mmWave Non-orthogonal Multiple Access Research Articles

Secure wireless communications for STAR‐RIS‐assisted millimetre‐wave NOMA uplink networks

AbstractThis paper explores the potential application of simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR‐RIS) in improving the physical layer security (PLS) of millimetre wave (mmWave) non‐orthogonal multiple access (NOMA) uplink communications. In particular, the legitimate users on both sides of STAR‐RIS send confidential information simultaneously to the base station (BS) while keeping it secret from the eavesdroppers near the BS by exploiting the STAR‐RIS to proactively regulate the electromagnetic propagation environment. By jointly designing the transmit power, the active beamforming at the BS, and the reflecting/transmitting coefficients at the STAR‐RIS, the authors' goal is to maximize the minimum secrecy capacity subject to the successive interference cancellation decoding order constraints. Due to the non‐convexity of the formulated problem, an efficient algorithm is proposed by capitalizing on alternating optimization, a penalty‐based approach, successive convex approximation, and semi‐definite relaxation. Firstly, a two‐layer iterative algorithm based on the penalty for the joint beamforming optimization sub‐problem is proposed. Then, the non‐convex problem is transformed into a convex positive semi‐definite programming problem for the transmit power sub‐problem. Numerical simulation results reveal that the STAR‐RIS‐aided system has more tremendous advantages and effectiveness in improving the PLS of mmWave NOMA uplink communication compared with the benchmark schemes.

Location-aided user selection and sum-rate analysis for mmWave NOMA

In this paper, we propose a user selection scheme based on location-aided interference prediction to reduce the training overhead in a non-orthogonal multiple access (NOMA) system. First, we cluster the users based on their location information, enabling the use of non-orthogonal pilot sequence within a cluster and orthogonal pilot sequence between different clusters to reduce the uplink pilot training length. Secondly, we exploit the location information in the computation of the covariance matrices, enabling the prediction of the interference between users. The predicted interference is employed to select the set of users with minimum interference for uplink channel estimation and downlink NOMA data transmission. Finally, the achievable sum-rate of the massive multiple-input multiple-output millimeter wave NOMA system is analyzed. The analytical and numerical results reveal that the location information can be exploited for user selection to reduce the effect of pilot contamination, enhancing the uplink channel estimation and downlink achievable sum-rate.

Angle-Domain Hybrid Beamforming-Based mmWave Massive MIMO-NOMA Systems

The millimeter-wave (mmWave) large-scale antenna arrays (LSAAs) systems play a vital role in increasing the beamforming (BF) gain and acquiring highly directional propagation. Recently, non-orthogonal multiple access (NOMA) has been integrated into these systems to manage massive connectivity and achieve spectral-efficient communications. This paper focuses on angle-domain (AD) hybrid beamforming (BF) for mmWave LSAAs and NOMA systems, thanks to the low complexity, power consumption, and channel estimation overhead. However, with limited radio-frequency chains, the hybrid BF-based single-beam (SB)-NOMA scheme generating a single beam to serve the NOMA users fails to exploit the multi-user diversity due to narrow beams with LSAAs. To tackle this limitation, we design schemes offering additional degrees of freedom. More importantly, they require only the knowledge of angular information and are suitable for either linear or rectangular antenna arrays, unlike those proposed in the literature. The first scheme exploits the time-domain resources to schedule groups having high spatial interference within distinct time slots. To minimize the need for fast and precise synchronization when applying time division multiple access (TDMA) with mmWave NOMA, we leverage the multi-beam (MB)-NOMA framework. And we propose a joint SB-and MB-NOMA scheme to benefit from NOMA multi-user diversity, whatever the cell load and the users’ positions. Using the New York University channel simulator (NYUSIM), we further validate the performance of the proposed schemes compared to the solution proposed in the literature and others using fully digital BF. Specifically, the proposed TDMA-based scheme achieves a sum-rate gain of up to 83% over the TDMA-based one existing in the literature. Moreover, we verify the superiority of applying both SB-and MB-NOMA instead of only MB-NOMA.

Genetic Algorithm for Optimizing Energy Efficiency in Downlink mmWave NOMA System with Imperfect CSI

Nonorthogonal multiple access (NOMA) is considered a promising technique for improving energy efficiency (EE) in beyond-5G wireless systems. In this paper, we investigate the maximization of EE of downlink wireless systems by combining mmWave with NOMA technologies, considering the asymmetric required data rate of user applications. We propose a genetic algorithm (GA) to solve the non-convex energy efficiency problem for an imperfect CSI downlink mmWave NOMA system. The studied mixed-integer optimization problem was converted to an integer optimization problem and solved using a GA, which determines the best clustering members in mmWave NOMA. The required population size of the proposed GA was determined to evaluate its effectiveness for a massive number of users. In addition, the GA’s convergence to the optimal solution for light traffic and relatively heavy traffic was also analyzed. Our results illustrate that the solution obtained solution via GA is almost equal to the optimal value and outperforms the conventional orthogonal multiple access, where the EE is improved by more than 75%. Finally, the impact of the estimation error of CSI on the system performance was evaluated at different required SINR scenarios. The results show that EE is degraded in the case of imperfect CSI case but is still close to the optimal solution.

