Performance of Cooperative NOMA Systems under Passive Eavesdropping

The full-duplex (FD) cooperative nonorthogonal multiple access (NOMA) achieves a superior throughput over the conventional half-duplex (HD) cooperative NOMA, wherein the strong users (SUs) with good channel conditions can act as an FD relay node for the weak users with poor channel conditions. However, the energy efficiency (EE) of a cooperative NOMA may be degraded due to the additional power consumption incurred at the SUs. We are, therefore, motivated to investigate the EE maximization problem of an FD cooperative NOMA system. More importantly, we investigate the “signal-to-inference-noise ratio gap reversal” problem of cooperative NOMA systems, which imposes successive interference cancellation (SIC) performance degradation at the SUs. This problem has not been documented in the existing cooperative NOMA literature. A low-complexity algorithm is proposed for maximizing the system's EE while guaranteeing a successful SIC operation. Our numerical results show that the proposed algorithm achieves both a higher EE and throughput over the existing HD cooperative NOMA and nonadaptive FD cooperative NOMA. More importantly, the proposed scheme guarantees a successful SIC operation at the SUs.

This paper investigates the application of simultaneous wireless information and power transfer (SWIPT) to cooperative non-orthogonal multiple access (NOMA). A new cooperative multiple-input single-output (MISO) SWIPT NOMA protocol is proposed, where a user with a strong channel condition acts as an energy-harvesting (EH) relay to help a user with a poor channel condition. The power splitting (PS) scheme is adopted at the EH relay. By jointly optimizing the PS ratio and the beamforming vectors, the design objective is to maximize the data rate of the "strong user" while satisfying the QoS requirement of the "weak user". It boils down to a challenging nonconvex problem. To resolve this issue, the semidefinite relaxation (SDR) technique is applied to relax the quadratic terms related with the beamformers, and then it is solved to its global optimality by two-dimensional exhaustive search. We prove the rank-one optimality, which establishes the equivalence between the relaxed problem and the original one. To further reduce the high complexity due to the exhaustive search, an iterative algorithm based on successive convex approximation (SCA) is proposed, which can at least attain its stationary point efficiently. In view of the potential application scenarios, e.g., IoT, the single-input single-output (SISO) case of the cooperative SWIPT NOMA system is also studied. The formulated problem is proved to be strictly unimodal with respect to the PS ratio. Hence, a golden section search (GSS) based algorithm with closed-form solution at each step is proposed to find the unique global optimal solution. It is worth pointing out that the SCA method can also converge to the optimal solution in SISO cases. In the numerical simulation, the proposed algorithm is numerically shown to converge within a few iterations, and the SWIPT-aided NOMA protocol outperforms the existing transmission protocols.

In this paper, secure communication in non-orthogonal multiple access (NOMA) downlink system is considered wherein two NOMA users with channel gain difference are paired in each transmission slot. The user with poor channel condition (weak user) is entrusted, while the user with good channel condition (strong user) is a potential eavesdropper. The weak user data can be intercepted by the strong user since the strong user needs to decode the weak user’s message for successive interference cancellation operation in NOMA. To impair strong user’s eavesdropping capability, weak user’s information-bearing signal is merged with an artificial signal (AS). Thus, the eavesdropping process requires extra decoding step at higher power level. The secrecy outage probability of the weak user is derived and provided in closed-form expression. The weak user faces a choice between transmitting the information-bearing signal with the total power and the deploying the AS technique, whereas the strong user can choose whether to eavesdrop the weak user’s message or not. To investigate users’ power-secrecy tradeoffs, their interactions are modeled as a non-cooperative zero-sum game. The existence of Nash equilibria (NEs) of the proposed game is first analyzed, and pure and mixed-strategy NE profiles are provided. In addition, numerical simulations are conducted to validate the analytical results and to prove that AS-Aided proposed scheme enhances the secrecy performance of NOMA systems while maintaining the NOMA superiority over OMA systems.

In this manuscript, we propose an alternating optimization framework to maximize the energy efficiency of a backscatter-enabled cooperative Non-orthogonal multiple access (NOMA) system by optimizing the transmit power of the source, power allocation coefficients (PAC), and power of the relay node under imperfect successive interference cancellation (SIC) decoding. A three-stage low-complexity energy-efficient alternating optimization algorithm is introduced which optimizes the transmit power, PAC, and relay power by considering the quality of service (QoS), power budget, and cooperation constraints. Subsequently, a joint channel coding framework is introduced to enhance the performance of far user which has no direct communication link with the base station (BS) and has bad channel conditions. In the destination node, the far user data is jointly decoded using a Sum-product algorithm (SPA) based joint iterative decoder realized by jointly-designed Quasi-cyclic Low-density parity-check (QC-LDPC) codes. Simulation results evince that the proposed backscatter-enabled cooperative NOMA system outperforms its counterpart by providing an efficient performance in terms of energy efficiency. Also, proposed jointly-designed QC-LDPC codes provide an excellent bit-error-rate (BER) performance by jointly decoding the far user data for considered BSC cooperative NOMA system with only a few decoding iterations.

