Multiple Access Downlink Research Articles

QAMA: Hierarchical QAM Based Downlink Multiple Access With a Simple Receiver

QAMA: Hierarchical QAM Based Downlink Multiple Access With a Simple Receiver

Resource allocation for practical phase shift STAR-RIS enabled MISO-NOMA wireless network with non ideal hardware

This study investigates the sum secrecy rate SRsum of a simultaneous transmission and reflection reconfigurable intelligent surface (STAR-RIS)-enabled multiple input single-output (MISO) non-orthogonal multiple access (NOMA) downlink network with non-ideal hardware defects (HWDs) at the transceivers. Earlier STAR-RIS studies assumed an ideal phase shift model, with each element reflect and transmit the signal without regard to its phase shift. The proposed practical phase shift model accounts for the phase-dependent amplitude variation in the reflection and transmission (RAT) coefficients. The goal is to maximize the SRsum through an alternating optimization (AO) iterative algorithm, leverages a classical semidefinite relaxation (SDR) for beamformer vector design (BVD), and employs an interior point method (IPM) to calculate NOMA power allocation factors αt,αr. The analysis focuses on the evaluation of the SRsum in scenarios with eavesdroppers, under the consideration of both ideal and practical phase shift cases. The study examines the impact of different non-ideal HWDs on the SRsum of the system and provides insights into vulnerabilities and limitations arising from these effects.

A new subcarrier‐index modulation schemes for downlink NOMA systems

SummaryIn this paper, we propose two downlink multiple access architectures for networks where human‐type communication users (HTCUs) and machine‐type communication devices (MTCDs) coexist. The proposed schemes combine non‐orthogonal multiple access (NOMA), orthogonal frequency division multiplexing (OFDM), and index modulation (OFDM‐IM) concepts. In the first scheme, the base station (BS) transmits bits of HTCUs using modulated symbols and bits of MTCDs by the subcarrier activation pattern (SAP). This approach called IM‐NOMA with null subcarriers (IM‐NOMA‐NS) ensures that inactive subcarriers are always null, which improves the system bit error rate (BER) performance. To improve the spectral efficiency (SE), we propose a second approach, termed IM‐NOMA with dual‐mode modulation (IM‐NOMA‐DM), in which the HTCUs' bits are transmitted using two‐dimensional modulation and the MTCDs' bits are transmitted using one‐dimensional modulation and the SAP. For each proposed system, a near‐optimal low‐complexity detector, based on the energy‐detection (ED) and the log‐likelihood ratio (LLR) criterion, is provided to mitigate the detection burden of the optimal maximum‐likelihood (ML) detector. The BER performances and SE of the proposed schemes are investigated. The average BERs of IM‐NOMA‐NS and IM‐NOMA‐DM are derived in closed‐form expressions corroborated by the simulation results. We have proved numerically that the proposed schemes achieve a good trade‐off between BER performance, SE, and the number of supported users, making them more suitable for Internet of Things (IoT) applications.

Optimisation of energy efficiency of ambient backscatter communication and reconfigurable intelligent surfaces in non‐orthogonal multiple access downlink

AbstractThe authors study the energy efficiency (EE) of ambient backscatter communication (AmBC) device‐assisted and reconfigurable intelligent surfaces (RIS)‐assisted non‐orthogonal multiple access (NOMA) downlinks. The authors establish two optimisation problems based on the two collaborative devices (AmBC devices, RIS) with the objective of maximising the EE of the system, taking into account the requirements of power limitation and rate limitation, etc. and also obtain the solutions of two problems by optimising the relevant performance metrics based on the alternating optimisation algorithm. For the backscatter device (BD)‐aided downlink NOMA network, the problem is first decoupled into three subproblems, where the power allocation optimisation subproblem is solved by using the quadratic transformation method and the subgradient algorithm. The maximum EE is obtained by iterating according to the Dinkelbach's algorithm. For the RIS‐aided downlink NOMA network, the power allocation problem is solved by the same method as above and the phase optimisation problem is solved by the successive convex approximation method. Numerical results show that the proposed algorithm can achieve convergence after several iterations, and the EE of systems with BD‐assisted and RIS‐assisted have different levels of sensitivity to different influencing factors.