Power Allocation and Equivalent Transmit FDA Beamspace for 5G mmWave NOMA Networks: Meta-Heuristic Optimization Approach

Most of the existing works in millimeter wave (mmWave) and non-orthogonal multiple access (NOMA) communications (mmWave NOMA) employs phased array (PA) antennas which yield only angle beamspace without range dependent, range-interference suppression and “dot-shaped” array resolution. In this paper, we propose a collocated transmitter (Tx) - receiver (Rx) mmWave NOMA system where frequency diverse array (FDA) antennas are utilized. FDA employs small frequency increments across the element indices which yields angle-range dependent beamspace. Herein, an equivalent radiate beamspace is designed to serve the users. Further, to ensure user equity, we formulate objective function to max-min achievable sum rate subject to power allocation and BS equivalent FDA transmit beamspace along the users. Since the problem is not convex, a boundary-compressed Bat meta-heuristic optimization algorithm is devised to provide a sub-optimal solution by optimizing the frequency increments to synthesis the BS beamspace along the users. The achievable secrecy rate, cost efficiency (CE), economic efficiency (ECE), and secrecy energy efficiency (SEE) are derived for the users. Both theoretical analysis and simulation results indicate that the proposed scheme would constitute an excellent nominee for 5G mmWave NOMA communication applications.

Secure and Reliable Transmission in mmWave NOMA Relay Networks With SWIPT

In this article, we consider the millimeter-wave simultaneous wireless information and power transfer nonorthogonal multiple access (NOMA) relay networks in the presence of multiple passive eavesdroppers, where the near and far Internet of Things (IoT) devices with different communication requirements are served by one source with the help of the relays, and stochastic geometry is used to characterize the distribution of multiple nodes. First, a NOMA pairing and relay selection scheme is designed. Then, the closed-form expressions for the energy-information coverage probability (EICP) and the effective secrecy throughput of the near IoT devices are derived. Finally, the EICPs of the far IoT device under random relay selection and opportunistic relay selection schemes are obtained. Moreover, the asymptotic expressions of the EICP for both the near and far IoT devices are derived, which show that the asymptotic EICP when the number of antennas at the source or relay tends to infinity is only dependent on the location distribution of nodes and blockage environment. The simulation results show that NOMA can outperform orthogonal multiple access, and the advantage of the opportunistic relay selection scheme over the random relay selection scheme is more obvious when the density of the relay is relatively large.

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.

Reconfigurable Intelligent Surfaces Aided mmWave NOMA: Joint Power Allocation, Phase Shifts, and Hybrid Beamforming Optimization

In this paper, a reconfigurable intelligent surface (RIS)-aided millimeter wave (mmWave) non-orthogonal multiple access (NOMA) system is analyzed. In particular, we consider an RIS-aided mmWave-NOMA downlink system with a hybrid beamforming structure. To maximize the achievable sum-rate under a minimum rate constraint for the users and a maximum transmit power constraint, a joint RIS phase shifts, hybrid beamforming, and power allocation problem is formulated. To solve this non-convex optimization problem, we develop an alternating optimization (AO) algorithm. Specifically, first, the non-convex problem is transformed into three subproblems, i.e., power allocation, joint phase shifts and analog beamforming optimization, and digital beamforming design. Then, we solve the power allocation problem by keeping fixed the phase shifts of the RIS and the hybrid beamforming. Finally, given the power allocation matrix, an alternating manifold optimization (AMO)-based method and a successive convex approximation (SCA)-based method are utilized to design the phase shifts, analog beamforming, and transmit beamforming, respectively. Numerical results reveal that the proposed AO algorithm outperforms existing schemes in terms of sum-rate. Moreover, compared to a conventional mmWave-NOMA system without RIS, the proposed RIS-aided mmWave-NOMA system is capable of improving the achievable sum-rate.

Performance of Multi-Cell mmWave NOMA Networks With Base Station Cooperation

Coordinated multipoint transmission is applied to multi-cell millimeter wave (mmWave) non-orthogonal multiple access (NOMA) networks in downlink transmission in this letter. Multiple cooperative base stations (BSs) jointly transmit same signal to a cell-edge user as a far user. And each BS individually serves a near user at the same beamwidth with NOMA. Subsequently, the outage probability, outage sum rate and energy efficiency are derived. Further, simulation results are provided to verify the correctness of analytical derivation and demonstrate the cell-edge user's performance improvement as well as the superiority of BS cooperation in multi-cell mmWave NOMA networks.