This paper investigates the solution to an optimisation problem to minimise the total transmission power at the transmitter in a cooperative non-orthogonal multiple access (NOMA) system with simultaneous wireless information and power transfer (SWIPT) and an energy-harvesting user. First, we formulate the optimisation problem to obtain the minimum transmission power at the transmitter under the constraints of minimum signal-to-interference-plus-noise ratio and minimum energy harvesting. Since the problem is not convex, we transform it into a bi-level optimisation problem. Then, conditions to guarantee the feasibility of the problem are provided, and we derive the analytical optimal solution via the Lagrange method meeting Karush–Kuhn–Tucker optimality conditions to solve the lower-level variables of the inner convex problem. Second, we use particle swarm optimisation to find the approximately optimal values of the upper-level variables. Next, we present two baseline schemes based on orthogonal multiple access (OMA) and equal power splitting for performance comparison with the proposed cooperative NOMA system with SWIPT. Finally, simulation results show that cooperative NOMA with SWIPT can reduce the transmit power at the transmitter, compared to two baseline schemes: OMA and EPS.

SummaryThe next generation wireless access technology highly relies on nonorthogonal multiple access (NOMA) technique. This paper proposes a novel power domain cyclic spread multiple access (PDCSMA) scheme for the design of NOMA system with power domain superposition coding (SC) and cyclic spreading at the transmitter concurrent with symbol level successive interference cancellation (SL‐SIC) at the receiver. Based on acceptable difference in channel gain, the users are grouped together to form PDCSMA clusters, and the unique power is allotted to each user in a cluster. The user with good channel condition is allotted less power, and the user with poor channel condition is allotted more power. Each PDCSMA cluster has its own spreading code, and the data of every user in a cluster are cyclic spread with the same code. Each cluster supports the number of multipath components equivalent to the length of the spreading code. The use of cyclic spreading makes the signal suffered by multipath fading less prone to intra cluster interference. The user signal is decoded by minimum mean square error‐frequency domain equalization (MMSE‐FDE) or maximal ratio combining (MRC)–based receiver in which weak user is detected with hard decision, and strong user is detected with SIC. Compared with conventional power domain NOMA (PDNOMA) and interleaved NOMA, the proposed PDCSMA achieves better bit error rate (BER) performance and assures guaranteed detection.

In this letter, a cooperative non-orthogonal multiple access (NOMA) method is utilized in millimeter wave (mmWave) communications to active non-line-of-sight (NLOS) users, where NOMA users are modeled with the aid of Poisson point process (PPP) and all users are severed by a central base station (BS). Closed-form expressions for outage probabilities of paired NOMA users are derived to characterize the performance of the proposed networks. Our numerical results show that the proposed system outperforms the traditional mmWave networks using non-cooperative NOMA.

Over the past decade, there has been notable attention directed towards spectrum and energy efficiency in conjunction with simultaneous wireless information and power transfer (SWIPT), aimed at extending the operational lifespan of energy-constrained wireless devices within cooperative non-orthogonal multiple access (C-NOMA) systems. This article delves into a system model comprising a transmitter, a relay, and two users, where the time-switching receiver (TSR) protocol is employed for energy harvesting at the relay. The objective of this study is to evaluate the performance of the system in terms of outage probability (OP), and system throughput (ST), and to determine the optimal time-switching (TS) factor. Our analysis encompasses the evaluation of OP, ST, and the determination of the optimal TS factor. Numerical simulation results reveal that the Nakagami-m fading channel exhibits the lowest outage probability, and that system throughput escalates with the signal-to-noise ratio (SNR) augmentation. These findings underscore the potential of the Nakagami-m fading channel to enhance energy efficiency in C-NOMA systems and suggest promising directions for future research.

Due to the successive interference cancellation (SIC) technique, Non orthogonal multiple Access (NOMA) users have the capability of decoding the other paired users sharing the same resource bloc. As a result, NOMA technique is not only exposed to external eavesdropping, but also to internal eavesdropping which raises security risks regarding NOMA users' privacy. Therefore, this research aims to analyse a Multi-antenna cooperative NOMA system where both Decode and Forward (DF) and Amplify and Forward (AF) strategies are studied. In this context, we investigate the secrecy performance of our system with both internal and external eavesdropping scenarios. Closed-form expressions of the secrecy outage probability (SOP) and the strictly positive secrecy capacity (SPSC) metrics are derived. Numerical results are given to illustrate the validity of the obtained theoretical results.

In this paper, we investigate the secrecy outage performance of a typical cooperative downlink non-orthogonal multiple access (NOMA) system over Nakagami-m fading channel, in which the base station transmits a superimposed signal to two users via a relay. First, the secrecy outage behavior of the considered system over Nakagami-$m$ fading channel under three wiretapping cases, e.g., one eavesdropper (Eve), non-colluding and colluding eavesdroppers, are studied, and both analytical and asymptotic expressions for the secrecy outage probability are derived. Next, by considering the availability of Eves' channel state information, we adopt the two-stage relay selection (RS) strategy to improve the system's secrecy outage performance. Finally, simulation results are provided to corroborate the accuracy of our derived expressions. The results show that: 1) there exists secrecy performance floor for cooperative NOMA system, and it was determined by the weak user's secrecy requirement and the channel conditions of the Eves; 2) the two-stage RS scheme can increase the secrecy outage performance significantly under three wiretapping cases; 3) the secrecy performance of cooperative NOMA network is superior to that of orthogonal multiple access network on the condition of low and medium signal-to-noise ratio regions.