Enhancing NOMA Network Security with RIS-UAV Integration: Exploring PPO Technique

In order to improve the security aspects of a nonorthogonal multiple access (NOMA) downlink network, we investigate the use of the proximal policy optimization (PPO) technique in this study. This network is uniquely augmented with reconfi- gurable intelligent surfaces (RIS) mounted on unmanned aerial vehicles (UAVs). The main objective of this article is to prevent possible eavesdroppers from accessing the network while still assuring continuous and secure connectivity for legitimate users. The network configuration includes a UAV outfitted with a RIS, which plays an important role in signal propagation. This work aims to increase the secrecy rates of all user communications in eavesdropping prone locations. By adaptively modifying the phase shifts and power distributions through the RIS the proposed PPO algorithm not only shows adaptability and effectiveness in securing wireless communications, but also highlights the significant advances made in communication system security through this technology.

Multi-UAV and IRS placement for secure data transmission in NOMA-enabled wireless networks

In this paper, we investigate multi-unmanned aerial vehicle (UAV)-enabled intelligent reflecting surface (IRS)-assisted non-orthogonal multiple access (NOMA) downlink networks for secure data transmission in the presence of an intruder. Since Line of Sight (LoS) communication is obstructed, UAVs are utilized as aerial base stations to transmit confidential data to ground users with the assistance of IRS. The objective is to optimize the placement of UAVs and IRS, phase shift of the IRS, and transmit beamforming of the UAVs in order to maximize the network secrecy rate. The formulated optimization problem is non-convex in nature, posing a challenge in finding the optimal solution using conventional techniques. To tackle this problem, we propose a novel algorithm called balanced grey wolf optimizer and differential evolution (B-GWODE). This algorithm not only leverages the advantages of both the GWO and DE algorithms but also incorporates parameter modifications to mitigate the issue of premature convergence and to strike a suitable balance between exploration and exploitation. This hybrid approach efficiently search the solution space and obtain the optimal configuration that maximizes the network secrecy rate. Simulation results demonstrate the superiority of our proposed B-GWODE algorithm over the benchmark methods.

On performance analysis of non-orthogonal multiple access downlink for cellular-connected unmanned aerial vehicle relaying assisted vehicle-to-everything system

This paper presents the unmanned aerial vehicle (UAV) relays’ assisted vehicle-to-everything (V2X) network to implement the internet of things (IoT) systems with improvement in the coverage area. Such a network benefits from many advantages of the non-orthogonal multiple access (NOMA) scheme. We have implemented a decode-and-forward (DF) scheme for these UAVs. Then, we characterize the channels as Nakagami-m fading to evaluate the performance of the system. We derive closed-form expressions of outage probability (OP), ergodic capacity (EC), and throughput. The results show that the performance of the system depends on the transmitted signal-to-noise ratio (SNR) at the base station and the heights of the UAV relays. Target rate and power allocation factors are two main parameters that can be adjusted to achieve better performance. The results also compare to the system without UAV and OMA technique that shows the advantages of deploying UAV-assisted NOMA. Therefore, the design of NOMA for UAV relay-assisted V2X systems provides sufficient demand. The simulation results verified the effectiveness of the proposed UAV network and the precision of the theoretical analysis.

An ADO‐OFDM with integrated precoding, companding and optimized power allocation methods for high‐speed data transmission in visible light communication

AbstractVisible light communications (VLC) has received a lot of attention in recent studies because of its benefits over radio‐frequency (RF) communications. It is a short‐range optical wireless communication system that uses light‐emitting diodes (LEDs) as transmitters. Optical orthogonal frequency division multiplexing (OFDM) is an auspicious technology for VLC high‐speed data transfer. The use of OFDM in a VLC system raises the system's peak‐to‐average power ratio (PAPR). The intrinsic non‐linearity of LED is a key concern in an asymmetrically clipped DC‐biased optical OFDM (ADO‐OFDM) system due to its high PAPR. Also, conventional ADO‐OFDM modulation scheme cannot be used for accommodating different demands of services in downlink multiple access due to the restriction of direct current biased optical OFDM (DCO‐OFDM) and asymmetrically clipped optical (ACO‐OFDM) transmission on odd and even subcarriers. To tackle this issue, a modified ADO‐OFDM (MADO‐OFDM) is proposed that adjusts the numbers of subcarriers required for the transmission of ACO‐OFDM and DCO‐OFDM adaptively based on the requests of services in downlink multiple access. Also, the proposed MADO‐OFDM is integrated with a discrete Hartley Matrix transform (DisHMT) precoder and Generalized Piecewise Linear Compander (GPLD) to provide high‐speed data transmission with less PAPR. In addition, the power is allocated to the proposed MADO‐OFDM system optimally by maximizing the channel capacity based on the Aquila optimizer algorithm. The simulation results reveal that the suggested system's PAPR is reduced by 2.4 dB and 0.8 dB, respectively, compared to conventional ADO and hybrid ADO‐OFDM. It also confirms that the suggested generalized PLC can greatly increase BER without affecting the PAPR performance.