Research on Cognitive Power Allocation for Secure Millimeter-Wave NOMA Networks

Millimeter-wave (mmWave) non-orthogonal multiple access (NOMA) networks are considered to be a promising network architecture to address spectrum scarcity, massive connectivity and low latency in the Internet of Things (IoT). This paper investigates the cognitive power allocation scheme to evaluate the reliability and secrecy performance of secondary user in mmWave NOMA networks, where a base station (BS) provides services for primary and secondary users distributed in different regions in the presence of randomly located eavesdroppers. In the proposed cognitive power allocation scheme, under the condition that both the security and quality of service (QoS) of the primary user are first satisfied, the opportunistic service is provided to the secondary user. In addition, a sector eavesdropper-exclusion zone is introduced around BS to ameliorate the secrecy performance of the networks. Considering the path loss and blockage model of mmWave channels, and the secrecy performance of secondary user is comprehensively studied by using the stochastic geometry from three aspects: connection outage probability (COP), secrecy outage probability (SOP), and secrecy throughput (ST). The performance evaluation results have demonstrated that when the security constraint of the primary user is heightened, the secondary user's secrecy performance is weakened to varying degrees, but the tradeoff between security and reliability is improved. Moreover, the ST of the mmWave NOMA communication networks with cognitive power allocation outperforms the equivalent networks with mmWave OMA scheme.

On the Physical Layer Security of Millimeter Wave NOMA Networks

For the capability of providing multi-giga BPS (bits per second) rates, millimeter wave (mmWave) communication is one of the key enabling technologies for the new and future generations of mobile communications, i.e., the fifth generation (5G) and beyond. Meanwhile, non-orthogonal multiple access (NOMA) can significantly increase the spectral efficiency by simultaneously serving multiple users in the same channel. Thus, mmWave NOMA networks have recently attracted considerable research attention. Meanwhile, a large number of confidential messages exchanged within highly interconnected systems has posed tremendous challenges on secure wireless communications, and thus in this article, we investigate the physical layer security of mmWave NOMA networks. Considering the limited scattering characteristics of mmWave channels and imperfect successive interference cancellation at receivers, we develop an analytic framework for the secrecy outage probability (SOP) for mmWave NOMA networks, in which legitimate users and eavesdroppers are randomly distributed. Based on the directional transmission property of mmWave signals, we propose a minimal angle-difference user pairing scheme to reduce the SOP of users. Considering the spatial correlation between the selected user pair and eavesdroppers, we develop two maximum ratio transmission (MRT) beamforming schemes to further enhance the secrecy performance of mmWave NOMA networks. Closed-form SOPs for the paired users with different eavesdropper detection capacities are derived. Numerical results show the effectiveness of our analysis and that there exists an optimal radius of network coverage ranges and transmit power to minimize the SOP of the user pair.

Joint users selection and beamforming in downlink millimetre‐wave NOMA based on users positioning

In this study, joint users selection and beamforming based on users positioning are proposed for enabling non-orthogonal multiple access (NOMA) in the millimetre-wave (mmWave) transmissions. In the proposed scheme, the mmWave users are arranged in descending order based on their estimated distances from the mmWave base station (BS), which guarantees the users ordering based on their line-of-sight availability, as well. Out of the ordered users, the two users having the minimum pairing angle and expected to lie within the specified mmWave beamwidth are selected as the two NOMA users. In this study, the users' pairing angle is defined as the maximum expected difference angle between two users considering their location-uncertainty areas. Consequently, the BS antenna beam is formed towards the two selected NOMA users by adjusting its boresight angle to lie in between their estimated location angles. Then, the fixed power allocation scheme is used to adjust the power allocation coefficients of the selected NOMA users. Mathematical analysis is performed to bound the performance of the proposed mmWave NOMA scheme. Also, numerical simulations are conducted to prove the mathematical findings and to show the superior performance of the proposed scheme over the well-known technique of using random beamforming.

Angle-Domain NOMA Over Multicell Millimeter Wave Massive MIMO Networks

The application of non-orthogonal multiple access (NOMA) in millimeter wave (mmWave) massive multiple-input multiple-output (MIMO) systems can enhance spectral efficiency. In this paper, we propose an angle-domain NOMA scheme over the multi-cell mmWave massive MIMO networks. This scheme is optimized through both user scheduling and precoders/decoders design to maximize the system sum rate, where the precoders are decomposed into outer and inner ones. We construct the outer precoders with the help of the users’ spatial angle information, i.e., beam signatures , and propose two design strategies for both inner precoders and decoders, i.e., joint optimization of precoders/decoders (JOPD) and cooperative NOMA (C-NOMA). Specifically, in JOPD, the precoders/decoders are obtained through maximizing a nonconvex function subject to the users’ quality-of-service (QoS) constraints, where an alternate optimization algorithm based on the constrained concave-convex procedure is proposed for its solutions. In C-NOMA, we adopt interference alignment to cooperatively serve the cell-edge users and achieve simplified yet effective precoders/decoders. Furthermore, we optimize C-NOMA through power allocation. Afterwards, user scheduling algorithms are proposed for both JOPD and C-NOMA. Extensive simulations verify that the proposed schemes exhibit improved performance in terms of both sum rate and users’ QoS compared to that of existing mmWave NOMA schemes.

