA system based on the primal decomposition method. Through simulation, we verify that our analysis is correct, and confirm that applying IGS improves the fairness of the NOMA system.

Performance Analysis of NOMA in 5G Systems With HPA Nonlinearities

In this paper, we provide an analytical performance assessment of downlink non-orthogonal multiple access (NOMA) systems over Nakagami-m fading channels in the presence of nonlinear high-power amplifiers (HPAs). By modeling the distortion of the HPA by a nonlinear polynomial model, we evaluate the performance the NOMA scheme in terms of outage probability (OP) and ergodic sum rate. Hence, we derive a new closed-form expression for the exact OP, taking into account the undesirable effects of HPA. Furthermore, to characterize the diversity order of the considered system, the asymptotic OP in the high signal-to-noise (SNR) regime is derived. Moreover, the ergodic sum rate is investigated, resulting in new upper and lower bounds. Our numerical results demonstrate that the performance loss in presence of nonlinear distortions is very substantial at high data rates. In particular, it is proved that in presence of HPA distortion, the ergodic sum rate cannot exceed a determined threshold which limits its performance compared to the ideal hardware case. Monte-Carlo simulations are conducted and their results agree well with the analytical results.

Performance Analysis of Downlink NOMA Systems Over $\kappa$-$\mu$ Shadowed Fading Channels

Non-orthogonal multiple access (NOMA) has emerged as a promising technology for 5G networks and beyond. In order to fully reap the benefits of NOMA, it is essential to characterize its performance for different channel conditions. In this paper, we carry out a performance analysis of downlink NOMA systems subject to $\kappa$ - $\mu$ shadowed fading channels. Specifically, ergodic capacity, outage probability, and average bit error rate expressions for a two-users NOMA scheme under $\kappa$ - $\mu$ shadowed fading links are derived. The accuracy of the analytical results has been validated by extensive simulations.

DSWIPT Scheme for Cooperative Transmission in Downlink NOMA System

In this paper, we focus on the issue how to reduce the throughput performance gap between a cell-center user and a cell-edge user in a two-user downlink non-orthogonal multiple access (NOMA) system. To this end, we propose a dynamic Simultaneous Wireless Information and Power Transfer cooperative NOMA (DSWIPT NOMA) scheme. In the proposed scheme, the cell-center user works as a relay to improve the throughput performance of the cell-edge user. The relaying operation is powered by the DSWIPT scheme, in which both the time allocation (TA) ratio and power splitting (PS) ratio can be adjusted dynamically to make full use of the time/power resource in each transfer frame. Specifically, the analytical expressions of the outage probability (OP) corresponding to the cell-center user and the cell-edge user are derived. And a joint optimization algorithm is also proposed to find the optimal TA ratio and PS ratio for maximizing the sum-throughput of the system. The numerical results show that the analytical results are in accordance with the Monte-Carlo simulation results exactly. Compared with the non-cooperative NOMA scheme and the SWIPT cooperative NOMA scheme, the DSWIPT NOMA scheme has a better performance in both the sum-throughput of the system and the OP of the cell-edge user.

Performance analysis at far and near user in NOMA based system in presence of SIC error

Non-orthogonal multiple access (NOMA) scheme is an emerging 5G technology to improve upon the spectral efficiency. It can support more users than the number of available frequency-,time-, and code-domain resources. The innovation behind NOMA technology is to exploit non-orthogonal resource allocation at the cost of receiver complexity to mitigate the inter-user interference arising due to non-orthogonal signals. In this paper, we present performance analysis of downlink NOMA (DL-NOMA) system in presence of successive interference cancellation (SIC) error over Rayleigh fading channels. In particular, we derive the exact closed-form expressions of bit error rate (BER) at near-user (U1) and far-user (U2) for the considered DL-NOMA system. Besides, we also present the achievable rate analysis of the considered NOMA system and it is observed that its achievable rate is higher as compared to conventional orthogonal multiple access (OMA) schemes. Further, we also determine the optimized power allocation coefficient for the NOMA system. The analytical results are corroborated with the Monte-Carlo simulations. The simulation results are presented for different modulation schemes and for different NOMA scenarios. A close argument between the analytical and simulation results validate our analysis.

