Cooperative Non-orthogonal Multiple Access Scheme Research Articles

Enhancing uplink NOMA cooperative networks with RIS-UAV assistance in Nakagami-[formula omitted] fading channels

Reconfigurable intelligent surfaces (RIS) emerges as a novel communication technology for the 6G networks, boasting advantages such as low cost, high energy efficiency, and wide coverage. To enhance spectrum utilization and ensure signal transmission stability in complex communication scenarios, this paper presents an uplink cooperative non-orthogonal multiple access (NOMA) transmission network assisted by unmanned aerial vehicles (UAV) equipped with RIS. Firstly, a direct link is established between near users and the base station, while a decode-and-forward (DF) relay R and RIS-UAV are deployed between far users and the base station to achieve reliable transmission in a dynamic environment through joint NOMA. Furthermore, an incremental cooperative NOMA scheme is proposed for the uplink transmission, where the relay R remains silent if the RIS-UAV successfully transmits the signal of distant users; if the RIS-UAV fails to transmit the signal to the base station, the relay R is activated to complete the communication between distant users and the base station. Then, the base station employs a selection combining (SC) scheme to process the received signals. Finally, exact expressions of the outage probability under various power allocation strategies are derived. The research results indicate that the outage performance improves with an increase in the number of RIS reflection elements. Moreover, the presence of RIS-UAV can reduce the impact of power allocation strategies on the performance of the uplink NOMA network compared to networks without RIS-UAV.

Beamforming for the Successive Relaying-Based Cooperative Non-Orthogonal Multiple Access Transmission

In this paper, a set of cooperative non-orthogonal multiple access (NOMA) transmission schemes is proposed, in which two half-duplex relaying user terminals (UTs) successively deliver the AP signals toward two NOMA UTs. The successive relaying (SR) of two HD relays emulates the full-duplex (FD) relaying transmission such that the whole transmission of one data block is finished in one time slot. Different from FD relaying, where the removal of self-interference (SI) at the relay is challenging, the SR is suffering from cross-relay interference (CRI) between the two HD relays and the handling of this CRI is the key technical challenge. This CRI suppression is implemented by beamforming with multiple antennas at the relaying UTs, while the beamforming also does the role of maximizing the NOMA transmission efficiency. Two beamforming schemes are proposed, where the first method employs semi-definite relaxation (SDR) with four dimensional (4-D) search. Here, the four dimensions come from the four parameters reflecting the CRI levels at the relaying UTs and signal strengths at the NOMA UTs. This scheme alternates until the 4-D search range is reduced and the sum throughput rate saturates. The second method is computationally simpler than the first one since it avoids the SDR and alternation, and it finds the beamformer set in one shot calculation. Both schemes rely on zero forcing of the CRI. These two schemes along with the HDR relay-based cooperative NOMA scheme are compared in the numerical experiments, and the results exhibit their pros and cons in several aspects.

CoMP-Assisted NOMA and Cooperative NOMA in Indoor VLC Cellular Systems

In this paper, we investigate the dynamic power allocation for a visible light communication (VLC) cellular system consisting of two coordinating attocells, each equipped with one access-point (AP). The coordinated multipoint (CoMP) between the two cells is introduced to assist users experiencing high inter-cell-interference (ICI). Specifically, the coordinated zero-forcing (ZF) precoding is used to cancel the ICI at the users located near the centers of the cells, whereas the joint transmission (JT) is employed to eliminate the ICI at the users located at the edge of both cells and to improve their receptions as well. Furthermore, two multiple access techniques are invoked within each cell, namely, non-orthogonal-multiple-access (NOMA) and cooperative non-orthogonal-multiple-access (C-NOMA). Hence, two multiple access techniques are proposed for the considered multi-user multi-cell system, namely, the CoMP-assisted NOMA scheme and the CoMP-assisted C-NOMA scheme. For each scheme, two power allocation frameworks are formulated each as an optimization problem, where the objective of the former is maximizing the network sum data rate while guaranteeing a certain quality-of-service (QoS) for each user, whereas the goal of the latter is to maximize the minimum data rate among all coexisting users. The formulated optimization problems are not convex, and hence, difficult to be solved directly unless using heuristic methods, which comes at the expense of high computational complexity. To overcome this issue, optimal and low complexity power allocation schemes are derived. In the simulation results, the performance of the proposed CoMP-assisted NOMA and CoMP-assisted C-NOMA schemes are compared with those of the CoMP-assisted orthogonal-multiple-access (OMA) scheme, the C-NOMA scheme and the NOMA scheme, where the superiority of the proposed schemes are demonstrated. Finally, the performance of the proposed schemes and the considered baselines is evaluated while varying various system parameters.