Best sum-throughput evaluation of cooperative downlink transmission nonorthogonal multiple access system

In cooperative simultaneous wireless information and power transfer (SWIPT) nonorthogonal multiple access (NOMA) downlink situations, the current research investigates the total throughput of users in center and edge of cell. We focus on creating ways to solve these problems because the fair transmission rate of users located in cell edge and outage performance are significant hurdles at NOMA schemes. To enhance the functionality of cell-edge users, we examine a two-user NOMA scheme whereby the cell-center user functions as a SWIPT relay using power splitting (PS) with a multiple-input single-output. We calculated the probability of an outage for both center and edge cell users, using closed-form approximation formulas and evaluate the system efficacy. The usability of cell edge users is maximized by downlink transmission NOMA (CDT-NOMA) employing a SWIPT relay that employs PS. The suggested approach calculates the ideal value of the PS coefficient to optimize the sum throughput. Compared to the noncooperative and single-input single-output NOMA systems, the best SWIPT-NOMA system provides the cell-edge user with a significant throughput gain. Applying SWIPT-based relaying transmission has no impact on the framework’s overall throughput.

Performance of STAR-RIS-aided cooperative NOMA networks under Nakagami-[formula omitted] fading

In this work, we investigate the performance of the cooperative non-orthogonal multiple access (CNOMA) downlink wireless communication system assisted by the newly emerged technology of simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) over Nakagami-m fading channels. Specifically, outage probability (OP), spectral efficiency (SE), and energy efficiency (EE) are used as the performance metrics. The near user’s mathematically intractable probability density and cumulative distribution functions were approximated through the Gamma moment-matching technique. This was followed by the derivation of the users’ closed-form OP and ergodic rates expressions, where the latter was used to determine the SE and EE of the system. Further, extensive Monte-Carlo simulations confirmed the correctness of the obtained analytical findings. In addition, to inspect the usefulness of CNOMA in the proposed communication system, the comparison with its NOMA counterpart was incorporated into the numerical results. Finally, it was found that the cooperative link can further enhance the SE and EE of the STAR-RIS-based NOMA network.

Hybrid Time-Switching and Power-Splitting EH Relaying for RIS-NOMA Downlink

Reconfigurable intelligent surface (RIS) offers a potential performance boost for next-generation wireless networks, with significant improvements in spectral and energy efficiencies. In this paper, considering a two-user multiple-input single-output non-orthogonal multiple access (MISO-NOMA) downlink, we propose a RIS-aided cooperative transmission scheme using hybrid simultaneous wireless information and power transfer (SWIPT) and transmit antenna selection (TAS) protocols. Particularly, in the proposed scheme, the cell-center user servering as a relay helps the cell-edge user with the support of hybrid time-switching (TS), power-splitting (PS) SWIPT protocols and RIS passive beamforming. To evaluate the network performance, we first derive the best- and worst-case of closed-form expressions in terms of outage probability. For gleaning further insights, we also investigate the diversity order and the high signal-to-noise ratio (SNR) slope for the relevant outage probability. Simulation and numerical results demonstrate the achievable performance improvement for our proposed scheme with respect to schemes of orthogonal multiple access (OMA), non-cooperative NOMA and cooperative NOMA without RIS. In addition, the proposed scheme achieves a higher diversity gain with more transmit antennas and RIS elements.

Partial Non-Orthogonal Multiple Access: A New Perspective for RIS-Aided Downlink

In this letter, a novel reconfigurable intelligent surface (RIS) aided partial non-orthogonal multiple access (P-NOMA) downlink is investigated, where the passive beamforming on the RIS is designed for serving paired user equipments (UEs), with their signal frequencies partially overlapped. We consider the general case that all UEs have both direct and reflection links undergoing Rayleigh and Nakagami- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${m}$ </tex-math></inline-formula> fadings. By approximating the effective channel gains of the P-NOMA UEs as Gamma random variables, we derive the closed-form expressions of the lower and upper bounds on the achievable rates. The accuracy of the derived close-form expressions are verified through simulation and numerical results, where the proposed RIS-aided P-NOMA downlink considerably outperforms the conventional NOMA and orthogonal counterparts.