Achievable Diversity Order of HARQ-Aided Downlink NOMA Systems

The combination between non-orthogonal multiple access (NOMA) and hybrid automatic repeat request (HARQ) is capable of realizing ultra-reliability, high throughput and many concurrent connections particularly for emerging communication systems. This paper focuses on characterizing the asymptotic scaling law of the outage probability of HARQ-aided NOMA systems with respect to the transmit power, i.e., diversity order. The analysis of diversity order is carried out for three basic types of HARQ-aided downlink NOMA systems, including Type I HARQ, HARQ with chase combining (HARQ-CC) and HARQ with incremental redundancy (HARQ-IR). The diversity orders of three HARQ-aided downlink NOMA systems are derived in closed-form, where an integration domain partition trick is developed to obtain the bounds of the outage probability specially for HARQ-CC and HARQ-IR-aided NOMA systems. The analytical results show that the diversity order is a decreasing step function of transmission rate, and full time diversity can only be achieved under a sufficiently low transmission rate. It is also revealed that HARQ-IR-aided NOMA systems have the largest diversity order, followed by HARQ-CC-aided and then Type I HARQ-aided NOMA systems. Additionally, the users’ diversity orders follow a descending order according to their respective average channel gains. Furthermore, we expand discussions on the cases of power-efficient transmissions and imperfect channel state information (CSI). Monte Carlo simulations finally confirm our analysis.

A New User Grouping and Power Allocation Scheme for Downlink Non-orthogonal Multiple Access Systems

In this paper, we propose a new user grouping and power allocation scheme based on beamforming for downlink non-orthogonal multiple access systems. The proposed user grouping scheme can effectively reduce the interference from the other user and other beams as well, and can effectively improve the weak user rate especially when the SNR is not large. In addition, a power allocation scheme that can maximize the sum capacity while satisfying a certain fairness index is proposed. From the simulation results, the user grouping and power allocation method proposed in this paper can not only improve the overall system throughput performance, but also improve the fairness of weak users.

Secrecy Analysis for Cooperative NOMA Networks With Multi-Antenna Full-Duplex Relay

In a downlink non-orthogonal multiple access (NOMA) system, the reliable transmission of cell-edge users cannot be guaranteed due to severe channel fading. On the other hand, the presence of eavesdroppers can severely threaten the secure transmission due to the open nature of wireless channel. Thus, a two-user NOMA system assisted by a multi-antenna decode-and-forward relay is considered in this paper, and a two-stage jamming scheme, full-duplex-jamming (FDJam), is proposed to ensure the secure transmission of NOMA users. In the FDJam scheme, using full-duplex, the relay transmits the jamming signal to the eavesdropper while receiving confidential messages in the first stage, and the base station generates the jamming signal in the second stage. Furthermore, we eliminate the self-interference and the jamming signal at the relay and the legitimate node, respectively, through relay beamforming. To measure the secrecy performance, analytical expressions for secrecy outage probability (SOP) are derived for both the cell-center and cell-edge users, and the asymptotic SOP analysis at high transmit power is presented as well. Moreover, two benchmark schemes, half-duplex-jamming and full-duplex-no-jamming, are also considered. Simulation results are presented to show the accuracy of the analytical expressions and the effectiveness of the proposed scheme.