Adaptive Multiuser Cooperative NOMA Scheme With Energy Buffer-Aided Near-Users for High Spectral and Energy Efficiency

In this article, a multiuser cooperative nonorthogonal multiple access (NOMA) network is considered. In order to improve their battery lifetime, the near-user (NU) Internet of Things (IoT) nodes relay information to the far-user (FU) IoT nodes using only the energy harvested from the ambience. The harvested energy at all the NU IoT nodes is stored in energy buffers. To improve the throughput performance, the best NU and best FU (BNBF) user selection scheme is employed. Moreover, unlike conventional orthogonal multiple access (OMA) networks, an adaptive NOMA network is considered in this article that switches between direct NOMA, NOMA with relaying, and OMA modes to maximize throughput. To improve accuracy and reduce the computational complexity, the energy buffer states at the NU IoT nodes are modeled using a continuous state-space Markov chain (CSMC) instead of discrete state-space Markov chain (DSMC). We derive the limiting distributions of the stored energy in energy buffers with either the best effort policy (BEP) or the on–off policy (OOP) applied for buffer energy management. Expressions for throughput are derived for both harvest-store-use (HSU) and harvest-use (HU) architectures. The Monte Carlo simulations are used to validate the derived analytical expressions.

Cooperative NOMA-Based User Pairing for URLLC: A Max–Min Fairness Approach

In this article, cooperative nonorthogonal multiple access (C-NOMA) is considered in short packet communications with finite blocklength codes. The performance of a decode-and-forward relaying along with selection combining (SC) and maximum ratio combining (MRC) strategies at the receiver side is examined. We explore joint user pairing and resource allocation to maximize fair throughput in a downlink scenario. In each pair, the user with a stronger channel (strong user) acts as a relay for the other one (weak user), and optimal power and blocklength are allocated to achieve max–min throughput. To this end, first, only one pair is considered, and optimal resource allocation is explored. Also, a suboptimal algorithm is suggested, which converges to a near-optimal solution. Finally, the problem is extended to a general scenario, and a suboptimal C-NOMA-based user pairing is proposed. Numerical results show that the proposed C-NOMA scheme in both SC and MRC strategies significantly improves the users’ fair throughput compared to the NOMA and orthogonal multiple access (OMA). It is also investigated that the proposed pairing scheme based on C-NOMA outperforms the hybrid NOMA/OMA scheme from the average throughput perspective, while the fairness index degrades slightly.

Performance Analysis of Cooperative Nonorthogonal Multiple Access Scheme in Two-Layer GEO/LEO Satellite Network

Multilayer satellite networks (MLSNs) are viewed as a promising framework of the future satellite communication networks by realizing cooperative communication among satellites in different orbits. In this article, we introduce the cooperative nonorthogonal multiple access (C-NOMA) scheme into a two-layer geostationary Earth orbit/low Earth orbit satellite network in a frequency coexistence scenario and investigate the performance of the considered network. Taking into account the dynamic properties and channel characteristic of the satellite nodes, we first establish the generic architecture and then obtain the equivalent end-to-end signal-to-interference-plus-noise ratios along with the feasible region of power allocation coefficients based on the C-NOMA principle. Furthermore, the analytical expressions of the system ergodic capacity and outage probability are derived and approximated using Meijer-G functions to evaluate the system performance efficiently. Finally, simulation results are provided to attest the validity of the theoretical results as well as show the superiority of the C-NOMA scheme and the impact of various parameters on the system performance.

