Multi-user Non-orthogonal Multiple Access System Research Articles

On performance of RIS-aided ground-to-air and air-to-ground communications in multi-user NOMA systems

This paper examines the performance of a wireless system where two reconfigurable intelligent surfaces (RISs) and an unmanned aerial vehicle (UAV) are combined to aid multiple ground users adopting nonorthogonal multiple access (NOMA) scheme. Specifically, the UAV acts as an aerial relay station, the first and the second RISs are respectively used to aid ground-to-air (G2A) and air-to-ground (A2G) communications. Besides the reflected link from the RISs, the direct link between a ground base station (S) and the UAV, the UAV and ground users are exploited to increase the received power at UAV and ground users. We successfully derive the theoretical formulas of outage probability (OP) and ergodic capacity (EC) of RIS-aided G2A/A2G-NOMA system over Nakagami-m fading channel. The derived formulas are validated by Monte-Carlo simulations, and the impacts of system parameters on the OP and EC are fully evaluated to achieve useful insights into the system behaviors. Numerical results clarify that by using two RISs where every RIS is equipped with 100 reflecting elements (REs), the OP of RIS-aided G2A/A2G-NOMA system is greatly lower than the OP of the G2A/A2G-NOMA system without RISs, in which there are only S-UAV and UAV-ground users direct links. In the case of EC, the benefit of using RISs focuses on a low transmit power regime. It is because the EC of the RIS-aided G2A/A2G-NOMA system is only higher than the EC of the system without RISs in the low transmit power region. For the high transmit power regions, the ECs with and without RISs are similar.

On the outage performance of multi-user NOMA system with adaptive SIC

The performance of non-orthogonal multiple-access (NOMA) is significantly determined by the successive interference cancellation (SIC) technique. However, the interference redundancy can happen during the NOMA receiver performing SIC. To tackle this issue, this paper proposes a novel adaptive successive interference cancellation (ASIC) method. Specifically, the decoding error is considered as interference during the SIC process, and an adaptive filter with various weights is introduced for detection error mitigation. For the multi-user downlink NOMA system, the outage probability is analyzed under conventional and the proposed SIC methods. Simulation results verify our analysis, which also demonstrate the superiority of the proposed ASIC method.

On performance of multi-RIS assisted multi-user nonorthogonal multiple access system over Nakagami-[formula omitted] fading channels

In practice, multiple reconfigurable intelligent surfaces (RISs) may be deployed to assist wireless systems. Unfortunately, the mathematical analysis of multiple RISs aided wireless systems is challenging. In this paper, multiple RISs are utilized in a multi-user nonorthogonal multiple access (NOMA) system. Particularly, multiple users combine signals traveling on the transmitter–user direct paths and the reflected paths from the RISs. We successfully derive the closed-form expressions of the outage probability (OP) and ergodic capacity (EC) of this multi-RIS aided multi-user NOMA (RIS-NOMA) system. We also obtain the OP and EC expressions of the RIS-NOMA system in the case that there are only two NOMA users, where the specific derived forms depend on the exact values of power allocation coefficients and threshold. We validate all derived expressions through Monte-Carlo simulations. Numerical results show that compared with the case without RISs, the performance in terms of OP and EC of the NOMA system with RISs are significantly improved. In particular, the OP in the case with RISs is greatly lower than that in the case without RISs for the considered range of transmission power. However, the EC in the case with RISs is only significantly higher than that in the case without RISs in the low transmission power regime. In the high transmission power regime, they are similar. Consequently, depending on the certain requirements in practical scenarios where the RIS-NOMA system is deployed, we can use a suitable transmission power to achieve the OP and EC performance target and save the power consumption.