Due to the increase in massive device connectivity and increasing demands of future technologies, the Fifth-Generation (5G) networks are expected to support high spectral efficiency and low latency. In this regard, Non-Orthogonal Multiple Access (NOMA) technique has grabbed drastic attention from researchers and industry. Furthermore, to improve the performance of the NOMA system, we have incorporated the idea of cooperative communication. This paper considers a downlink NOMA system consisting of a Base Station (BS), two relays named R1 and R2, employed with Decode and Forward (DF) protocol, one far user, and one near user. In this work, an algorithm to investigate the Symbol Error Probability (SEP) performance of cooperative downlink NOMA system with Quadrature Phase Shift Keying (QPSK) modulation in the presence of the Rayleigh channel fading environment is developed. From the simulation results, it is observed that the SEP performance of the proposed NOMA system is significantly enhanced compared to that of the existing downlink NOMA system due to cooperation from the relay nodes. In addition to that, the SEP performance analysis has been made by varying the distance between the relays and BS resulting in increased SNR and reduced SEP.

In this paper, a novel communication scheme that combines beamforming, energy harvesting, and cooperative non-orthogonal multiple access (NOMA) system is introduced. In the proposed scheme, NOMA is first combined with beamforming to serve more than one user in each beamforming vector. Later, simultaneous wireless information and power transfer (SWIPT) technique is exploited to encourage the strong user to relay the weak user's information messages in full-duplex (FD) manner. However, one major drawback of FD communications is the self-interference (SI) signal due to signal leakage from terminal's output to the input. Three different cases are adopted. In the first case, the SI signal is assumed to be fully cancelled at the strong user using SI cancellation techniques. In the second case, SI is only cancelled at the information decoding circuits, while being harvested in the energy harvester to provide extra energy. The third case investigates the impact of imperfect SI cancellation on system performance is investigated. It is proved that introducing SWIPT doesn't only motivate users to collaborate, but also mitigates the impact of the SI signal. In addition, the problem of total sum rate maximisation is formulated and solved locally by means of two-step difference of convex functions (DC)-based procedure. Furthermore, the outage probability of both weak and strong user is derived and provided in closed-form expressions. Numerical simulations are presented to illustrate the sum rate gain of the proposed scheme if compared with existing schemes, and to verify the derived formulas.

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.

Cooperative non-orthogonal multiple access (NOMA) is heavily studied in the literature as a solution for 5G and beyond 5G applications. Cooperative NOMA transmits a superimposed version of all users’ messages simultaneously with the aid of a relay, after that, each user decodes its own message. Accordingly, NOMA is deemed as a spectral efficient technique. Another emerging technique exploits orbital angular momentum (OAM), where OAM is an attractive character of electromagnetic waves. OAM gathered a great deal of attention in recent years (similar to the case with NOMA) due to its ability to enhance electromagnetic spectrum exploitation, hence increasing the achieved transmission throughput. However, OAM-based transmission suffers from wave divergence, especially at high OAM orders. This OAM limitation reduces the transmission distance. The distance can be extended via cooperative relays (part of cooperative NOMA). Relay helps the source to transmit packets to the destination by providing an additional connection to handle the transmission and provide a shorter distance between source and destination. In this paper, we propose employing OAM transmission in the cooperative NOMA network. Simulation experiments show that OAM transmission helps cooperative NOMA in achieving higher throughput compared to the conventional cooperative NOMA. Concurrently, the cooperation part of cooperative NOMA eases the divergence problem of OAM. In addition, the proposed system outperforms the standalone cooperative OAM-based solution.

In non-orthogonal multiple access (NOMA) systems, serving multiple users in shared resource blocks can allow untrusted users to overhear the messages of other users. In this context, we study a network consisting of a base station (BS), a near user and a far user, where the latter attempts to overhear the message of the former. The near user is a full-duplex (FD) node that can also act as a relay. Two operating scenarios are considered: 1) friendly jammer (FJ), where the FD node broadcasts noise for degrading the channel between the BS and the far user, while receiving data from the BS; and 2) friendly jammer relay (FJR), where, in addition to degrading the channel between the BS and the far user, the FD node relays the message of the far user. We investigate the secrecy performance of the network by characterizing the secrecy outage probability (SOP) in both scenarios. We obtain the exact SOP of the FJ case, and an approximation for the SOP of the FJR scenario, both expressed in closed-form. Numerical results confirm the accuracy of the analytical results. For a given BS power budget and power allocation to the users, it is demonstrated that the jamming and relaying powers are prominent factors to make the NOMA-FJR scheme superior to NOMA-FJ, as well as to conventional and cooperative NOMA schemes.