Orthogonal Multiple Access Systems Research Articles

Hybrid Multiple Access Techniques Performance Analysis Of Dynamic Resource Allocation

The Non-Orthogonal Multiple Access (NOMA) and the Orthogonal Frequency Division Multiple Access (OFDMA) are promising techniques for next-generation wireless systems. Although much attention in the literature considered these techniques, an effective combination between such systems as hybrid multiple access (HMA) needs more attention. For this reason, this paper devotes to proposing two different scenarios of HMA using multi-user OFDM (MU-OFDM) and power domain (PD) NOMA for downlink transmission to overcome the limitations of both Orthogonal Multiple Access (OMA) and NOMA. Due to the randomness of a wireless channel, different user grouping strategies and dynamic power allocation (DPA) strategies are employed to satisfy users’ requirements. The proposed systems give high flexibility utilizing bitrate allocation and user fairness. System results show superior performance to traditional OMA and NOMA systems. The achieved variety of fairness is helpful for the diversity of applications which is a principal requirement for beyond-fifth-generation (B5G) networks. The next step in this analysis is to enhance the proposed systems’ spectral efficiency (SE) by introducing beamforming for massive Multiple-Input Multiple-Output (MIMO) systems. The bit error rate (BER) result of the proposed system achieves almost the same error floor with a benefit of approximately 10 dB signalto-noise ratio (SNR) when 90 resource blocks (RBs) are used.

Examination of Multiples Users MIMO-NOMA over Robust Fading Channel Conditions

The purpose of this study is to analyze the performance of the multiple-input multiple-output (MIMO) system in combination with the non-orthogonal multiple access (NOMA) system, taking into consideration the robust fading channel circumstances that are characterized by considerable variations in signal quality. This study examines the susceptibility of the system to various variables that impact channel impairments and the achievable data rate. It is assumed that the fading connections, which are independent and identically distributed (i.i.d.), adhere to Nakagami-m distributions. These links also have successive interference cancellation (SIC) errors, and the channel coefficients exhibit temporal variability. The variables examined in this research include the transmission power of the base station (BS), the presence of imprecise channel state information (CSI), the number of antennas used at the BS, and the power ratio assigned to NOMA users. To assess the efficacy of the system, the conventional time division multiple access (TDMA) approach has been chosen as a means of comparing the performance of the two users inside each beam. The results of the simulations also show that MIMO-NOMA systems have a higher channel capacity than traditional orthogonal multiple access (OMA) systems. This study utilizes a comprehensive dataset to demonstrate the congruence between the analytical findings and the simulation outcomes.

A novel resource allocation scheme for full-duplex NOMA systems with power splitting-based SWIPT

This paper introduces novel resource allocation (RA) algorithms for optimizing wireless network performance, focusing on a full-duplex (FD) base station (BS) employing non-orthogonal multiple access (NOMA) technology. Motivated by the intriguing trade-off introduced by the simultaneous wireless information and power transfer (SWIPT) scheme, in which achievable data rates are sacrificed to enhance harvested energy, this study aims to quantify the delicate equilibrium between two vital aspects meticulously: the overall achievable data rate facilitated by NOMA and FD technologies and the energy harvested via the SWIPT approach. Specifically, our focus is on jointly optimizing the enhancement of both data rate and harvested energy for downlink (DL) users where a multi-objective optimization approach is developed in the proposed RA algorithm to study the existing trade-off between these conflicting objective functions. This problem optimizes the power allocation for both the BS and the users, allocating subcarriers, and optimizing the power splitting factor for SWIPT scheme. We developed efficient iterative solutions for the intended RA problems because they involve complex mixed-integer nonlinear optimization challenges that are generally difficult to solve. The effectiveness of the proposed RA framework is demonstrated using simulation results. These results investigate whether FD MC-NOMA systems, enabled by the proposed RA algorithms, provide a significant increase in system throughput compared to both conventional half-duplex (HD) multicarrier orthogonal multiple access (MC-OMA) systems and HD MC-NOMA systems, as well as other baseline methods. The results also demonstrate that our proposed scheme allows for a greater power harvest than the OMA-FD configuration and other baseline schemes.