Latency-Critical Downlink Multiple Access: A Hybrid Approach and Reliability Maximization

In this work, we study a downlink multi-user network, where a single access point (AP) is supposed to accomplish data transmissions to all users under low latency constraints. To more effectively cope with the multiple access demand, we consider a hybrid strategy for the multi-user downlink service in finite blocklength (FBL) regime, which combines broadcasting with time-division multiple access (TDMA). In the hybrid strategy, the users are first clustered into different groups. Different groups are served in a TDMA manner with dedicated time slots, while users within each group are served together via a broadcasting signal from the AP. By taking into account the fairness of transmission reliability among all users, we formulate a problem minimizing the maximum error probability among users via jointly determining the user grouping and allocating blocklength among all groups. To address the complicated non-convex problem, we first characterize the optimal blocklength allocation under each given grouping decision, which leads to an optimal closed-form allocation solution via solving an equation system. Based on the characterized features, we are enabled to efficiently distill out the optimal grouping from all possible groupings, which forms the efficient optimal solution for the optimal joint design. Afterwards, aiming at a complexity reduction, we further propose a low-complexity iterative solution, in which the grouping is iteratively improved via the introduced operations until a convergence to a suboptimum. Finally, via simulations, we validate the proposed solutions and reveal the close optimality of the iterative solution. In addition, the hybrid strategy has shown a significant reliability advantage in comparison to pure broadcasting or TDMA, and this performance advantage becomes further enlarged in case of more users.

Non-Orthogonal Multiple Access Technique Performance Characterization in Downlink Network

In this paper, a Non Orthogonal Multiple Access (NOMA) downlink multiuser communication system is, composed of a single base station (BS) over a wireless channel to M users equipped with single antenna is considered. The BS is located at the center of a disc denoted by D with radius R. A Gaussian-Chebyshev quadrature approximation of the cumulative density function, of unordered channel gain is applied in the case of two users independent and identically distributed, located in a disk of radius R, under two techniques OMA and NOMA, in a downlink network, to evaluate ergodic capacities of NOMA and OMA users at the bounds of the fairness region of NOMA are calculated, and from a comparison between NOMA and OMA ergodic capacities as functions of SNR ratio, one can observe that NOMA always outperforms OMA technique. In this system model, NOMA users are sorted according to their locations from the BS, so that an mth user is able to detect its own signal, after performing Successive Interference Cancellation (SIC) algorithm based on an iterative cancellation of the ith user information. In the case of the failure of mth user to detect the ith user signal, mth user meets outage probability. This outage probability under an increasing SNR ratio of two NOMA users is evaluated for different values of N parameter, disk radius R, path loss exponent and power allocation coefficients. One can note that, NOMA users achieve more outage probability with increasing values of disk radius R, and the path loss exponent tends to increase, as there are more obstructions, and NOMA users tend to achieve more outage probability.

Deep Reinforcement Learning-Based Optimization for RIS-Based UAV-NOMA Downlink Networks (Invited Paper)

This study investigates the application of deep deterministic policy gradient (DDPG) to reconfigurable intelligent surface (RIS)-based unmanned aerial vehicles (UAV)-assisted non-orthogonal multiple access (NOMA) downlink networks. The deployment of UAV equipped with a RIS is important, as the UAV increases the flexibility of the RIS significantly, especially for the case of users who have no line-of-sight (LoS) path to the base station (BS). Therefore, the aim of this study is to maximize the sum-rate by jointly optimizing the power allocation of the BS, the phase shifting of the RIS, and the horizontal position of the UAV. The formulated problem is non-convex, the DDPG algorithm is utilized to solve it. The computer simulation results are provided to show the superior performance of the proposed DDPG-based algorithm.

Outage probability computation in multi-backscatter systems with multi-modes of operation

In this article, we study the outage performance of an ambient multi-backscatter system supported by non-orthogonal multiple access (NOMA) downlink transmission, where legacy users receive information either directly from the base station (BS) or from multiple backscatter devices (BDs) as well. Specifically, we characterize the network connections between the BS and network users into three modes of operation. In the first mode, the BS communicates with the users via the BDs by exploiting NOMA. In the second mode, the BS communicates directly with the users by employing selection combining (SC) beamforming. In the third mode, the users receive signals from both the BS and BDs simultaneously, by using maximal ratio combining (MRC) to combine the BS and BDs links at the users. Consequently, we derive the exact outage expressions of each mode and utilize Monte Carlo simulations to validate the individual exact expressions.

