Cognitive Non-orthogonal Multiple Access Research Articles

A Fast Algorithm for Fractional QCQP and Applications to Secure Beamforming in Cognitive Nonorthogonal Multiple Access Networks

In this paper, we investigate a nonconvex fractional quadratically constrained quadratic problem (fractional QCQP), which has a wide application to the resource allocation optimization in wireless communication systems. Different from the state-of-the-art methods needing to solve semidefinite programmings or second-order cone programmings, we propose a fast algorithm for solving fractional QCQP by combining the successive convex approximation method and the consensus alternating direction method of multipliers, which has only simple computations and works very fast in applications with modest accuracy. We also apply the proposed fast algorithm to secure beamforming design for enhancing physical layer security in cognitive nonorthogonal multiple access (NOMA) networks, where secrecy rate optimization problems in both underlay and overly cognitive NOMA networks are typical fractional QCQPs and have not been well studied. Simulation results have shown that our proposed fast algorithm achieves almost the same performance as the state-of-the-art methods, however, the proposed fast algorithm has very low computation complexity.

Performance Analysis of Energy Harvesting-Assisted Overlay Cognitive NOMA Systems With Incremental Relaying

In this paper, we analyze the performance of an energy harvesting (EH)-assisted overlay cognitive non-orthogonal multiple access (NOMA) system. The underlying system consists of a primary transmitter-receiver pair accompanied by an energy-constrained secondary transmitter (ST) with its intended receiver. Accordingly, ST employs a time switching (TS) based receiver architecture to harvest energy from radio-frequency signals of the primary transmissions, and thereby uses this energy to relay the primary information and to transmit its own information simultaneously using the NOMA principle. For this, we propose two cooperative spectrum sharing (CSS) schemes based on incremental relaying (IR) protocol using amplify-and-forward (AF) and decode-and-forward (DF) strategies, viz., CSS-IAF and CSS-IDF, and compare their performance with the competitive fixed relaying based schemes. The proposed IR-based schemes adeptly avail the degrees-of-freedom to boost the system performance. Thereby, considering the realistic assumption of the NOMA-based imperfect successive interference cancellation, we derive the expressions of outage probability for the primary and secondary networks under both CSS-IAF and CSS-IDF schemes subject to the Nakagami-m fading. In addition, we quantify the throughput and energy efficiency for the considered system. The obtained theoretical findings are finally validated through numerous analytical and simulation results to reveal the advantages of the proposed CSS schemes over the baseline direct link transmission and orthogonal multiple access schemes.

Underlay Cognitive NOMA-Based Coordinated Direct and Relay Transmission

In this letter, a cooperative underlay cognitive non-orthogonal multiple access (NOMA)-based coordinated direct and relay transmission is investigated. Specifically, exact closed-form expressions for the outage probabilities (OPs) of the primary and SNs are derived, based on which the respective asymptotic analysis are carried out. Our results show the superiority of the proposed system when compared with conventional cooperative schemes. Also, it is shown that there exists an optimal point where the transmit powers of the primary and STs can be set in order to ensure a good performance for both networks.

Cognitive Backscatter NOMA Networks With Multi-Slot Energy Causality

In this letter, we consider a multi-user cognitive backscatter network. Backscatter devices (BDs) not only share the spectrum with the primary user, but also harvest energy from and transmit their own information over the primary signal. The multi-BD system adopts non-orthogonal multiple access (NOMA) to further improve the spectral efficiency. Moreover, BDs involved are assumed to have energy storage capability so that the energy harvested in the current block can be stored for future use. Under this assumption, a multi-slot energy causality constraint is proposed. The transmit power of the primary user and the reflection coefficients of BDs are jointly optimized to maximize the sum-rate of the secondary system over multiple slots. Based on the concave-convex procedure (CCCP), we obtain a near-optimal solution by iteratively maximizing the sum-rate. Besides, the effect of imperfect SIC on our proposed resource allocation schemes is also studied. Numerical results show that the multi-slot operation can achieve a remarkable performance gain over the single-slot operation.

