This paper investigates physical layer security (PLS) in cognitive radio inspired non-orthogonal multiple access (CR-NOMA) networks with multiple primary and secondary users. For managing the interferences among the users and guaranteeing the quality of services (QoS) of primary users, a new secure NOMA transmission strategy is designed, where the primary and secondary users are paired by their channel gains, respectively, and employing power-domain NOMA to transmit signal. Then, the closed-form expressions for connection outage probability (COP), secrecy outage probability (SOP) and effective secrecy throughput (EST) are derived of the primary users over Nakagami-m fading channels when the secondary users are considered as eavesdroppers. Typically, the secrecy performance can be improved by pairing the primary users with best channel gains or reducing the number of secondary users. In addition, we have also investigated the performances of secondary users by deriving the closed-form expressions of COP and throughput of secondary network. Significantly, the power allocation factors of primary users and secondary users provide a tradeoff between the performance of primary and secondary users. Furthermore, simulations are conducted to verify our analysis results and provide insights into the impact of the parameters on system performance.

In this article, we investigate covert millimeter-wave (mmWave) communications with finite blocklength, where a multiantenna transmitter sends covert messages to a legitimate receiver in the presence of spatially random wardens. Both the phase array (PA) and linear frequency diverse array (LFDA) beamforming schemes, which are designed to maximize the antenna gain from the transmitter to the legitimate receiver, are investigated to improve the covert communication performance. First, the novel expressions of covert communication constraint and average effective covert throughput (AECT) are derived for both beamforming schemes. Then, taking into account the constraint of maximal available blocklength, the optimal transmit power and blocklength are determined for maximizing the AECT. Typically, comparing to the benchmark with fixed blocklength, the enhancement of AECT by utilizing the optimized blocklength enlarges as the density of wardens increases. In addition, it is observed that increasing the maximal available blocklength cannot always improve the maximum AECT due to the tradeoff between the transmit power and blocklength. Furthermore, it is shown that the maximum AECT varies for different directions of the legitimate receiver under both the beamforming schemes, and the transmitter can adaptively choose the PA or LFDA beamforming scheme to improve the covertness performance against spatially random wardens.

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.

Considering the diversified communication requirements in Internet of Things (IoT) networks, this paper proposes a non-orthogonal multiple access (NOMA) scheme which pairs two types of typical users, i.e., delay-sensitive user (DSU) and security-required user (SRU) to share the same non-orthogonal communication resources. We focus on the investigation of the secure transmission for the SRU while guaranteeing the communication requirements of the DSUs at the high priority. Specifically, the delay requirement of the DSUs and the security demand of the SRU are guaranteed by using short-packet communications and maximal ratio transmission (MRT) scheme, respectively. Since the packets cannot always be detected correctly in the short-packet communications, a novel power allocation strategy is designed sophisticatedly to avoid the outage performance floor at the SRU caused by the short-packet communications. A set of closed-form expressions of the connection outage probability (COP), secrecy outage probability, and effective secrecy throughput (EST) of the SRU are derived over Nakagami-m channels in the proposed NOMA scheme and also the benchmark OMA scheme. Besides, we provide the security-reliability tradeoff (SRT) and security-efficiency tradeoff (SET) results for obtaining more insights into system performance. Results demonstrate the superiority of the proposed NOMA scheme over the benchmark OMA scheme in terms of the COP, EST, SRT, and SET when the transmitter is equipped with multiple antennas.

This letter investigates covert communication in uplink NOMA systems assisted by an energy harvesting jammer (EHJ), where a reliability-required user (RRU) and a covertness-required user (CRU) are paired to perform NOMA to transmit messages to Bob, in the presence of a warden (Willie) who tries to detect CRU’s transmission behavior. In particular, the EHJ with time switching (TS) scheme has been employed for the purpose of confusing Willie and compensating for the energy supply. We derive the closed-form expressions of expected minimum detection error probability (EMDEP) and connection outage probabilities (COPs) at RRU and CRU, then we analyse the maximum effective covert rate (ECR) under the covertness and RRU’s quality of service (QoS) constraints. Our analysis indicates that there exist an optimal transmit power at Bob and time switching factor to maximize the maximum ECR, and RRU’s QoS constraint has a remarkable effect on the maximum ECR as the transmit power at Bob or time switching factor increases.

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.