Resource Allocation in UAV-Enabled NOMA Networks for Enhanced Six-G Communications Systems

Enhancing energy efficiency, content distribution, latency, and transmission speeds are vital components of communication systems. Multiple access methods hold great promise for boosting these performance indicators. This manuscript evaluates the effectiveness of Non-Orthogonal Multiple Access (NOMA) and Orthogonal Multiple Access (OMA) systems within a single cell, where users are scattered randomly and rely on relays for dependability. This paper presents a model for improving energy efficiency, content distribution, latency, and transmission speeds in communication systems using NOMA and OMA systems within a single cell. Additionally, this paper also proposes a caching strategy using unmanned aerial vehicles (UAVs) as aerial base stations for ground users. These UAVs distribute cached content to minimize the overall latency of content demands from ground users while modifying their positions. We carried out simulations using various cache capacities and user counts linked to their respective UAVs. Furthermore, we evaluated OMA and NOMA in terms of the achievable rate and energy efficiency. The proposed model has achieved noteworthy enhancement across various scenarios including different sum rates, numbers of mobility users, diverse cache sizes, and amounts of power allocation.

An Uplink Cooperative NOMA Based on CDRT With Hardware Impairments and Imperfect CSI

Hardware impairments in wireless transceivers can cause severe performance degradation, particularly for high-rate communication networks. Few studies have explored the impact of these frameworks on recent wireless networks, such as uplink nonorthogonal multiple access (NOMA) with imperfect channel estimation. In this article, we investigate the effect of two practical and malignant imperfections on the performance of coordinated direct and relay transmission (CDRT) using uplink NOMA. Based on these imperfections, we derive an exact closed-form expression for the ergodic sum capacity and outage probability of an uplink NOMA CDRT network. Our model accounts for all the hardware distortions caused by nonideal transceiver nodes as well as residual interference from imperfect channel estimates and imperfect successive interference cancellations. Furthermore, in order to establish a benchmark, we provide analytical expressions and numerical results that demonstrate the superiority of the CDRT-assisted uplink NOMA system with respect to an orthogonal multiple access system. To verify the accuracy of the derived analytical expressions, Monte Carlo simulations are performed.

MCIK-NOMA: A novel index modulation based approach to improve spectral efficiency and error performance over generalized [formula omitted] and [formula omitted] fading channels with imperfect CSI

Non-orthogonal multiple access (NOMA) scheme has emerged as a promising multiple access technology in future wireless communications as it provides massive connectivity than the existing orthogonal multiple access (OMA) methods. Index modulation (IM) permits additional information to be conveyed through the indices of active sub carriers along with the M-ary data symbols thereby increasing the spectral efficiency through selection diversity gain. The union of NOMA and IM called a novel multi-carrier index keying based non-orthogonal multiple access (MCIK-NOMA) system is proposed in this paper and its performance is analyzed over generalized α−μ and k−μ fading environments. The impact of various fading factors such as non-linearity (α), ratio of powers (k) and number of multi-path clusters (μ) on the system performance is investigated and discussed through simulations. Furthermore, the error performance of Rayleigh, Weibull, Gamma, exponential and Nakagami-m fading environment is investigated by changing the fading parameters of generalized fading channels. The impact of perfect, fixed and variable channel state information (CSI) conditions on the proposed system is explored. Simulation results verify the theoretical expressions. The results demonstrate that the proposed MCIK-NOMA system outperforms the existing NOMA and OMA systems in terms of bit error rate (BER), spectral efficiency, and energy efficiency.

Throughput maximization for irregular reconfigurable intelligent surface assisted NOMA systems

AbstractReconfigurable intelligent surface (RIS) is an emerging technology to improve the spectral efficiency of wireless communication systems. However, the high complexity of beam design and the non-negligible overhead associated with RIS limit the number of elements that can be deployed in practice. In this paper, we investigate the downlink communications of irregularly deployed intelligent reflecting surfaces that assist non-orthogonal multiple access (NOMA) systems. To address this challenge, we propose a novel four-step resource allocation algorithm. Specifically, we first obtain a sub-optimal solution for the sparse deployment of RIS elements using a Simulated Annealing Algorithm. We then solve the power allocation problem by employing an integer optimization algorithm that continuously iterates the immobile point. To simplify and optimize the reflection coefficient matrix, we propose a construction inequality algorithm. Finally, we optimize the channel assignment using a genetic algorithm. The simulation results demonstrate that the proposed irregular RIS-assisted NOMA system outperforms the traditional RIS-assisted orthogonal multiple access system, with a maximum throughput increase of approximately 30%.