Coverage Analysis of Cooperative NOMA in Millimeter Wave Networks

This letter analyzes the coverage probability of a cooperative non-orthogonal multiple access (NOMA) millimeter wave (mmWave) network consisting of source ( $S$ ) and destination ( $D$ ) pair without a direct link. A cooperative relay ( $R$ ), which is selected from a set of active users, helps $D$ to receive its data. The transmission from $S$ to $R$ is based on NOMA, by which $R$ can receive its own data and $D$ ’s data simultaneously. Two relay selection schemes are developed. For each scheme, the favorable users that meet a rate threshold are selected to be in the decoding set. Then from the decoding set, a relay is selected that is nearest to $S$ (in the 1st selection scheme), or nearest to $D$ (in the 2nd selection scheme). For both schemes, we characterize the spatial density (location dependent) of decoding sets and derive the coverage probability. Both schemes outperform orthogonal multiple access. We also derive the coverage of randomly picking a relay (i.e., no selection) to quantify the benefits of relay selection in mmWave NOMA.

Secure Transmission in mmWave NOMA Networks With Cognitive Power Allocation

Millimeter wave (mmWave) and non-orthogonal multiple access (NOMA) are two key technologies for future wireless networks. In this paper, secure communication in mmWave NOMA networks based on cognitive power allocation scheme is studied by preferentially satisfying the quality of service (QoS) of primary user. Considering directional beamforming, a corresponding sector guard zone is invoked to against the random distributed eavesdroppers. We give a comprehensive investigation of secrecy performance through stochastic geometry in terms of connection outage probability (COP), secrecy outage probability (SOP), and secrecy throughput (ST). The simulation results validate our derivations and show that the secondary user in NOMA schemes can obtain better connection performance than that in orthogonal multiple access (OMA) schemes, while the primary user achieves the same connection performance in NOMA and OMA schemes. In addition, when the system parameters are reasonable set, the secondary user in NOMA schemes can achieve a higher ST than that in OMA. Moreover, blockages can be also utilized to improve secrecy performance on some occasions.

Unveiling Capacity Gains in Ultradense Networks: Using mm-Wave NOMA

Millimeter-wave (mm-wave) communications and nonorthogonal multiple access (NOMA) are two important techniques to achieve high data rates in fifth-generation (5G) ultradense networks (UDNs). Due to interference that is intentionally added during the superpositioned transmissions with NOMA, an additional power budget is required to maintain the target block error rate (BLER). This necessitates the consideration of new approaches to ensure the power efficiency of NOMA systems. In this article, we show that this additional required power can be implemented using the directional transmission capabilities offered by mm-wave antenna arrays. Through the use of our small cells cluster simulations, we investigate the performance of NOMA in mm-wave frequency bands with consideration to the total system capacity, hybrid resource allocation, pairing probability, and power requirements. We demonstrate that a 30d beamwdith, as opposed to a typical 120? beamwidth, can result in a 20% system-capacity gain without requiring any extra transmission power. Our results indicate novel tradeoffs between system capacity, pairing probability, and transmission power in mm-wave NOMA networks owing to the effect of beamwidth variations. We conclude by summarizing the future challenges of NOMA in mm-wave bands.

Optimal User Scheduling and Power Allocation for Millimeter Wave NOMA Systems

This paper investigates the application of non-orthogonal multiple access (NOMA) in millimeter wave (mm-Wave) communications by exploiting beamforming, user scheduling, and power allocation. Random beamforming is invoked for reducing the feedback overhead of the considered system. A non-convex optimization problem for maximizing the sum rate is formulated, which is proved to be NP-hard. The branch and bound approach is invoked to obtain the $\epsilon$ -optimal power allocation policy, which is proved to converge to a global optimal solution. To elaborate further, a low-complexity suboptimal approach is developed for striking a good computational complexity-optimality tradeoff, where the matching theory and successive convex approximation techniques are invoked for tackling the user scheduling and power allocation problems, respectively. Simulation results reveal that: 1) the proposed low complexity solution achieves a near-optimal performance and 2) the proposed mm-Wave NOMA system is capable of outperforming conventional mm-Wave orthogonal multiple access systems in terms of sum rate and the number of served users.