Multi-user Non-orthogonal Multiple Access Research Articles

Closed-form BER expression for multi-user downlink NOMA

Massive machine-type communication (MMTC) is an essential element for realizing the Internet of Things (IoT). However, certain challenges arise because MMTC should support a massive number of users. Recently, non-orthogonal multiple access (NOMA) has been highlighted as a technology for MMTC in a limited frequency band. To design and operate a NOMA-based communication system for a massive number of users, an accurate performance analysis for an arbitrary number of users and a practical power assignment method for the channel condition of each user are required. In this study, the exact bit error rate (BER) expression for an arbitrary user number is derived in an explicit closed form for NOMA-based communication. In addition, for analytical purposes, an approximate expression with high accuracy is proposed. To achieve an extension to the fading channel, the average BER expressions for the Rayleigh and Nakagami-m fading channels are presented in closed forms. Based on the derived BER expression, a power assignment algorithm that can achieve the target performance is proposed when each user is associated with a different signal-to-noise ratio. The derived BER expressions and the power assignment algorithm were verified based on computer simulations.

Combining transmit antenna selection and full-duplex aerial relay drone for performance enhancement of multiuser NOMA millimeter-wave communications

In this article, multiple antennas are proposed to utilize the millimeter-wave (MW) communication in a full-duplex aerial relay drone (FD-ARD) multi-user non-orthogonal multiple access (NOMA) system. Notably, the transmit antenna selection (TAS) technique is also incorporated to yield a significant boost in the system performance and a noteworthy reduction in signal processing complexity at the user end. The mathematical formulas of outage probability (OP) and achievable capacity (AC) of the proposed system are derived under the effect of channel and system parameters that are recommended for the fifth generation (5G) standard and the beyond 5G (B5G) proposals. All derived formulas are validated by using Monte-Carlo simulations. Numerical results show that TAS offers a substantial advantage over the scenarios without TAS. This holds even in the presence of several adverse factors, including the attributes of NOMA, the residual self-interference (RSI) stemming from full-duplex (FD) transmissions, the utilization of high carrier frequencies within MW bands, and the elevated altitude of the drone. The OP of systems equipped with TAS remains impressively below 10−11, whereas systems lacking TAS exhibit a substantially higher OP value of 10−4. Furthermore, the AC is notably elevated with the incorporation of TAS, as compared to scenarios where TAS is absent. However, the gap between the performance of the systems with and without TAS becomes insignificant when the RSI is sufficiently high. Consequently, this work can recommend the suitable transmit power corresponding to a specific RSI level and other system requirements to save energy and avoid saturated performance. In addition, the specific effect of high carrier frequencies in MW bands, positions of the drone, numbers of transmit/receive antennas, and RSI levels are deeply investigated to achieve useful insights into the behaviors of the proposed system.

Multi-User Downlink NOMA Systems Aided by Ambient Backscattering: Achievable Rate Regions and Energy-Efficiency Maximization

In this paper, we investigate the energy efficiency of a multi-user downlink non-orthogonal multiple access (NOMA) system aided by an ambient backscatter device that modulates its own information by reflecting the incident signal coming from the NOMA transmitter. Because of the multiplicative operation at the ambient backscatter device when reflecting the transmitter’s signal, the achievable sum rate of the system is not known. Hence, we first derive the information-theoretic achievable rate region for a discrete memoryless channel and, subsequently, for Gaussian channels. We then propose a joint optimization framework for maximizing the system energy efficiency as the tradeoff and ratio between the overall sum rate and the power consumption, under user minimum rate constraints. For this, we propose a modification that simplifies the resulting non-convex optimization problems, which enables us to obtain the optimal reflection coefficient and power allocation policy analytically (up to a one line search). Numerical results demonstrate the negligible impact of the introduced modification on the optimality of our solution. Remarkably, our results show that the ambient backscatter-aided NOMA significantly outperforms OMA: the relative gain increases with the number of receivers, reaching up to 14× improvement over OMA. At last, we show the pertinence of our solution also under imperfect channel state information.