Combining multi-RIS and relay for performance improvement of multi-user NOMA systems

In this paper, we propose a theoretical framework to examine the performance of a non-orthogonal multiple access (NOMA) system with multiple users under the support of multiple advanced reconfigurable intelligent surfaces (RISs) and a traditional relay. We derive the analytical expressions of main performance metrics of the proposed multi-RIS-and-relay aided multiple-user NOMA system (so called the RIS-R-NOMA system), including the outage probability (OP) and the achievable data rate (ADR) over Nakagami-m fading channels. We conduct the Monte-Carlo simulations to confirm the exactness of our mathematical expressions. Numerical results reveal that the OP of the proposed RIS-R-NOMA system is significantly lower than the OPs of the traditional NOMA systems without either RIS or relay (the R-NOMA and RIS-NOMA systems), and the ADR of the proposed RIS-R-NOMA system is much higher than the ADR of the R-NOMA system. Importantly, at high transmit power, the ADRs of both RIS-R-NOMA and R-NOMA systems are similar due to the property of NOMA. Additionally, the effects of various parameters such as the number of reflecting elements (REs), the locations of the RISs, the power allocation coefficients, and the data transmission rates on the performance of the RIS-R-NOMA system are demonstrated.

The Impact of User-Pairing on the Performance of Non-Orthogonal Multiple Access (NOMA) System

Non-orthogonal multiple access (NOMA) is being considered as a candidate to meet the expected traffic demand growth in 5G and beyond 5G (5GB) networks. Due to the complexity of applying multiuser detection at the NOMA receiver, a number of works have suggested applying user-pairing to simplify dealing with all users as a whole. This paper addresses the effect of applying user-pairing on the performance of NOMA assisted system in general proposes three different user pairing approaches to examine which one is best for boosting the performance of multiuser NOMA system and making it reaches its potential. This paper evaluates the performance of NOMA system with organized (uniform), random, and distance-based user pairing schemes. Simulation results show that the organized pairing assisted NOMA scheme offers the best performance as compared to its pairing peer techniques in terms of the overall sum rate and the energy efficiency.

Statistical Beamforming Techniques for Power Domain NOMA System

Power-domain non-orthogonal multiple access (NOMA) assigns different power levels for near and far users in order to discriminate their signals by employing successive interference cancellation (SIC) at the near user. In this context, multiple-input-single-output NOMA (MISO-NOMA), where the base station (BS) is equipped with multiple antennas while each mobile user has a single antenna receiver, is shown to have a better overall performance by using the knowledge of instantaneous channel state information (CSI). However, this requires prior estimation of CSI using pilot transmission, which increases the transmission overhead. Moreover, its performance is severely degraded in the presence of CSI estimation errors. In this work, we provide statistical beamforming solutions for downlink power-domain NOMA that utilize only knowledge of statistical CSI, thus reducing the transmission overhead significantly. First, we derive the outage probabilities for both near and far users in the multi-user NOMA system without imposing strong assumptions, such as Gaussian or Chi-square distribution. This is done by employing the exact characterization of the ratio of indefinite quadratic form (IQF). Second, this work proposes two techniques to obtain the optimal solution for beam vectors which rely on the derived outage probabilities. Specifically, these two methods are based on (1) minimization of total beam power while constraining the outage probabilities to the QoS threshold, and (2) minimization of outage probabilities while constraining the total beam power. These proposed methods are non-convex function of beam vectors and, hence, are solved using numerical optimization via sequential quadratic programming (SQP). Since the proposed methods do not require pilot transmission for channel estimation, they inherit better spectral efficiency. Our results validate the theoretical findings and prove the supremacy of the proposed method.