Energy Efficiency Optimization for SLIPT-Enabled NOMA System

For the upcoming sixth generation (6G) networks, the application of simultaneous lightwave information and power transfer (SLIPT) in a non-orthogonal multiple access (NOMA) system is a potential solution to improve energy efficiency (EE). In this paper, we propose a novel SLIPT-enabled NOMA multi-user system with power splitting (PS) protocol and investigate the effect of system parameters on EE. In addition, to enhance the energy harvesting and information receiving performance of the proposed system, we build up an optimization framework that aims to maximize the EE of the system by jointly optimizing the power allocation of the users and the PS coefficient. We introduce a two-step particle swarm optimization (PSO) algorithm to solve this problem while satisfying the constraints of maximum transmit power, the minimum achievable data rate, and the minimum harvested energy. The numerical results demonstrate the SLIPT-enabled NOMA system using PSO algorithm has significantly improved up to 3.83 ×106 bit/s/J in terms of EE over the traditional orthogonal multiple access (OMA) systems.

Adaptive Clustering of Users in Power Domain NOMA.

By enabling multiple non-orthogonal transmissions, power domain non-orthogonal multiple access (PD-NOMA) potentially increases a system's spectral efficiency. This technique can become an alternative for future generations of wireless communication networks. The efficiency of this method fundamentally depends on two previous processing steps: an appropriate grouping of users (transmission candidates) as a function of the channel gains and the choice of power levels that will be used to transmit each signal. Thus far, the solutions presented in the literature to address the problems of user clustering and power allocation do not consider the dynamics of communication systems, i.e., the temporal variation in the number of users and the channel conditions. In order to consider these dynamic characteristics in the clustering of users in NOMA systems, this work proposes a new clustering technique based on a modification of the DenStream evolutionary algorithm, chosen for its evolutionary capacity, noise robustness and online processing. We evaluated the performance of the proposed clustering technique considering, for simplicity, the use of an already widely known power allocation strategy called improved fractional strategy power allocation (IFSPA). The results show that the proposed clustering technique can follow the system dynamics, clustering all users and favoring the uniformity of the transmission rate between the clusters. Compared to orthogonal multiple access (OMA) systems, the proposed model's gain was approximately 10%, obtained on a challenging communication scenario for NOMA systems since the channel model adopted does not favor a large difference in the channel gains between users.

Outage Performance and Ergodic Capacity of User Pairing in Downlink MIMO-NOMA systems with Imperfect SIC Over Nakagami-m Fading Channels

Non-orthogonal multiple access (NOMA) has been envisaged as a key driving scheme for future wireless communication systems because of its huge connecting ability and superior spectrum efficiency. Integrating the concepts of Multiple-input-multiple-output (MIMO) into NOMA enhance the system capacity at the cost of hardware complexity. In this article, we consider an MIMO-NOMA-based system in which the users are partitioned to form a cluster. A beamforming vector that helps to maximize the channel gain has been designed to remove the inter-cluster interference. This work presents the effect of user pairing in downlink MIMO-NOMA systems. User pairing has a significant impact on the performance of MIMO-NOMA systems. Outage probability and Ergodic Capacity (EC) are analyzed for MIMO-NOMA systems. Exact outage probability expressions are provided for user pairing to reveal some useful insights that pairing two users with dissimilar channel conditions increases the probability that NOMA achieves a larger sum rate than traditional orthogonal multiple access (OMA) systems. This mathematical investigation is carried out by considering Nakagami-m fading channel. Furthermore, this notion of user pairing is presented for imperfect successive interference cancellation (SIC). Diversity order is also derived for the strong and weak users. Extensive simulation results are provided to validate the derived analytic expressions. Furthermore, it is also demonstrated that the EC of MIMO-NOMA systems is improved significantly when compared with traditional OMA systems.