Deep Reinforcement Learning Based Beam Selection for Hybrid Beamforming and User Grouping in Massive MIMO-NOMA System

This paper presents a deep reinforcement learning-based beam-user selection and hybrid beamforming design for the multiuser massive multiple-input multiple-output (MIMO) non-orthogonal multiple access (NOMA) downlink systems. The conventional hybrid beamforming in massive MIMO provides multiple directional beams, but each beam serves only one user. The integration of NOMA with the massive MIMO enables power domain multiplexing within a beam, hence increasing the system capacity. In this paper, we first design a channel gain and correlation-based users grouping algorithm per beam, and then using the deep reinforcement learning-based beam selection, a beamspace orthogonal analog precoder is obtained. The deep Q-network consists of a main network and target network with Adam optimizer. Finally, optimal power is allocated to the users in each beam. Simulation results show that at transmit SNR of 10 dB, the proposed scheme provides a 42% increase in sum-rate and energy efficiency performance as compared to the state-of-the-art <i>K</i>-means users&#x2019; grouping and Stable Matching-based beam selection NOMA scheme.

Machine Learning for User Partitioning and Phase Shifters Design in RIS-Aided NOMA Networks

A novel reconfigurable intelligent surface (RIS) aided non-orthogonal multiple access (NOMA) downlink transmission framework is proposed. We formulate a long-term stochastic optimization problem that involves a joint optimization of NOMA user partitioning and RIS phase shifting, aiming at maximizing the sum data rate of the mobile users (MUs) in NOMA downlink networks. To solve the challenging joint optimization problem, we invoke a modified object migration automation (MOMA) algorithm to partition the users into equal-size clusters. To optimize the RIS phase shifting matrix, we propose a deep deterministic policy gradient (DDPG) algorithm to collaboratively control multiple reflecting elements (REs) of the RIS. Different from conventional training-then-testing processing, we consider a long-term self-adjusting learning model where the intelligent agent is capable of learning the optimal action for every given state through exploration and exploitation. Extensive numerical results demonstrate that: 1) The proposed RIS-aided NOMA downlink framework achieves enhanced sum data rate compared with the conventional orthogonal multiple access (OMA) framework. 2) The proposed DDPG algorithm is capable of learning a dynamic resource allocation policy in a long-term manner. 3) The performance of the proposed RIS-aided NOMA framework can be improved by increasing the granularity of the RIS phase shifts. The numerical results also show that increasing the number of reflecting elements (REs) is an efficient method to improve the sum data rate of the MUs.

Backscatter Cooperation in NOMA Communications Systems

In this paper, a backscatter cooperation (BC) scheme is proposed for non-orthogonal multiple access (NOMA) downlink transmission. The key idea is to backscatter the surplus power of the received downlink signals at one user to enhance the reception of the user who cannot recover its information. To evaluate the performance of the proposed BC-NOMA scheme, three benchmark schemes are introduced, which include the non-cooperation (NC)-NOMA scheme, the conventional relaying (CR)-NOMA scheme, and the incremental relaying (IR)-NOMA scheme. For all these schemes, the outage performance, the expected rate, and the diversity-multiplexing trade-off (DMT) performance are analyzed. Additionally, the optimal power allocation to maximize the expected rate of the BC-NOMA scheme in the high signal-to-noise ratio region is derived. The theoretical results show that the proposed BC-NOMA can enhance both transmission reliability (outage performance) and transmission effectiveness (expected rate) compared with NC-NOMA. Furthermore, representative numerical results are presented to validate the theoretical results. It is shown that unlike the two relaying-based cooperative schemes (CR/IR-NOMA), which improve the outage performance at the cost of transmission rate, the proposed BC-NOMA scheme can enhance the transmission reliability without impairing the spectrum efficiency, which makes backscattering an appealing solution to cooperative NOMA downlinks.

Joint Resource Allocation and Trajectory Optimization With QoS in UAV-Based NOMA Wireless Networks

Replacing base stations with unmanned aerial vehicles (UAVs) to serve the communication of ground users has attracted a lot of attention recently. In this paper, we study the joint resource allocation and UAV trajectory optimization for maximizing the total energy efficiency in UAV-based non-orthogonal multiple access (NOMA) downlink wireless networks with the quality of service (QoS) requirements. To handle the user scheduling problem, a heuristic algorithm based on matching and swapping theory is proposed first to allocate users that access UAV in each subperiod, then the transmit power allocation problem which considers the maximum transmit power and minimum user date rate is transformed to a convex optimization problem using logarithmic approximation. Meanwhile, the successive convex optimization is used in UAV trajectory optimization problem and a joint optimization algorithm is presented with the algorithm’s convergence and computational complexity. Finally, numerical results are provided to support the rationality of the proposed algorithm.