Fairness-Aware Throughput Maximization for Underlaying Cognitive NOMA Networks

To improve the radio spectral efficiency for 5G and beyond, novel radio access techniques need to be designed to accommodate unprecedented number of connected devices, and nonorthogonal multiple access (NOMA) has become a promising candidate. Additionally, power allocation and NOMA-secondary user (SU) assignment technology is an efficient way to enhance the resource utilization efficiency at the power domain and the spectral domain for underlaying cognitive NOMA networks. In this article, first, a joint power allocation and SU assignment problem is formulated for the NOMA downlink transmission in an underlaying cognitive radio network. The worst-case achievable rate for the NOMA-SU is maximized. To solve this mixed-integer nonlinear programming problem, we divide the original optimization problem into two subproblems: NOMA-SU assignment and power allocation. Next, a heuristic algorithm is adopted to solve the NOMA-SU assignment subproblem, and successive convex approximation based method is utilized to design a suboptimal power allocation algorithm. Furthermore, an alternative joint NOMA-SU assignment and power allocation scheme are proposed with its average computational complexity analysis given. Finally, numerical results show that the total throughput for the proposed algorithm outperforms more than 30% compared with an existing benchmark scheme at least.

Channel-Aware Power Allocation and Decoding Order in Overlay Cognitive NOMA Networks

To further improve spectral efficiency, this paper considers a non-orthogonal multiple access (NOMA)-assisted overlay cognitive radio network consisting of a pair of primary users and a pair of secondary users. In this network, communication between the pair of primary users is achieved with the help of the secondary transmitter (ST). In return, ST can communicate with the secondary receiver by sending a superimposed signal consisting of its own message and the received primary message. We intend to minimize the outage probability of the secondary system under quality-of-service constraints of the primary system by jointly optimizing the power allocation (PA) factor at ST and the decoding order (DO) at the receivers. When full channel state information (CSI) is available at ST, the optimal PA and DO are derived in closed form under the constraint that the target rate of the primary system should be guaranteed. When only statistical CSI is available at ST, the optimal PA and DO are derived in closed form or by using a bisection method, under the constraints that the outage probability of the primary system is not more than a predetermined threshold and is not larger than that of the secondary system. For the proposed strategies, outage probability expressions of the primary and secondary systems are derived in closed form or semi-closed form. Numerical results are shown to verify our analysis and demonstrate the superiority of the proposed strategies in different CSI-availability scenarios.

Performance Analysis of Overlay Cognitive NOMA Systems With Imperfect Successive Interference Cancellation

Non-orthogonal multiple access (NOMA) and cognitive radio (CR) are envisioned as promising solutions to achieve high spectral efficiency for future wireless networks. This work investigates the outage performance of an overlay cognitive NOMA system with imperfect successive interference cancellation (SIC). The outage probability of primary user and secondary user are derived in closed forms. To obtain further insights, asymptotic expressions of outage probability and system throughput are evaluated when the transmit signal-to-noise ratio approaches infinity. An optimal power allocation coefficient is provided to maximize the system throughput. Moreover, the overlay cognitive NOMA system is compared with the underlay CR system and the overlay cognitive orthogonal multiple access (OMA) system in terms of outage probability. Finally, the performance analysis is validated by simulations. The simulation results demonstrate that the outage performance and system throughput of overlay cognitive NOMA system are superior to those of overlay cognitive OMA system and underlay CR system when the imperfect SIC satisfies certain conditions.

Robust Energy-Efficient Maximization for Cognitive NOMA Networks Under Channel Uncertainties