Energy Efficient Uplink Transmission in Cooperative mmWave NOMA Networks With Wireless Power Transfer

In 5G wireless networks, cooperative non-orthogonal multiple access (NOMA) and wireless power transfer (WPT) are efficient ways to improve the spectral efficiency (SE) and energy efficiency (EE). In this paper, a new cooperative NOMA scheme with WPT is proposed, where EE optimization with a constrained maximum transmit power and minimum required SE is considered for the user grouping and transmit power allocation of users. We obtain a sub-optimal solution by decoupling the original problem in two sub-problems: an iterative algorithm is considered for the user grouping, while, in addition, we utilize the Bat Algorithm (BA) for solving the power allocation problem, where BA was proved to be able to achieve a higher accuracy and efficiency with respect to other meta-heuristic algorithms. Furthermore, to validate the performance of the proposed system, analytical expressions for the energy outage probability and outage probability of users are derived, confirming the effectiveness of the simulation results. It is demonstrated that the proposed cooperative NOMA with WPT offers a considerable improvement in terms of SE and EE of the network compared to other methods. Finally, the effectiveness of BA in solving the EE optimization problem is demonstrated through a high convergence speed by comparing it with other methods.

On the Comparison of Optimal NOMA and OMA in a Paradigm Shift of Emerging Technologies

Non-orthogonal multiple access (NOMA) is a better multiple access technique than orthogonal multiple access (OMA), precisely orthogonal frequency division multiple access (OFDMA) scheme, at the conceptual level for fifth-generation (5G) networks and beyond 5G (B5G) networks. We investigate the potentials of the schemes by comparing the proposed NOMA scheme with the traditional cooperative communication NOMA (CCNOMA) scheme, rather than the comparison between NOMA and OMA only. To probe the effectiveness of NOMA as a multiple access technique, we propose a novel NOMA scheme considering two adjacent BSs with a special design of the transceiver architecture. The proposed scheme provides a reasonable data rate to both near user (NU) and far user (FU) without compromising the quality of service (QoS) to anyone of them. The conclusive analyses on the optimization framework of multi-user sum rate, capacity, transmit power, spectral efficiency (SE), and energy efficiency (EE) trade-off for NOMA and OFDMA schemes have been established to a succession of derivations. Under the analytical optimization framework, we also prove quite a few properties for them. Simulation results confirm the theoretical findings and show that the two schemes can efficiently approach the optimal power allocation, minimization of power consumption, and optimal SE-EE trade-off, and the proposed NOMA scheme provides comparatively better data sum rates than the baseline OMA scheme.

Cooperative Non-Orthogonal Multiple Access Scheme for Cellular Two-Way Relay Networks

Cooperative Non-Orthogonal Multiple Access Scheme for Cellular Two-Way Relay Networks

Improving Physical Layer Security of Cooperative NOMA System with Wireless-Powered Full-Duplex Relaying

Non-orthogonal multiple access (NOMA) and wireless energy harvesting are two promising technologies for improving spectral efficiency and energy efficiency, respectively. In this paper, we study the physical layer security of a wireless-powered full-duplex (FD) relay-aided cooperative NOMA system. In particular, the source is wiretapped by an eavesdropper, and the FD relay assists the transmission from the source to a near user and a far user with self-energy recycling. To enhance the security performance of the system, we propose an artificial noise (AN)-aided cooperative transmission scheme, in which the relay emits a jamming signal to confuse the eavesdropper while receiving the signal from the source. For the proposed scheme, the ergodic secrecy sum rate (ESSR) is derived to characterize the secrecy performance and a lower bound of ESSR is obtained. Finally, numerical results verify the accuracy of the theoretical analysis of the proposed AN-aided secure transmission scheme. The superiority of the proposed scheme is also demonstrated since this scheme can achieve better secrecy performance, compared to the conventional cooperative NOMA scheme.