Distortion-Aware Cross-Layer Power Allocation for Video Transmission Over Multi-User NOMA Systems

Non-orthogonal multiple access (NOMA) is promising to enable growing video services with requirements of massive traffic and low latency. In this paper, we investigate a novel multi-user NOMA system design for video delivery. Different from data delivery considered in existing NOMA systems, video distortion is taken into consideration for the design and optimization of the proposed scheme. We first propose a cross-layer scalable video delivery scheme over NOMA wireless networks based on given user grouping. By adopting a semi-analytical rate distortion model for encoded video sequences and investigating the physical layer model for the multi-carrier NOMA network, the distortion-aware power allocation problem is formulated with the goal to minimize the system end-to-end distortion with quality-of-service requirements. To solve such a non-convex problem, we design an efficient algorithm by exploiting its hidden monotonic property to approximate the optimal solution. Inspired by the decoding order in NOMA, a fast algorithm is developed by combining water-filling and greedy strategies to further tame the computational complexity. Simulations results have verified the advantages of the proposed cross-layer scheme with distortion-aware power allocation algorithms, against two existing NOMA schemes and an OMA scheme.

Optimized Shallow Neural Networks for Sum-Rate Maximization in Energy Harvesting Downlink Multiuser NOMA Systems

This article considers a power allocation problem in energy harvesting downlink non-orthogonal multiple access (NOMA) systems in which a transmitter sends desired messages to their respective receivers by using harvested energy. To tackle this problem, we make use of a reinforcement learning approach based on a shallow neural network structure. We prove that the optimal power allocation policy and the optimal action-value function depend monotonically on some of their input variables and the shallow neural network structure is designed based on properties revealed in the proof. Different from inefficient deep learning methods that tend to require tremendous computational resources, this structure is capable of fully capturing the characteristics of the desired function with a single hidden layer. The optimized structure also allows learning agents to be robust and highly reliable in learning about randomly occurring data. Furthermore, we provide comprehensive experimental results in harsh environments where various arbitrary factors are assumed in order to demonstrate the robustness of the proposed learning approach compared with deep neural networks without proper grounds. It is also shown that the proposed learning process converges to a policy that outperforms existing power allocation algorithms.

DRL-Based Energy-Efficient Resource Allocation Frameworks for Uplink NOMA Systems

Nonorthogonal multiple access (NOMA) is one of the promising technologies to meet the huge access demand and high data-rate requirements of the next-generation networks. In this article, we investigate the joint subchannel assignment and power allocation problem in an uplink multiuser NOMA system to maximize the energy efficiency (EE). Different from conventional model-based resource allocation methods, we propose three deep-reinforcement-learning (DRL)-based frameworks to solve this nonconvex optimization problem, referred to as the discrete DRL-based resource allocation (DDRA) framework, continuous DRL-based resource allocation (CDRA) framework, and joint DRL and optimization resource allocation (DORA) framework. Specifically, for the DDRA framework, a multi-DQN-based network is designed to dynamically allocate resources discretely, which can reduce the output dimension and improve the learning efficiency. To overcome the loss of power discretization in DDRA, a joint DQN and deep deterministic policy-gradient (DDPG)-based network (CDRA framework) is designed to generate the resource allocation policy. The DORA framework is then proposed as a performance boundary. Finally, an event-triggered learning method is combined with all three frameworks to further reduce the computational consumption. The numerical results show that the proposed frameworks can improve the EE performance of the uplink NOMA system and reduce the computation time.

Performance analysis of downlink NOMA over η–µ and κ–µ fading channels

Non-orthogonal multiple access (NOMA) is recognised as an improved multiple access technique as compared with OMA technique to fulfil the requirements of fifth-generation wireless communication systems. In this study, the outage probability and sum-rate analysis of downlink multi-user NOMA system over the generalised fading channels, i.e. η–µ and κ–µ, are presented. Specifically, the outage probability of NOMA is analysed with fixed target rate and OMA rate. The mathematical expressions for the outage probability and sum rate with a fixed set of power allocation coefficients are derived and verified through the simulation results. The impact of system parameters such as number of users, power allocation coefficients, fading parameters etc. on the performance of NOMA system is investigated. NOMA with given fixed target rate offers better outage performance as compared with OMA. However, NOMA with OMA rate offers better outage performance for the near user only. The sum rate and near user rate of NOMA improves with signal-to-noise ratio; however, the far user rate does not follow the same trend.