Power-Balanced Non-Orthogonal Multiple Access Based on Virtual Channel Optimization

In this brief, we propose a virtual channel optimization method for downlink non-orthogonal multiple access (NOMA) systems. With this method, we show that superposed users’ data can be separated successfully in power-balanced scenario for NOMA. We derive the optimum relative phase offset for two users as well as three users based on the minimum Euclidean distance (MED) among the superposed constellation points without channel state information at the transmitter (CSIT). We exploit maximum likelihood (ML) detector at the receiver as successive interference cancelation (SIC) is not applicable for power-balanced scenario. Since the complexity of ML is high, we design a low-complexity ML algorithm to substantially reduce the computational complexity. Simulation results for quadrature phase-shift keying (QPSK) modulation demonstrate the effectiveness and the performance of the proposed method.

Rethinking Outage Constraints for Resource Management in NOMA Networks

In non-orthogonal multiple access (NOMA) systems, the outage is regarded to happen when a user cannot correctly decode the messages for users with higher decoding order and hence cannot perform the successive interference cancelation (SIC). However, in this case, the user may still correctly decode its message by treating the uncanceled signal as interference and avoid the outage. By considering this behavior, the outage probability should be redefined. In this paper, we investigate user scheduling and power allocation for a downlink NOMA system with imperfect SIC by using the alternative outage probability as a constraint. In order to tackle the complicated non-convex resource allocation problem, we propose a two-phase algorithm, in which the user scheduling is first optimized through a matching theory based algorithm, and then power allocation is performed with the aid of the branch and bound technique and the concave–convex procedure method. Simulation results show that the performance of the proposed low-complexity algorithm is near-optimal and the algorithm based on the alternative outage probability outperforms that based on the traditional one when the residual interference from imperfect SIC significantly affects the decoding.

The Application of Antenna Diversity to NOMA With Statistical Channel State Information

In this paper, antenna diversity is used to improve the outage performance of downlink non-orthogonal multiple access (NOMA) systems combined with single-user multiple-input multiple-output (SU-MIMO) due to the statistical channel state information (CSI), that the transmitter only knows the statistical characteristic associated with each user's channel matrix. We design the precoding and detection vectors, which makes the proposed MIMO-NOMA system reduce to a single-antenna NOMA system with statistical CSI. Next, a dynamic power allocation scheme is proposed to maximize the ergodic sum rate of the considered NOMA group with the minimum ergodic rate constraints of the weaker users. Under this power allocation, we analyze the impact of user clustering and then develop a novel user clustering algorithm. Also, the outage analysis indicates that the proposed system only obtains receive diversity and under the proposed power allocation, the users' diversity gains are linear with their successive interference cancellation decoding orders. Simulation results confirm our proposed analysis.

Secrecy analysis of a cooperative NOMA network using an EH untrusted relay

ABSTRACTIn this paper, a down-link non-orthogonal multiple access (NOMA) system with imperfect successive interference cancellation (SIC) using Energy-Harvesting untrusted relays is investigated. These relaying nodes use in this study use a power-switching architecture to harvest energy from the sources signals and apply an amplify-and-forward protocol to forward the signals. In addition, transmit jamming or artificial noise, is generated by a source node to improve the security of the system and protect confidential source information from untrusted relays. Likewise, three relaying selection strategies are employed to examine the secrecy performance of the proposed system. In order to evaluate the performance evaluation of the proposed system, closed-form expressions of the Secrecy Outage Probability (SOP) are studied over Rayleigh fading channels and a Monte Carlo simulation is used to confirm the analytical results. Furthermore, we study the effects of various parameters, such as power allocation factors, relay node selection, the number of relays, energy harvesting efficiency and the location of relay nodes on the secure outage performances for two users of NOMA system and conventional orthogonal multiple access (OMA). These results show that NOMA offers the better security performance with multiple users.

Sum-Rate Maximization of NOMA Systems Under Imperfect Successive Interference Cancellation

This letter addresses the sum-rate maximization for a downlink non-orthogonal multiple access (NOMA) system in the presence of imperfect successive interference cancellation (SIC). We assume that the NOMA users adopt improper Gaussian signaling (IGS), and hence derive new expressions of their rates under residual interference from imperfect SIC. We optimize the circularity coefficient of the IGS-based NOMA system to maximize its sum-rate subject to quality-of-service requirements. Compared to the NOMA with proper Gaussian signaling, simulation results show that the IGS-based NOMA system demonstrates considerable sum-rate performance gain under imperfect SIC.