Optimal Multi-Antenna Transmission for the Cooperative Non-Orthogonal Multiple-Access System

We investigate the beamforming for the multi antenna cooperative non-orthogonal multiple access (NOMA) system, where an access point (AP) delivers messages for multiple user terminals (UT) with successive interference cancellation (SIC) reception method. Some UTs with multiple antennas cooperate with the AP transmission to improve the diversity and the average power performance. We formally present two optimal beamforming schemes at the AP and at the cooperative UTs. One scheme has no power limitation for the cooperative UTs, while the other one does have such limitation. We guarantee that the rank one beamformer is sufficient to achieve the optimal points so that the proposed schemes have rank one semi-definite programming (SDP) structure. Simulation results show the performance gain of the multi-antenna cooperative NOMA schemes in the sense of diversity and the average power.

Security Enhancement With a Hybrid Cooperative NOMA Scheme for MEC System

This article investigates how to exploit the cooperative mechanism between non-orthogonal multiple access (NOMA) user pairs to enhance the security of the mobile-edge computing (MEC) system. Considering the different delay requirements of Internet of Things (IoT) users, we propose a two-slot hybrid cooperative NOMA security (THCNS) scheme that utilizes the cooperative interference between NOMA user pairs to enhance the security of offloading. In the first time slot, the interference to the eavesdropper comes from the task signals (TSs) transmitted by the two users, while in the second time slot comes from jamming signal (JS) transmitted by the user who has completed the offloading tasks. The weighted sum secrecy outage probability (wSOP) and secrecy computation probability (SCP) are utilized to measure the security performance of users in the proposed scheme. To confirm the feasibility and applicability of the proposed scheme, we analyze the impact of some key parameters on users’ security, including the users’ local computation time and offloading time, transmit power, task allocation coefficients of two time slots. The analysis results indicate that the proposed scheme is more suitable for the case where the wiretap channel outperforms the main channel, and it can enhance the system security when selecting the optimal parameters. Finally, the numerical simulation results reveal how the analysis parameters affect the security performance of users and validate the theoretical analysis, it provides good insights about how to design parameters of IoT users to enhance their security performance in MEC system.

Power allocation for D2D aided cooperative NOMA system with imperfect CSI

Non-orthogonal multiple access (NOMA) has become one of the promising technologies for 5G, which can improve the spectrum resource utilization and system throughput along with the support of effective resource allocation algorithms. Most previous works on NOMA-enhanced cooperative relay systems assume perfect channel state information (CSI), which degrades the performance of their schemes severely when used in imperfect CSI environment. In this article, a power allocation algorithm in D2D aided cooperative NOMA communications with imperfect CSI is proposed. In the proposed algorithm, the probabilistic non-convex optimization problem is transformed into a non-probabilistic convex optimization problem by evaluating the channel gains and using the successive convex programming (SCP) which approximately gives the lower bound of the maximum transmission rate. In the SCP enhanced power allocation algorithm (SCPPAA), we iteratively obtain the sub-optimal power allocation coefficients for the optimization problem by Lagrangian dual multiplier method and Karush–Kuhn–Tucker conditions. This program is divided into two layers for updating the power allocation coefficients and the multipliers respectively. Numerical results demonstrate that our algorithm has a fast convergence performance and the algorithm in D2D-based cooperative NOMA scheme has significant sum-data-rate advantages compared with it in traditional ways.

