This letter studies physical layer security (PLS) of an intelligent reflecting surface (IRS) aided wireless powered communication (WPC) system in the presence of a passive eavesdropper, and proposes three secure IRS-WPC modes. In mode-I, the IRS is deployed between hybrid access point (HAP) and wireless user (U) for co-located power station (PS) and access point (AP). In mode-II, the IRS is deployed between AP and U for separate PS and AP, while in mode-III, the IRS is deployed between PS and U for separate PS and AP. For each mode, the optimal phase shift is designed to maximize the reception of energy and information at legitimate receiver. The accurate and asymptotic ergodic secrecy capacity (ESC) of each mode is analyzed, and various new closed-form expressions are obtained. The results reveal that mode-I and mode-II significantly outperform benchmark without IRS, while mode-III outperforms benchmark without IRS under lower transmission power or small number of IRS elements.

Nonorthogonal multiple access (NOMA) assisted semi-grant-free transmission admits grant-free users to access the channels otherwise solely occupied by grant-based users, and has been recently attracting considerable attention in terms of accommodating massive connectivity and reducing access delay in Internet of Things (IoT). In this work, we investigate the security of semi-grant-free NOMA transmission in the presence of passive and active eavesdropping attacks. In particular, for the scenario-I with strong grant-based user and weak grant-free users, the scenario-I-based maximal user scheduling (IbMUS) and scenario-I-based optimal user scheduling (IbOUS) schemes are proposed to combat the passive and active eavesdropping, respectively. For the scenario-II with weak grant-based user and strong grant-free users, two parallel schemes, namely, the scenario-II-based maximal user scheduling (IIbMUS) and scenario-II-based optimal user scheduling (IIbOUS) schemes, are proposed to combat the passive and active eavesdropping, respectively. These proposed schemes enhance the security by scheduling a grant-free user with maximal main channel capacity/maximal secrecy capacity to access the NOMA channel on the premise of ensuring the grant-based user’s Quality of Service. Based on these proposed schemes, the exact secrecy outage probability (SOP) are analyzed to evaluate the system performance. The simulation results validates the theoretic analysis and the superiority of the proposed schemes. The IbOUS and IIbOUS schemes can achieve better performance than the IbMUS and IIbMUS schemes owing to the use of active eavesdropper’s channel state information (CSI). The SOP achieved by the proposed schemes can be further improved with the increasing number of grant-free users and decreasing target rate (or target secrecy rate).

Reconfigurable intelligent surface (RIS) enables performance enhancement of communication networks due to its significant reflective gain, which could also be used to strengthen secure transmission. In this paper, we consider a RIS enabled unmanned aerial vehicle (UAV) communication network, in which the UAV serves as a base station for a terrestrial legitimate user accompanied by a terrestrial eavesdropper. Specifically, security performance of the considered network is enhanced under both instantaneous and statistical eavesdropper channel state information (CSI). To maximize the average secrecy rate and minimize the secrecy outage duration, the trajectory and beamformer of UAV and the phase shift matrix of RIS are jointly optimized. For the instantaneous CSI scenario, we propose a successive convex approximation (SCA) based algorithm as a baseline solution, where the collective optimization problem is decomposed into three subproblems and solved separately. After that, benefiting from the state-of-the-art proximal policy optimization (PPO) algorithm, a deep reinforcement learning (DRL) based algorithm is developed to solve the average secrecy rate maximization problem from a holistic perspective. For the statistical CSI case, the close-form expression of secrecy outage probability (SOP) is derived to expound the secrecy outage duration. Since the secrecy outage duration minimization problem is hard to be solved via the SCA based algorithm, the developed DRL based algorithm is utilized again. Simulation results demonstrate that compared with benchmark algorithms, both the security performance and complexity of the proposed DRL based algorithm are competitive.

In view of the scarce spectrum resources and inconvenient deployment in wireless communications, this work focuses on an unmanned aerial vehicle (UAV) enabled non-orthogonal multiple access (NOMA) system where a UAV deployed as a base station serves two users by NOMA. A joint power allocation and aerial jamming (PAAJ) scheme is proposed to achieve reliable and secure communications for the system in the presence of a malicious eavesdropper. To be specific, dynamic power allocation is adopted to ensure the reliability of the system and another friendly UAV jammer is introduced to improve the security of the system. The closed-form expressions of connection outage probability (COP), secrecy outage probability (SOP), and effective secrecy throughput (EST) are derived to evaluate the performance of the security-required user in the practical condition of probabilistic line-of-sight (LoS) and non-line-of-sight (NLoS) air-to-ground (A2G) channels. Moreover, the asymptotic EST is analyzed to obtain further insights. Numerical results show the validity of theoretical derivations and the superiority of the proposed PAAJ scheme over benchmark schemes. The EST of the UAV-enabled NOMA communication system outperforms the conventional terrestrial NOMA system at the low transmitting power from the UAV base station. There exists an optimal height of the UAV base station to maximize the EST, which provides a useful framework for designing UAV-enabled communication systems with heterogeneous service requirements.

This paper proposes a reconfigurable intelligent surface (RIS) segmented symbiotic ambient backscatter system, which is composed of a primary source (S), a RIS, and a cooperative receiver (D) that decodes the signals from S and RIS with the aid of successive interference cancellation (SIC). For such, the RIS is segmented into two zones, namely, an EP zone and a BD zone, where the EP zone is used to enhance the received primary signal at D while the BD zone is utilized to backscatter RIS's signal to D. The coexistence outage probability (COP) is first developed, which is shown to be dominated by the minimum number of reflectors in the two zones for sufficiently high transmit signal-to-noise ratio (SNR). In contrast, the sum ergodic capacity (EC) can only be improved by increasing the transmit power and the number of reflectors in the EP zone at high transmit SNR. Finally, two algorithms are designed to minimize the number of reflectors in the two zones while achieving acceptable COP or sum EC.

This article investigates physical-layer security (PLS) of a typical wireless-powered communication (WPC) system with the aid of intelligent reflecting surface (IRS) in the presence of a passive eavesdropper for Internet of Things (IoT), and proposes three IRS-aided secure WPC modes. Specifically, in mode-I, the IRS is deployed between hybrid access point (HAP) and wireless user (U) for co-located power station (PS) and access point (AP). In mode-II, the IRS is deployed between AP and U for separate PS and AP, while in mode-III, the IRS is deployed between PS and U for separate PS and AP. For each mode, the optimal phase shift is designed to maximize the reception of energy and information at the legitimate receiver. We comprehensively analyze the performance of each mode, and derive the closed-form expressions of connection outage probability (COP), secrecy outage probability (SOP), and effective secrecy throughput (EST) for each mode, respectively. The theoretical analysis and simulation results reveal that from the perspective of reliability, mode-I can achieve the best COP with the increased number of IRS elements, while mode-II and mode-III have a similar COP. From the perspective of security, mode-II can achieve the best SOP with the increased number of IRS elements, while mode-I and mode-III have a similar SOP. Moreover, under the condition of small transmission power at HAP/PS or small number of IRS elements, mode-I has the best EST, while as the power or the number increases, the EST of mode-II becomes the best one.