User Pairing and Power Allocation in Untrusted Multiuser NOMA for Internet of Things

In the Internet-of-Things (IoT), massive sensitive and confidential information is transmitted wirelessly, making security a serious concern. This is particularly true when technologies, such as non-orthogonal multiple access (NOMA), are used, making it possible for users to access each other’s data. This paper studies secure communications in multiuser NOMA downlink systems, where each user is potentially an eavesdropper. Resource allocation is formulated to achieve the maximum sum secrecy rate, meanwhile satisfying the users’ data requirements and power constraint. We solve this non-trivial, mixed-integer non-linear programming problem by decomposing it into power allocation with a closed-form solution, and user pairing obtained effectively using linear programming relaxation and barrier algorithm. These subproblems are solved iteratively until convergence, with the convergence rate rigorously analyzed. Simulations demonstrate that our approach outperforms its existing alternatives significantly in the sum secrecy rate and computational complexity.

Spatial Modulation Aided Physical Layer Security for NOMA-VLC Systems

We consider the physical layer security (PLS) problem in multi-user non-orthogonal multiple access (NOMA) enabled multiple-input multiple-output (MIMO) visible light communication systems intercepted by a passive eavesdropper (Eve). We propose a novel transmit precoding scheme based on receive spatial modulation (RSM) to degrade the signal-to-interference-noise ratio (SINR) of Eve by exploiting only the slow-fading characteristics of the visible light channel of the legitimate users (Bobs). The proposed PLS precoder is reinforced with secret parameter exchange with Bobs and a CSI acquisition model is proposed to reduce the PLS algorithm's computational load substantially at the transmitter. The closed-form expressions for the achievable secrecy rates and their upper and lower bounds are derived. Via Monte Carlo simulations, we confirm that Bobs can successfully decode their information in various user configurations while Eve's received SINR is significantly worsened by the jamming signal induced by the proposed precoder with secret key exchange. It is also shown that Eve's bit error rate (BER) is increased to the 0.5-level for almost any position in the considered indoor environment. Finally, we corroborate the derived secrecy expressions by computer simulations and show that the proposed scheme provides PLS for Bobs.

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.

FD-NOMA Enabled Multiuser Transmission With Backscatter Communication

This letter investigates a full-duplex (FD) non-orthogonal multiple access (NOMA) enabled multiuser transmission with backscatter communication (FD-MTBC), where a multiuser downlink NOMA network is served by an FD source while backscatter devices transmit their information to the source simultaneously. Considering the constraints for successive interference cancellation (SIC) and the desired target uplink sum rate, we formulate an optimization problem to maximize the downlink sum rate. Further, we propose an iterative algorithm that jointly optimizes the power allocation coefficient and reflection coefficient to maximize the downlink sum rate. The effectiveness of the proposed algorithm is demonstrated using simulation results.

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.

A novel user clustering and a low-complexity power allocation in multi-user and multi-cluster NOMA system via Stackelberg game competition

Non-orthogonal multiple access (NOMA) has drawn wide attentions in multi-user, resource-constrained mobile systems to improve the system spectral efficiency. In the conventional resource assignment approaches, the optimal power allocation solutions are usually performed among multiple clusters, which, however, incur a high computational complexity due to the diversity of multiple users. In this work, we aim at the practical scenarios of multi-user and multi-cluster NOMA networks with the proposed a low-complexity power allocation and a suboptimal user clustering approach. By using Stackelberg game competition, a closed-form solution with no iterative operations is given first to the power allocation problem. The main idea of the authors’ proposed scheme is to have the base station (BS) allocating resources to all the users freely to guarantee their required minimum transmission-rates, and obtaining the maximal revenue through selling the remaining resources. Then, a suboptimal user clustering approach is further proposed to obtain more revenue for selling the resource. Numerical results show that the proposed power allocation scheme approximates the optimal sum-rates performance, but with lower computational floating-point operations (FLOPs) for the multi-user scene. And, the given user clustering scheme can pursue more revenue of the BS.