Cancel‐Decode‐Encode Processing on Two‐Way Cooperative NOMA Schemes in Realistic Conditions

This paper considers the effects of perfect/imperfect successive interference cancellation (SIC) and perfect/imperfect ` information (CSI) in a multiple‐relay two‐way cooperative network using nonorthogonal multiple access (NOMA) and digital network coding (DNC). In this model, a relay is selected by maximizing estimated channel gains to enhance the decoding capacity of the nearer source and minimize the collection time of imperfect CSI. Spectrum utilization efficiency is enhanced two times by a mixture of the SIC and DNC techniques at the selected relay (called as the SIC‐2TS protocol). The system performance is considered through analysis of the exact and asymptotic expressions of the system outage probabilities and throughput. The major thing is exposed as the proposed SIC‐2TS protocol can reach the best performance at optimal positions of the selected relay. Besides, the system throughput of the proposed protocol outperforms a SIC‐utilized two‐way relaying scheme without the DNC (called as the SIC‐3TS protocol) and a conventional two‐way scheme (called as the CONV‐4TS protocol) for all signal‐to‐noise ratio regions. Lastly, the validity of the analytical expressions is verified by the Monte Carlo simulation results.

Max Completion Time Optimization for Internet of Things in LEO Satellite-Terrestrial Integrated Networks

In this article, we investigate max completion time optimization for Internet of Things (IoT) in LEO satellite-terrestrial integrated networks (STINs), in which IoT devices use non-orthogonal multiple access (NOMA) scheme to transmit data to central earth stations (CESs), and orthogonal multiple access (OMA) scheme is used for data transmission from CESs to LEO satellite. We decouple this problem into two subproblems: 1) max completion time optimization in terrestrial networks and 2) max completion time optimization among satellite beams. Different from the existing works about NOMA data transmission in terrestrial networks, we propose a cooperative NOMA scheme, and derive the closed expressions of the optimal cooperative data and the optimal transmit power of IoT devices. Based on the closed-form expressions, a joint subcarrier assignment and cooperative NOMA pairing (JSACNP) approach is proposed to minimize the max completion time in terrestrial networks by utilizing matching theory. Then, to minimize the max completion time among satellite beams, the optimal linear receiver expression is derived with fixed transmit power. Convex optimization is utilized to solve transmit power optimization, we propose an algorithm to solve it by CVX tool. An iterative algorithm is proposed for improved performance. Finally, numerical results are provided to evaluate our proposed algorithms, compared with some other proposed approaches or algorithms.

Design and Evaluation of Buffer-Aided Cooperative NOMA With Direct Transmission in IoT

The high spectrum efficiency of nonorthogonal multiple access (NOMA) is attractive to solve the massive number of connections in the Internet of Things (IoT). This article investigates a buffer-aided cooperative NOMA (C-NOMA) system in the IoT, where the intended users are equipped with buffers for cooperation. The direct transmission from the access point to the users and the buffer-aided cooperative transmission between the intended users are coordinated. In particular, a novel buffer-aided C-NOMA scheme is proposed to adaptively select a direct or cooperative transmission mode, based on the instantaneous channel state information and the buffer state. Then, the performance of the proposed scheme, in terms of the system outage probability and average delay, is theoretically derived with closed-form expressions. Furthermore, the full diversity order of three is demonstrated to be achieved for each user pair if the buffer size is not less than three, which is larger than conventional nonbuffer-aided C-NOMA schemes whose diversity order is only two in the considered C-NOMA system in the IoT.

Power Allocation in CoMP-Empowered C-NOMA Networks

In this letter, the dynamic power allocation problem of a cellular network consisting of two adjacent and coordinating cells is investigated. The joint transmission coordinated multipoint (JT-CoMP) between the two-cell is introduced to assist users experiencing high inter-cell interference, where each cell invokes cooperative non orthogonal multiple access (C-NOMA) to serve its associated devices. Both effects of imperfect successive interference cancellation (SIC) and imperfect channel estimation are considered within the proposed scheme. A power allocation framework is formulated as an optimization problem with the objective of maximizing the network sum-rate while guaranteeing a certain quality-of-service (QoS) for each user. The formulated optimization problem is neither concave nor quasi-concave, which is difficult to be solved directly unless using heuristic methods, which comes with the expense of high computational complexity. To overcome this issue, a near-optimal closed-form expressions for the power allocation are derived. The simulation results show that our purposed scheme achieves an average sum-rate that is 3% less than the one of the optimal power control but it can save up to 99% in the computational time. In addition, the superiority of the proposed CoMP C-NOMA scheme is demonstrated when compared to the well known C-NOMA scheme.