LS-DN Algorithm Based User Matching and Power Minimization in NOMA Disaster Communication

<p>To address the limited and time delay disaster communication, a joint optimization scheme integrates the advantages of differential evolution algorithm (DE) and naked mole- rat algorithm (NMR), and proposes Lévy and sigmoidal DE-NMR, namely LS-DN. LS-DN applies the Lévy flight parameters of adaptive features and sigmoidal selection factor (λ) to the worker of NMR phase, and optimizes the crossover rate (CR) and variation parameter (F) in the DE algorithm, to obtain a balance the exploration and development capabilities. The proposed LS-DN algorithm is used to optimize the user aggregation scheme, since an effect aggregation of disaster victims can reduce power consumption and improve system performance. An value of power external function (Cfn ) is defined for each disaster victim, which is expressed as the system power consumption value for each disaster victim under different aggregation schemes. To minimize the microcell power without deteriorating the quality of service (QoS), it is demonstrated by analyzing the relevant characteristics of non-orthogonal multiple access（NOMA）disaster communication that the power consumption strongly depends on user aggregation method and power allocation. The significance of joint optimization for improving the performance of NOMA disaster communication systems is also emphasized. Simulation results show that LS-DN is able to significantly reduce the power consumption of the system. With the application of LS-DN, the throughput of NOMA system increases by 65% compared to the conventional orthogonal multiple access (OMA) system.</p> <p> </p>

Energy-Efficient Resource Allocation for NOMA HetNets in Millimeter Wave Communications

The integration of millimeter-wave (mmWave) and non-orthogonal multiple access (NOMA) has attracted academic and industrial interest as a promising enabling approach due to their massive bandwidth support and greater spectral efficiency. In this paper, we consider NOMA heterogeneous network (HetNet) in mmWave communications, consisting of the macro-cell and small cell tiers connected through the wireless backhaul. Following this, we propose a user grouping algorithm to significantly simplify the user clustering process where each cluster consists of highly correlated users to suppress the inter-cluster interference. We then propose a hybrid analog/digital precoding scheme at the macro base station. The goal is to maximize the system’s total energy-efficiency (EE) in NOMA HetNet by jointly optimizing the transmission of hybrid precoding, power allocation, and bandwidth partition. Next, we formulate an optimization problem within the proposed framework under the minimum data rate and total maximum transmission power constraints. Due to the non-convexity nature of the formulated problem, it is challenging to obtain the optimal solution. As a result, we transform the original problem into a quasi-convex equivalent by analyzing its structure. Then we propose energy-efficient distributed power allocation algorithms to optimize the resource allocation of macro-cell and small cells. Finally, we exhibit the simulation results to demonstrate the proposed algorithm’s performance gains. The simulation results reveal that the proposed algorithmic approach surpasses state-of-the-art and traditional mmWave orthogonal multiple access systems in both capacity and EE.

Performance Analysis of Ambient Backscatter NOMA Systems

This paper analyzed the performance of an ambient-backscatter-(AmBC)-assisted non-orthogonal multiple access (NOMA) system, where a backscatter device (BD) broadcasts its signal to numerous users. More specifically, the realistic assumptions of imperfect successive interference cancellation (ipSIC) and residual hardware impairments (RHIs) for AmBC–NOMA systems were taken into consideration. We further derived the closed-form and asymptotic expressions of outage probability for the BD and the d-th user. Based on the asymptotic expressions, the diversity orders of the BD and the d-th user were obtained in the high SNR regime. Furthermore, throughput and energy efficiency are further discussed for AmBC-assisted orthogonal multiple access (OMA) systems in the delay-limited transmission model. The numerical results revealed that: (i) AmBC–NOMA systems have the ability to achieve better outage behavior than AmBC–OMA; (ii) due to the existence of the backscatter link, the error floors of outage probability for the BD and the d-th user appear at a high signal-to-noise ratio; (iii) AmBC–NOMA systems are able to achieve higher energy efficiency and throughput than AmBC–OMA systems.