Resource allocation (RA) is a key technique to guarantee the quality of service of users and maximize the system capacity in cognitive nonorthogonal multiple access (NOMA) networks. However, traditional RA approaches have been proposed under perfect channel state information (CSI) which is too ideal in practical systems due to the impact of link delays, quantization errors, etc. In this article, with imperfect CSI, a downlink robust RA algorithm is proposed for robust transmission to maximize the sum energy efficiency (EE) of secondary users (SUs) under channel uncertainties. Meanwhile, it protects the minimum data rates of SUs, satisfying the maximum transmission power constraints of base stations and the maximum interference temperature constraint of each primary user (PU). The formulated problem is nonconvex, thus challenging to solve. For delay-tolerant services, to deal with the intractability of the problem caused by outage probability constraints, the closed-form expressions of outage probabilities of SUs and PUs are derived under Gaussian CSI error models. For delay-sensitive services, the robust constraints with bounded uncertainty sets are transformed into convex ones. Based on successive convex approximation and the parameter transformation approach, the original problem is converted into a closed-form geometric programming problem solved by dual decomposition methods and subgradient methods. Additionally, users’ outage probabilities and the minimum required transmit power under the two modeling approaches are analyzed. Computational complexity and sensitivity analysis are provided. The simulation results show the proposed algorithm serves good robustness and EE.

Cognitive Radio-Assisted NOMA Broadcasting for 5G Cellular V2X Communications: Model of Roadside Unit Selection and SWIPT.

The outage performance is a significant problem to implement the Cognitive Radio (CR) paradigm in the Vehicle to Everything (V2X) networks. Recently, more interest has focused on Non-Orthogonal Multiple Access (NOMA) in wireless-powered communication. In the conventional CR-enabled V2X-NOMA network, spectrum sensing and limited battery capacity at the Roadside Unit (RSU) may cause serious outage performance. In this study, RSU selection scheme is adopted. This paper presents an interesting model of a system with Simultaneous Wireless Information and Power Transfer (SWIPT) and a CR-enabled V2X-NOMA network. In the downlink, the RSU harvests wireless energy from Radio Frequency (RF) signals and senses the spectrum state at the same time. A CR-enabled V2X-NOMA system performance is presented by deriving exact expressions of outage probability of distant vehicles. In the overlay CR-enabled V2X-NOMA network, the constraints are transmit power and the number of designed RSU that make significant impacts on system performance. Simulation results show that the CR-enabled V2X-NOMA get benefits from energy harvesting and RSU selection scheme.

Outage analysis of SWIPT‐based full‐duplex cognitive NOMA downlink system over Nakagami‐m fading channels

SummaryCognitive nonorthogonal multiple access (NOMA) technique allows multiple users to share the same time and same frequency resources to fulfil the reliability and spectral efficiency requirements of 5G communication standards. In this paper, simultaneous wireless information and power transfer (SWIPT)–based full‐duplex cognitive NOMA downlink system is proposed. In this system, secondary source (SS) serves as a relay to far primary user as there is no direct link from the primary source. NOMA technique is used at SS to transmit information to far primary user and secondary user. The time switching mechanism is adopted at SS for harvesting energy and information decoding. Analytical closed‐form expressions are derived for the outage probabilities of both primary and secondary users. Outage analysis is carried out in Nakagami‐ fading environment in the presence of self‐interference at SS. In addition to that, the optimal harvesting time to maximize the instantaneous throughput of the far primary user is also derived. Numerical results are plotted to validate the derived expressions. It is inferred that the outage probability of the proposed system depends on the fading environment, harvesting parameters, and self‐interference at SS.

Ensuring equal outage performance for down‐link secondary users in full/half duplex cognitive NOMA systems

This study considers an underlay cognitive radio (CR) system, where the secondary network employs cooperative non-orthogonal multiple access (NOMA) to transfer information for downlink secondary users (SUs). Here the near user is enabled to act as a full duplex/half duplex (FD/HD) decode-and-forward (DF) relay for the far user. We derive analytical expressions for the outage probabilities experienced by the SUs in the considered FD/HD – CR-NOMA network. The maximum transmit power constraint of the secondary nodes and the tolerable interference power constraints at the primary receiver are considered for the analysis. We provide numerical and simulation results to identify the impact of key system parameters on the outage performance of SUs. Further, we provide insights on the selection of key system parameters such as NOMA power allocation coefficient for ensuring that both the SUs experience equal outage performance. Finally, we derive an analytical expression for the system outage probability of the FD-CR-NOMA network. We determine the optimal power allocation (OPA) factor at the secondary transmitter that minimises the system outage probability. We establish that OPA can reduce the system outage significantly as compared to the default choice of power allocation factor. The analytical results are validated by extensive simulation studies.

