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.

This work considers covert communication in downlink NOMA systems, where a public action (communication from Alice to the NOMA-weak user U1) is used to provide a cover for a secret action (communication from Alice to the NOMA-strong user U2). To aid system design, we first derive the detection error probability (DEP) at Willie and the connect outage probabilities (COPs) at U1 and U2. Then, considering the covertness and outage constraints, the optimization of power allocation factor ratio between U1 and U2 is tackled in order to maximize the effective covert rate. The results demonstrate that the minimum DEP is the power allocation factor for U1. In addition, for a given covert constraint, relaxing the outage constraint will effectively increase the maximum effective covert rate within a certain range of the maximum transmit power of U1.

Future wireless networks have requirements of large connections, low power consumption, high reliability, and strong security. Considering the abundant available bandwidth of millimeter wave (mmWave) and the large line-of-sight (LOS) link probability of air-ground channel, in this paper, we develop a framework to study security, reliability and energy coverage performance of the downlink mmWave SWIPT UAV networks under non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) schemes, where a UAV serves two types of authorized IoT devices in the presence of multiple passive eavesdroppers. Directional modulation (DM) is also used to improve the physical layer security performance. First, the analytical expressions for connection outage probability (COP), secrecy outage probability (SOP) and effective secrecy throughput (EST) of the users with high rate security requirement (HRSR) under NOMA and OMA schemes are derived using stochastic geometry. Then, we optimize the active antenna selection of DM using adaptive genetic simulated annealing algorithm to further improve the secrecy performance based on the obtained analytical results. Finally, the closed-form expressions for the COP and energy-information coverage probability (EICP) of the energy-constrained users with low-rate requirement (ECLR) under NOMA and OMA schemes are obtained. Numerical and simulation results provide interesting insights into the influence of various parameters on the tradeoff between the reliability and security for the HRSR user, and the tradeoff between the reliability and energy coverage for the ECLR user. Moreover, the EST of NOMA scheme outperforms OMA at low transmit power and high codeword transmission rate.

In this letter, we investigate the robust and secure beamforming (RSBF) in an intelligent reflecting surface (IRS) aided millimeter wave (mmWave) multiple input single output (MISO) system, where multiple single antenna eavesdroppers (Eves) are arbitrarily distributed nearby the legitimate receiver (Bob). Considering the channel state information (CSI) of cascaded wiretap channels is imperfectly known at the legitimate transmitter (Alice), the RSBF design problems to maximize the worst case of achievable secrecy rate (ASR) are formulated under the total transmission power and unit-modulus constraints. Since the problems are difficult to solve optimally due to their non-convexity and coupled variables, we substitute the wiretap channels by a weighted combination of discrete samples and propose a RSBF scheme based on alternating optimization and semidefinite relaxation (SDR) techniques, for both colluding and noncolluding eavesdropping scenarios. Simulation results show that the proposed RSBF scheme can effectively improve the ASR and also outperforms other benchmark schemes.

In this letter, we develop a new framework to jointly characterize covertness and timeliness of short-packet communications, in which a new metric named covert age of information (CAoI) is first proposed and then a closed-form expression for the average CAoI is derived. Our examination explicitly reveals the tradeoff between communication covertness and timeliness affected by the block-length, transmit power and prior transmission probability. Multiple transmission designs are tackled in order to minimize the average CAoI subject to covertness constraint, where the resultant differences relative to the designs with traditional metrics (e.g., effective covert rate, AoI) as objective functions are clarified. Our examination demonstrates that the optimal block-length is not the largest one, which is optimal in delay-constrained covert communication without considering the communication timeliness, and the optimal prior transmission probability may not be 1/2, which has been widely assumed in the literature of covert communications.

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.