Performance Analysis of IOS-Assisted NOMA System With Channel Correlation and Phase Errors

In this paper, we investigate the performance of an intelligent omni-surface (IOS) assisted downlink non-orthogonal multiple access (NOMA) network with phase quantization errors and channel estimation errors, where the channels related to the IOS are spatially correlated. First, upper bounds on the average achievable rates of the two users are derived. Then, channel hardening is shown to occur in the proposed system, based on which we derive approximations of the average achievable rates of the two users. The analytical results illustrate that the proposed upper bound and approximation on the average achievable rates are asymptotically equivalent in the number of elements. Furthermore, it is proved that the asymptotic equivalence also holds for the average achievable rates with correlated and uncorrelated channels. Additionally, we extend the analysis by evaluating the average achievable rates for IOS assisted orthogonal multiple access (OMA) and IOS assisted multi-user NOMA scenarios. Simulation results corroborate the theoretical analysis and demonstrate that: i) low-precision elements with only two-bit phase adjustment can achieve the performance close to the ideal continuous phase shifting scheme; ii) The average achievable rates with correlated channels and uncorrelated channels are asymptotically equivalent in the number of elements; iii) IOS-assisted NOMA does not always perform better than OMA due to the reconfigurability of IOS in different time slots.

Placement and Resource Allocation of Wireless-Powered Multiantenna UAV for Energy-Efficient Multiuser NOMA

This paper investigates a new downlink nonorthogonal multiple access (NOMA) system, where a multiantenna unmanned aerial vehicle (UAV) is powered by wireless power transfer (WPT) and serves as the base station for multiple pairs of ground users (GUs) running NOMA in each pair. An energy efficiency (EE) maximization problem is formulated to jointly optimize the WPT time and the placement for the UAV, and the allocation of the UAV’s transmit power between different NOMA user pairs and within each pair. To efficiently solve this nonconvex problem, we decompose the problem into three subproblems using block coordinate descent. For the subproblem of intra-pair power allocation within each NOMA user pair, we construct a supermodular game with confirmed convergence to a Nash equilibrium. Given the intra-pair power allocation, successive convex approximation is applied to convexify and solve the subproblem of WPT time allocation and inter-pair power allocation between the user pairs. Finally, we solve the subproblem of UAV placement by using the Lagrange multiplier method. Simulations show that our approach can substantially outperform its alternatives that do not use NOMA and WPT techniques or that do not optimize the UAV location.

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.

An SCA and Relaxation Based Energy Efficiency Optimization for Multi-User RIS-Assisted NOMA Networks

Reconfigurable intelligent surface (RIS) assisted non-orthogonal multiple access (NOMA) transmission can effectively improve the energy/spectrum efficiency in wireless networks. This paper designs a low-complexity scheme to achieve the balanced tradeoff between the sum-rate and power consumption in a RIS-assisted NOMA system, which can be measured by energy efficiency. To solve the formulated problem effectively, the original non-convex problem is first decomposed into two subproblems, i.e., beamforming optimization and phase shift optimization. Alternating optimization is proposed to solve these two subproblems iteratively. In particular, successive convex approximation (SCA) is utilized to convert the non-convex constraints to convex ones. The provided simulation results demonstrate that the proposed scheme can achieve superior performance on energy efficiency compared to the random phase shifts and orthogonal multiple access (OMA) schemes.

Adaptive bitrate streaming in multi-user downlink NOMA edge caching systems with imperfect SIC

Recently, mobile networks have witnessed an increasing dominance of video traffic streams to simultaneously distribute real-time contents to massive number of user devices. In this context, fifth-generation (5G) technologies and beyond expose their advanced new radio access interfaces to facilitate multi-user associations with successive interference cancellation (SIC)-based non-orthogonal multiple access (NOMA) mechanisms. However, the imperfectness of the SIC in reality may result in significant performance decrease in NOMA owing to inter-channel interference existence. Motivated by this observation, this paper studies adaptive bitrate streaming services given such a challenging transmission environment assumed. In this context, our objectives are to maximize the video bitrate (accordingly the video resolution) of the online streams while retaining the playback smoothness. The problem is transformed into a drift-plus-penalty balancing optimization, which is then resolved by an approximation algorithm. Numerical results highlight the outperformance of the proposed approach compared to existing algorithms in terms of video quality and smoothness in various system scenarios.