Deep Learning-Based Long-Term Power Allocation Scheme for NOMA Downlink System in S-IoT

In this paper, we formulate a long-term resource allocation problem of non-orthogonal multiple access (NOMA) downlink system for the satellite-based Internet of Things (S-IoT) to achieve the optimal decoding order and power allocation. This long-term resource allocation problem of the satellite NOMA downlink system can be decomposed into two subproblems, i.e., a rate control subproblem and a power allocation subproblem. The latter is a non-convex problem and the solution of which relies on both queue state and channel state. However, the queue state and the channel state continually change from one time slot to another, which makes it extremely strenuous to characterize the optimal decoding order of successive interference cancellation (SIC). Therefore, we explore the weight relationship between the queue state and the channel state to derive an optimal decoding order by leveraging deep learning. The proposed deep learning-based long-term power allocation (DL-PA) scheme can efficiently derive a more accurate decoding order than the conventional solution. The simulation results show that the DL-PA scheme can improve the performance of the S-IoT NOMA downlink system, in terms of long-term network utility, average arriving rate, and queuing delay.

Null-Steering Beamforming for Enhancing the Physical Layer Security of Non-Orthogonal Multiple Access System

This paper addresses enhancing the physical layer security of a downlink non-orthogonal multiple access systems. The proposed scheme consists of a base station, multiple legitimate users, and an eavesdropper. In each transmission slot, the base station communicates with two paired users, under the malicious attempts of the eavesdropper to intercept the information messages. In order to impair the eavesdropper's channel without affecting the legitimate paired users, a jamming signal is injected into the system by means of null-steering beamforming. In null-steering precoding, the jamming signal is directed toward the malicious node while being suppressed in the legitimate users' directions. Null-steering jamming is exploited in two different mainstreams, namely, self-cooperative and nonself-cooperative jamming. In the self-cooperative strategy, the base station implements the null-steering beamformer to transmit the jamming signal during the information exchange with the legitimate users, whereas in the nonself-cooperative jamming scheme, idle legitimate users (helpers) utilize the harvested energy in the first phase to transmit the jamming signal toward the eavesdropper in the second phase during the base station communication with the legitimate users. The outage behavior in the two considered scenarios is investigated, and the secrecy outage probability for both paired users is derived and provided in closed-form expressions. Numerical simulations are performed to validate the derived analytical results and to compare the two considered strategies secrecy performance. Comparisons yield that self-cooperative jamming has better outage behavior unless the number of helpers in the system is large enough.

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.

Backscatter-NOMA: A Symbiotic System of Cellular and Internet-of-Things Networks

Non-orthogonal multiple access (NOMA) is envisioned as a key technology to enhance the spectrum efficiency for 5G cellular networks. Meanwhile, ambient backscatter communication is a promising solution to the Internet of Things (IoT), due to its high spectrum efficiency and power efficiency. In this paper, we are interested in a symbiotic system of cellular and IoT networks and propose a backscatter-NOMA system, which incorporates a downlink NOMA system with a backscatter device (BD). In the proposed system, the base station (BS) transmits information to two cellular users according to the NOMA protocol, while a BD transmits its information over the BS signals to one cellular user using the passive radio technology. In particular, if the BS only serves the cellular user that decodes BD information, the backscatter-NOMA system turns into a symbiotic radio (SR) system. We derive the expressions of the outage probabilities and the ergodic rates and analyze the corresponding diversity orders and slopes for both backscatter-NOMA and SR systems. Finally, we provide the numerical results to verify the theoretical analysis and demonstrate the interrelationship between the cellular networks and the IoT networks.