Performance analysis of hybrid half and full duplex user relaying non orthogonal multiple access system with advanced successive interference cancellation

Performance analysis of hybrid half and full duplex user relaying non orthogonal multiple access system with advanced successive interference cancellation

Underwater optical wireless communication system: Deep learning CNN with NOMA-based performance analysis

This research is looking forward improving the performance for underwater optical wireless communication (UOWC) by applying a Non-orthogonal multiple access (NOMA) technique. We also get the benefit of the advantage the transmission based on convolutional neural network hybrid with a long short-term memory cell. The relays selection and power optimization are two main parameters to enhance the UOWC system performance. In this work, we suppose a pairing method for NOMA nodes. By replacing the inner dense connections with convolution layers, this model is proposed to overcome high complexity and over fitting to improve the model performance. The obtained performance for sum rates show that NOMA outperforms the orthogonal multiple access system by ~ 6%. Applying a step-by-step sub-optimization algorithm (SSOPA) yields better results than using fixed power allocation (FPA), while using a global optimal power allocation algorithm (GOPA) increases the sum rates over both FPA and SSOPA. It is found that the improvement when using GOPA combined with CNN approach enhances the performance of sum rates by ~ 2.5% than using the independent-relay-aided NOMA (ICNOMA) for UOWC. The GOPA improvement is 1.2%, 2.5%, 8.7% over FPA and is 0.12%, 0.34%, 2.09% over SSOPA, for clear, pure, and coastal water, respectively. The ICNOMA outperforms both ordinary NOMA (ONOMA) and cooperative NOMA (CNOMA) without independent relay nodes. The ICNOMA achieves an improvement over ONOMA and CNOMA by 20.4% and 3.2%, respectively.

Research on Power Allocation in Multiple-Beam Space Division Access Based on NOMA for Underwater Optical Communication.

To meet the transmission requirements of different users in a multiple-beam access system for underwater optical communication (UWOC), this paper proposes a novel multiple-beam space division multiple access (MB-SDMA) system by utilizing a directional radiation communication beam of the hemispherical LED arrays. The system's access users in the different beams are divided into two categories: the users with a single beam and the users with multiple beams. We also propose a power allocation algorithm that guarantees the quality of service (QoS) for single beam and multiple beam access, especially the QoS for edge users, and fairness for all users. An optimization model of power distribution under the constraints of specific light-emitting diode (LED) emission power is established for two scenarios, which ensure the user QoS for edge users and the max-min fairness for fair users. Using the Karush-Kuhn-Tucker (KKT) condition and the bisection method, we obtain the optimal power allocation expression for the two types of users in the optimization model. Through simulation, we verify that the proposed user classification and power allocation method can ensure the fairness of fair users on the premise of ensuring the QoS of edge users. At the same time, we know that the number of users will affect the improvement of the minimum rate, and the throughput of the non-orthogonal multiple access (NOMA) system is greatly improved compared with the traditional orthogonal multiple access (OMA) systems.

HD/FD Cooperative NOMA under UAV Deployment for a Novel Disaster-Management Model

This work presents a novel system model consisting of an unmanned aerial vehicle (UAV) equipped with a half/full-duplex relay (HDR/FDR) operating as a near-user in the downlink non-orthogonal multiple access (NOMA) systems. In a disaster situation, there is no direct connectivity of the active base station (BS) to the far user due to the out-of-coverage range. Therefore, UAV communication is established to aid the transmission from the same BS to the far user via the UAV. To quantify the effect, outage probability and throughput expressions in the exact and asymptotic forms were developed over the Weibull distribution (WD) fading channel. Additionally, the separation distance of the UAV from the base station is considered to quantify the effect. In particular, this paper helps to determine the optimal location of the UAV deployment from the BS at a fixed height from the ground to either maximize the far-user throughput or attain far-user throughput over different conditions of the WD fading channel. In addition, the performance results of the UAV-HDR/FDR-NOMA system are compared with those of a conventional downlink orthogonal multiple access (OMA) system. The comparison reveals that the UAV-HDR/FDR-NOMA systems outperform corresponding OMA systems in terms of outage probability and throughput over different values of the Weibull shaping index. The analytical results are then validated through numerical simulations on MATLAB.