Deep Multi-Task Learning for Cooperative NOMA: System Design and Principles

Envisioned as a promising component of the future wireless Internet-of-Things (IoT) networks, the non-orthogonal multiple access (NOMA) technique can support massive connectivity with a significantly increased spectral efficiency. Cooperative NOMA is able to further improve the communication reliability of users under poor channel conditions. However, the conventional system design suffers from several inherent limitations and is not optimized from the bit error rate (BER) perspective. In this article, we develop a novel deep cooperative NOMA scheme, drawing upon the recent advances in deep learning (DL). We develop a novel hybrid-cascaded deep neural network (DNN) architecture such that the entire system can be optimized in a holistic manner. On this basis, we construct multiple loss functions to quantify the BER performance and propose a novel multi-task oriented two-stage training method to solve the end-to-end training problem in a self-supervised manner. The learning mechanism of each DNN module is then analyzed based on information theory, offering insights into the explainable DNN architecture and its corresponding training method. We also adapt the proposed scheme to handle the power allocation (PA) mismatch between training and inference and incorporate it with channel coding to combat signal deterioration. Simulation results verify its advantages over orthogonal multiple access (OMA) and the conventional cooperative NOMA scheme in various scenarios.

Customized NOMA and Sector Model for Battery Efficient Beyond 5G Green Networks

Recent developments in technologies like the internet of Things (ioT) and massive machine-type communications (mMTC), the number of wireless communication devices, the demand for spectrum resources, the power consumed in transmission, processing, and reception will proliferate. Non-orthogonal multiple access (NOMA) and cooperative schemes are attracting researchers across the globe to use NOMA to solve the problem of spectrum scarcity by serving multiple users simultaneously, whereas cooperative schemes can help in reducing transmission power consumption by employing relay nodes in between. Therefore, to condense the system's power consumption, this article integrates NOMA and cooperative technology. This article proposes two schemes: a battery efficient cooperative NOMA scheme and a customized sector model. The introduced battery efficient cooperative NOMA takes advantage of relay and device-to-device (D2D) cooperation for introducing different modes of operation in the NOMA scheme, aiming to reduce the battery power consumption at the transmitter and the receiver side, whereas the customized sector model intends to optimize the energy of the system through efficient deployment of the NOMA cell-sector model. Finally, the discussion of various challenges and future opportunities for unveiling the NOMA technique is presented.

Cooperative NOMA for Downlink Asymmetric Interference Cancellation

This letter advances the non-orthogonal multiple access (NOMA) technique for cellular downlink co-channel interference mitigation, via exploiting the (limited) cooperation among base stations (BSs). Specifically, we consider a simplified but practically relevant scenario of two co-channel cells with asymmetric interference, i.e., only the user in one cell receives the strong interference from the BS in the other cell. To mitigate such interference, we propose a new cooperative NOMA scheme, where the interfered user's serving BS sends a superposed signal comprising both the desired message and the co-channel user's message (shared by the interfering BS). The co-channel user's signal is aimed to add constructively with the interfering BS's signal at the interfered user's receiver so that the combined interference with enhanced power can be effectively decoded and cancelled. This thus leads to a new problem on how to optimally allocate the transmit power for the two superposed messages. We provide the closed-form solution to this problem and investigate the conditions under which the performance of the proposed scheme is superior over the existing schemes.