Cognitive NOMA for UAV-Enabled Secure Communications: Joint 3D Trajectory Design and Power Allocation

In the paper, we investigate the physical layer security (PLS) of an unmanned aerial vehicle (UAV) enabled communication system that integrates non-orthogonal multiple access (NOMA) with cognitive radio (CR), namely CR-NOMA. Specifically, the secondary UAV transmits confidential information to multiple secondary legitimate receivers with NOMA by sharing the spectrum with the primary networks, while an eavesdropper (Eve) attempts to wiretap the communication. We aim to jointly optimize the three-dimensional (3D) UAV trajectory and power allocation to maximize the worst-case average secrecy sum rate of all secondary receivers (SRs) while keeping the interference to primal receivers (PRs) below a certain threshold. The formulated problem is mixed-binary non-convex and is generally NP-hard. To address this issue, we first equivalently transform the binary variables into continuous ones by introducing tractable inequality constraints. Then, we decompose the optimization problem into two subproblems and develop an efficient iterative algorithm based on the successive convex approximate (SCA) technique, block coordinate descent (BCD) technique, and penalty function method. Numerical results are extensively studied to show that the proposed joint design significantly outperforms the benchmark schemes in terms of secrecy rate.

Secure Transmission Design in HARQ Assisted Cognitive NOMA Networks

In this paper, we design a secure transmission scheme in hybrid automatic repeat request (HARQ) assisted cognitive non-orthogonal multiple access (NOMA) networks, where a security-required user (SRU) is paired with a quality of service (QoS)-sensitive user (QSU) to perform NOMA. To elaborate, the QoS requirement of the QSU is guaranteed by a cognitive power allocation scheme, while the HARQ technique is employed to mitigate the successive interference cancellation (SIC) errors and improve the secrecy performance of the SRU. For reducing information leakage, a randomized retransmission NOMA (RR-NOMA) scheme is designed, where the retransmitted signals are generated from independent randomized codebooks. In this scheme, the closed-form expressions for the connection outage probability (COP), the average number of transmission (ANT), the secrecy outage probability (SOP), and effective secrecy throughput (EST) of the SRU are derived. In addition, as benchmarks, the performance analyses for the fixed retransmission (FR-NOMA) scheme and the randomized retransmission orthogonal multiple access (RR-OMA) scheme are also provided. Results show a trade-off between SOP and COP or EST, which is denoted by security-reliability trade-off (SRT) or security-efficiency trade-off (SET). Furthermore, simulation results show that the HARQ technique improves SRT and the RR-NOMA scheme achieves better SET than the FR-NOMA scheme in the low SOP region. We further conduct asymptotic analysis in the RR-NOMA, FR-NOMA and RR-OMA schemes. Asymptotic results demonstrate that the three schemes achieve the same ANT and the RR-NOMA scheme obtains better secrecy performance than the RR-OMA scheme and equal secrecy performance to the FR-NOMA scheme in terms of both EST and SOP.

Outage Performance Improvement by Selected User in D2D Transmission and Implementation of Cognitive Radio-Assisted NOMA.

In this paper, we investigate the outage performance in secondary network of cognitive radio (CR) employing non-orthogonal multiple access (NOMA) wireless networks over Rayleigh fading channels. The considered system model adopts device-to-device (D2D) transmission together with traditional communication to form a new system model, namely CR-D2DNOMA network. The specific user is selected from multiple D2D-Tx users (D2Ds) to communicate with far NOMA users to form qualified D2D connection with assistance of the Relay user (). The main metric in such CR-D2DNOMA network needs to be considered and we particularly introduce the closed-form expressions for outage probability in the secondary network where it is designed to serve two far NOMA users. The perfect Successive Interference Cancellation (SIC) and imperfect SIC can be further examined at the second NOMA user who detects signal based on the ability of SIC. The results show the positive impact of increasing the fading parameters on the system performance. More importantly, numerical results are provided to verify the correctness of our derivations. Additionally, the effects of asymptotic expressions on insights evaluation are also further analyzed.