RETRACTED: Energy-aware resource management for uplink non-orthogonal multiple access: Multi-agent deep reinforcement learning

Abstract Non-orthogonal multiple access (NOMA) is one of the promising technologies to meet the huge access demand and the high data rate requirements of the next generation networks. In this paper, we investigate the joint subchannel assignment and power allocation problem in an uplink multi-user NOMA system to maximize the energy efficiency (EE) while ensuring the quality-of-service (QoS) of all users. Different from conventional model-based resource allocation methods, we propose two deep reinforcement learning (DRL) based frameworks to solve this non-convex and dynamic optimization problem, referred to as discrete DRL based resource allocation (DDRA) framework and continuous DRL based resource allocation (CDRA) framework. Specifically, for the DDRA framework, we use a deep Q network (DQN) to output the optimum subchannel assignment policy, and design a distributed and discretized multi-DQN based network to allocate the corresponding transmit power of all users. For the CDRA framework, we design a joint DQN and deep deterministic policy gradient (DDPG) based network to generate the optimal subchannel assignment and power allocation policy. The entire resource allocation policies of these two frameworks are adjusted by updating the weights of their neural networks according to feedback of the system. Numerical results show that the proposed DRL-based resource allocation frameworks can significantly improve the EE of the whole NOMA system compared with other approaches. The proposed DRL based frameworks can provide good performance in various moving speed scenarios through adjusting learning parameters.

Joint Spectral and Energy Efficiency Optimization for Downlink NOMA Networks

Non-orthogonal multiple access (NOMA) holds the promise to be a key enabler of 5G communication. However, the existing design of NOMA systems must be optimized to achieve maximum rate while using minimum transmit power. To do so, this paper provides a novel technique based on multi-objective optimization to efficiently allocate resources in the multi-user NOMA systems supporting downlink transmission. Specifically, our unique optimization technique jointly improves spectrum and energy efficiency while satisfying the constraints on users quality of services (QoS) requirements, transmit power budget and successive interference cancellation. We first formulate a joint problem for spectrum and energy optimization and then employ dual decomposition technique to obtain an efficient solution. For the sake of comparison, a low complexity single-objective NOMA optimization scheme is also provided as a benchmark scheme. The simulation results show that the proposed joint approach not only performs better than the traditional benchmark NOMA scheme but also significantly outperforms its counterpart orthogonal multiple access (OMA) scheme in terms of both energy and spectral efficiency.

Efficient Resource Management for Sum Capacity Maximization in 5G NOMA Systems

The modern cellular technologies are expected to provide high data rates and massive connectivity in fifth generation (5G) systems; however, this may be impossible through traditional radio access techniques. Recently, non-orthogonal multiple access (NOMA) has emerged as one of the promising cellular techniques for modern cellular communications with its ability to provide access for multiple users to the network over the same system resources. This paper studies resource management problem for downlink transmission of multiuser NOMA system. Our objective is to optimize both frequency and power resources for sum capacity maximization while taking into account each user minimum capacity requirement. Firstly, the problem of resource management decouples into two subproblems, that is, efficient sub-channel assignment and optimal power allocation, respectively. Secondly, for given power at base station, we design two sub-optimal algorithms for sub-channel assignment based on user channel condition and user minimum capacity requirement, respectively. Lastly, for any given sub-channel assignment, the problem first transforms into standard convex optimization problem and then we employ duality theory. To evaluate our proposed NOMA scheme, the enhanced version of existing NOMA optimization scheme is also presented as a benchmark. Results demonstrate that the proposed NOMA resource management scheme outperforms the benchmark NOMA optimization scheme in terms of sum capacity.