Joint Design for Simultaneously Transmitting and Reflecting (STAR) RIS Assisted NOMA Systems

Different from traditional reflection-only reconfigurable intelligent surfaces (RISs), simultaneously transmitting and reflecting RISs (STAR-RISs) represent a novel technology, which extends the half-space coverage to full-space coverage by simultaneously transmitting and reflecting incident signals. STAR-RISs provide new degrees-of-freedom (DoF) for manipulating signal propagation. Motivated by the above, a novel STAR-RIS assisted non-orthogonal multiple access (NOMA) (STAR-RIS-NOMA) system is proposed in this paper. Our objective is to maximize the achievable sum rate by jointly optimizing the decoding order, power allocation coefficients, active beamforming, and transmission and reflection beamforming. However, the formulated problem is non-convex with intricately coupled variables. To tackle this challenge, a suboptimal two-layer iterative algorithm is proposed. Specifically, in the inner-layer iteration, for a given decoding order, the power allocation coefficients, active beamforming, transmission and reflection beamforming are optimized alternatingly. For the outer-layer iteration, the decoding order of NOMA users in each cluster is updated with the solutions obtained from the inner-layer iteration. Moreover, an efficient decoding order determination scheme is proposed based on the equivalent-combined channel gains. Simulation results are provided to demonstrate that the proposed STAR-RIS-NOMA system, aided by our proposed algorithm, outperforms conventional RIS-NOMA and RIS assisted orthogonal multiple access (RIS-OMA) systems.

Simultaneously Transmitting and Reflecting Reconfigurable Intelligent Surface Assisted NOMA Networks

Simultaneously transmitting/refracting and reflecting reconfigurable intelligent surface (STAR-RIS) has been introduced to achieve full coverage area. This paper investigate the performance of STAR-RIS assisted non-orthogonal multiple access (NOMA) networks over Rician fading channels, where the incidence signals sent by base station are reflected and transmitted to the nearby user and distant user, respectively. To evaluate the performance of STAR-RIS-NOMA networks, we derive new approximate expressions of outage probability and ergodic rate for a pair of users, in which the imperfect successive interference cancellation (ipSIC) and perfect SIC (pSIC) schemes are taken into consideration. Based on the asymptotic expressions, the diversity orders of the nearby user with ipSIC/pSIC and distant user are achieved carefully.The high signal-to-noise ratio slopes of ergodic rates for nearby user with pSIC and distant user are equal to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$one$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$zero$ </tex-math></inline-formula> , respectively. In addition, the system throughput of STAR-RIS-NOMA is discussed in delay-limited and delay-tolerant modes. Simulation results are provided to verify the accuracy of the theoretical analyses and demonstrate that: 1) The outage probability of STAR-RIS-NOMA outperforms that of STAR-RIS assisted orthogonal multiple access (OMA) and conventional cooperative communication systems; 2) With the increasing of reflecting elements <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> and Rician factor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\kappa $ </tex-math></inline-formula> , the STAR-RIS-NOMA networks are capable of attaining the enhanced performance; and 3) The ergodic rates of STAR-RIS-NOMA are superior to that of STAR-RIS-OMA.

Spectrum Efficient Resource Allocation of NOMA Downlink System With MMSE Receiver

Non-orthogonal multiple access (NOMA) with the effective successive interference cancellation (SIC) is proved as a promising solution to improve the spectrum efficiency (SE). However, since the effective SIC usually requires complex algorithm and leads to higher processing delay, it may not be accepted by the B5G wireless communication due to the limited hardware resource of the mobile terminal and strict real-time request of service. Therefore, whether the NOMA with simple receiver could keep performance advantage directly relate to the promotion and application of NOMA in future. In this paper, we mainly investigate the SE performance of the NOMA downlink system with Minimum Mean Squared Error (MMSE) receiver (MMSE-NOMA) under the imperfect SIC (ipSIC) and CSI (ipCSI) assumption. Based on the analysis of residual interference caused by the ipSIC and ipCSI, the alterative first-order approximation iteration (A-FOAI) algorithm is proposed to optimal the SE performance of MMSE-NOMA. To make the research more objective and comprehensive, we evaluate the SE performance of NOMA with perfect SIC (pSIC) and CSI (pCSI) by adopting the first-order approximation iteration (FOAI) algorithm firstly. Numerical results show that the performance advantage of NOMA heavily depends on the differential degree among various channel gains. Especially, under the ipSIC and ipCSI conditions, the SE of MMSE-NOMA may even be lower than that of the traditional orthogonal multiple access (OMA) system with MMSE receiver when the differential degree among various channel gains is relatively insignificant. This conclusion can help us correctly adopt MMSE receiver to avoid high hardware resource consumption and processing delay in the future NOMA system.