Cognitive Non-Orthogonal Multiple Access With Energy Harvesting: An Optimal Resource Allocation Approach

We consider a cognitive radio system, in which a secondary transmitter harvests energy from a primary transmitter's RF signal. The secondary transmitter, which provides decode-and-forward relaying service for the primary system, transmits its own data by using downlink non-orthogonal multiple access (NOMA). A time-switching protocol is used by the secondary transmitter to harvest energy and decode the primary transmitter's information. Our objective is to achieve maximal secondary throughput, by optimally selecting the time portion used for energy harvesting and the secondary transmitter's power allocation in NOMA transmission. In this paper, two optimization problems are formulated, in which the secondary receiver performs or does not perform successive interference cancellation (SIC), respectively. Although the two problems are nonconvex, we devise a method to transform the problems into equivalent problems under different cases. Then, we theoretically prove that the objective functions of the equivalent problems are quasiconcave, based on which we develop two-level bisection search algorithms to solve the equivalent problems. Interestingly, we show that performing SIC at the secondary receiver does not always guarantee a higher secondary throughput than the case without performing SIC. Computer simulation demonstrates that our algorithm performs better than an equal power allocation algorithm and an orthogonal multiple access algorithm.

Outage Capacity Analysis for Cognitive Non-Orthogonal Multiple Access Downlink Transmissions Systems in the Presence of Channel Estimation Error

Outage Capacity Analysis for Cognitive Non-Orthogonal Multiple Access Downlink Transmissions Systems in the Presence of Channel Estimation Error

Cognitive Non-Orthogonal Multiple Access with Cooperative Relaying: A New Wireless Frontier for 5G Spectrum Sharing

Two emerging technologies toward 5G wireless networks, namely non-orthogonal multiple access (NOMA) and cognitive radio (CR), will provide more efficient utilization of wireless spectrum in the future. In this article, we investigate the integration of NOMA with CR into a holistic system, namely a cognitive NOMA network, for more intelligent spectrum sharing. Design principles of cognitive NOMA networks are perfectly aligned to functionality requirements of 5G wireless networks, such as high spectrum efficiency, massive connectivity, low latency, and better fairness. Three different cognitive NOMA architectures are presented, including underlay NOMA networks, overlay NOMA networks, and CR-inspired NOMA networks. To address inter-network and intra-network interference, which largely degrade the performance of cognitive NOMA networks, cooperative relaying strategies are proposed. For each cognitive NOMA architecture, our proposed cooperative relaying strategy shows its potential to significantly lower outage probabilities. We discuss open challenges and future research directions on implementation of cognitive NOMA networks.

Antenna Selection for MIMO Nonorthogonal Multiple Access Systems

This paper considers the joint antenna selection (AS) problem for a classical two-user MIMO non-orthogonal multiple access (NOMA) system, where both the base station (BS) and users (UEs) are equipped with multiple antennas. Specifically, several computationally-efficient AS algorithms are developed for two commonly-used NOMA scenarios: fixed power allocation NOMA (F-NOMA) and cognitive radio-inspired NOMA (CR-NOMA). For the F-NOMA system, two novel AS schemes, namely max-max-max AS (A$^3$-AS) and max-min-max AS (AIA-AS), are proposed to maximize the system sum-rate, without and with the consideration of user fairness, respectively. In the CR-NOMA network, a novel AS algorithm, termed maximum-channel-gain-based AS (MCG-AS), is proposed to maximize the achievable rate of the secondary user, under the condition that the primary user's quality of service requirement is satisfied. The asymptotic closed-form expressions of the average sum-rate for A$^3$-AS and AIA-AS and that of the average rate of the secondary user for MCG-AS are derived, respectively. Numerical results demonstrate that the AIA-AS provides better user-fairness, while the A$^3$-AS achieves a near-optimal sum-rate in F-NOMA systems. For the CR-NOMA scenario, MCG-AS achieves a near-optimal performance in a wide SNR regime. Furthermore, all the proposed AS algorithms yield a significant computational complexity reduction, compared to exhaustive search-based counterparts.