Energy-Efficient Power Allocation for NOMA With Imperfect CSI

An optimum user power allocation solution to maximize the energy efficiency (EE) in a non-orthogonal multiple access (NOMA) system with imperfect channel state information at transmitter (CSIT) and user quality of service constraint is proposed. The EE objective function is a non-convex nonlinear function that leads to a fractional programming problem. To deal with this problem, an auxiliary variable is defined, which converts the problem into a convex one. A closed form optimal solution for the power allocated to each user is found using the Karush–Kuhn–Tucker conditions. Furthermore, difference of two concave functions—DC—programming and Dinkelbach structure are used to find close-to-optimal total power value (sum of powers being allocated to all users) for the multiuser NOMA system. Simulation results show that in terms of EE, for users with low rate requirement and with CSIT error, NOMA has a better performance compared with conventional orthogonal multiple access (OMA).

Joint User Pairing and Subchannel Allocation for Multisubchannel Multiuser Nonorthogonal Multiple Access Systems

In this paper, we investigate the issues of joint user pairing and subchannel allocation (JUP-SA) in a multisubchannel multiuser nonorthogonal multiple access (NOMA) system. We first investigate the uplink and propose a JUP-SA scheme so that the minimum achievable diversity order of users is maximized. By deriving the upper and lower bounds of the outage probability of the worst-performance user, the achievable diversity order of the scheme is obtained. Following this, the downlink of the system is considered and a JUP-SA scheme that maximizes the minimum diversity order is presented. The closed-form expression of the worst-case outage probability is then derived and validated by simulation results. Numerical results show that both schemes in uplink and downlink NOMA systems can achieve the same diversity order as that of exhaustive search.

Spectral and Energy-Efficient Wireless Powered IoT Networks: NOMA or TDMA?

Wireless powered communication networks (WPCNs), where multiple energy-limited devices first harvest energy in the downlink and then transmit information in the uplink, have been envisioned as a promising solution for the future Internet-of-Things (IoT). Meanwhile, non-orthogonal multiple access (NOMA) has been proposed to improve the system spectral efficiency (SE) of the fifth-generation (5G) networks by allowing concurrent transmissions of multiple users in the same spectrum. As such, NOMA has been recently considered for the uplink of WPCNs based IoT networks with a massive number of devices. However, simultaneous transmissions in NOMA may also incur more transmit energy consumption as well as circuit energy consumption in practice which is critical for energy constrained IoT devices. As a result, compared to orthogonal multiple access schemes such as time-division multiple access (TDMA), whether the SE can be improved and/or the total energy consumption can be reduced with NOMA in such a scenario still remains unknown. To answer this question, we first derive the optimal time allocations for maximizing the SE of a TDMA-based WPCN (T-WPCN) and a NOMA-based WPCN (N-WPCN), respectively. Subsequently, we analyze the total energy consumption as well as the maximum SE achieved by these two networks. Surprisingly, it is found that N-WPCN not only consumes more energy, but also is less spectral efficient than T-WPCN. Simulation results verify our theoretical findings and unveil the fundamental performance bottleneck, i.e., "worst user bottleneck problem", in multiuser NOMA systems.

Resource Allocation in Non-Orthogonal and Hybrid Multiple Access System With Proportional Rate Constraint

Non-orthogonal multiple access (NOMA), which has attracted a lot of attention recently due to its superior spectral efficiency, could play a vital role in improving the capacity of future networks. In this paper, a resource allocation scheme is developed for a downlink multi-user NOMA system. An optimization problem is formulated to maximize the sum rate under the total power and proportional rate constraints. Due to the complexity of computing the optimal solution, we develop a low complexity sub-optimal solution for a two-user scenario and then extend it to the multi-user case by proposing a user-pairing approach as well as a number of power allocation techniques that facilitate dealing with a large number of users in NOMA system. Simulation results support the effectiveness of the proposed approaches and show the close performance to the optimal one. In addition, we propose a new hybrid multiple access technique that combines the properties of NOMA and the orthogonal frequency division multiple access. Simulation results show that the proposed hybrid method provides better performance than NOMA in terms of the overall achievable sum rate and the coverage probability.