Effective Throughput Maximization of NOMA With Practical Modulations

Non-orthogonal multiple access (NOMA) has been considered as a promising technology for future wireless communications. In most of the existing NOMA schemes, the ideal information rate based on Shannon capacity is used as the performance metric, assuming perfect successive interference cancellation (SIC) and Gaussian transmit signals without considering practical modulations. The implicit assumptions and the resulting schemes may lead to suboptimal performance in practical NOMA systems. In this paper, we consider multi-user multi-channel NOMA systems using practical quadrature amplitude modulation (QAM) with imperfect SIC. We aim to maximize a more practical performance metric, namely the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">effective throughput</i> , which takes into account the data rate and error performance. To achieve this goal, we derive both the exact and approximate expressions of the effective throughput. We also formulate a joint resource optimization problem of the power allocation, channel assignment, and modulation selection to maximize the effective throughput. We develop an efficient power allocation solution by proposing a closed-form power allocation within channels and a waterfilling-form power budget allocation among channels. We also develop efficient channel assignment and modulation selection methods with the aid of matching theory and machine learning, respectively. Consequently, we provide an efficient joint resource allocation algorithm via iterative optimization to maximize the effective throughput. Numerical results are presented to verify the superiority of the proposed NOMA scheme over orthogonal multiple access (OMA) and other NOMA schemes.

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.

Demonstration of Performance Improvement in Multi-User NOMA VLC System Using Joint Transceiver Optimization

The bit error ratio (BER) performance of a non-orthogonal multiple access (NOMA) visible light communication (VLC) system is poor due to the unequal distances between adjacent points in the superposition constellation (SC). In this paper, we propose a novel scheme to improve the BER performance by adjusting parameters to change the shape of SC at the transmitter and by adjusting the parameters of successive interference cancellation (SIC) decoding at the receiver simultaneously, which is called a SC and SIC adjustment (SC-SIC-A) scheme. For multi-user NOMA VLC system, we derive the closed-form BER expression for each user, where the modulation format is four-quadrature amplitude modulation. According to the derived BER expressions, we formulate an optimization problem that minimizes the average BER for all users by adjusting the obtained parameters of SC and SIC decoding via differential evolution algorithm. The improvement of capacity performance is investigated consequently. In order to verify the feasibility and effectiveness of the proposed SC-SIC-A scheme, we carried out theoretical analysis, Monte Carlo simulation and experiments of two-user and three-user NOMA VLC systems. Results show that the SC-SIC-A scheme outperforms the existing schemes in NOMA VLC system, where the signal-to-noise ratio (SNR) reductions to achieve BER of 10−3 are 1.3 dB and 0.8 dB for both users in the two-user NOMA VLC system, respectively, and the SNR reductions to achieve BER of 10−3 are 5.7 dB, 4.3 dB and 4.6 dB for all users in the three-user NOMA VLC system, respectively.

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.

On the Effective Rate of NOMA in Underlay Spectrum Sharing

In this paper, we present the delay-constrained performance analysis of a multi-antenna-assisted multiuser non-orthogonal multiple access (NOMA) based spectrum sharing system over Rayleigh fading channels. We derive analytical expressions for the sum effective rate (ER) for the downlink NOMA system under a peak interference constraint. In particular, we show the effect of the availability of different levels of channel state information (instantaneous and statistical) on the system performance. We also show the effect of different parameters of interest, including the peak tolerable interference power, the delay exponent, the number of antennas and the number of users, on the sum ER of the system under consideration. An excellent agreement between simulation and theoretical results confirms the accuracy of the analysis.