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This work investigates the security issue of the energy-constrained untrusted relay network with imperfect channel reciprocity, where the relay without the built-in power supply can only scavenge energy from radio-frequency signals radiated by the source and destination. A hybrid power-splitting (PS)- and time-switching (TS)-based relaying (HPTR) protocol is presented to improve the degraded secrecy performance due to the incomplete self-interference cancellation at the destination. To evaluate the secrecy throughput (ST) of the studied system, the analytical expression of the secrecy outage probability under the delay-limited transmission mode and the closed-form lower bound of the ergodic secrecy capacity under delay-tolerant transmission mode are derived. Both linear and nonlinear models for the energy harvester at the relay are compared. The optimal PS and TS ratios are evaluated numerically. The theoretical derivations are validated by numerical results, revealing that the residual jamming has a negative effect on the secrecy performance of untrusted relay networks, which can be alleviated by the HPTR protocol. Besides, we compare the ST performance of the HPTR protocol with that of the PS and TS relaying schemes, and the results show that the HPTR protocol outperforms both PS and TS relaying protocols in terms of the ST.

Simultaneous wireless information and power transfer (SWIPT) is a major breakthrough in the field of low-power wireless information transmissions. In this paper, the secrecy performance of the SWIPT-enabled relay network with full-duplex destination-aided jamming is assessed, where both the power-splitting (PS) and time-switching (TS) schemes at the relay are considered with the linear and nonlinear energy harvesting models. The relay harvests energy from the confidential signal and artificial noise sent by the source and destination, respectively, and forwards the amplified signal to the destination, in the presence of an eavesdropper. The analytical closed-form expressions of the connection outage probability (COP), secrecy outage probability (SOP), and transmission outage probability (TOP) for PS- and TS-based schemes are derived, and the closed-form expression of the lower bound of ergodic secrecy capacity (ESC) is calculated. The asymptotic-form expressions of the COP, SOP, TOP, and ESC are further analyzed to capture the valuable information in the high SNR regime. Numerical results verify the correctness of analytical results, reveal the effects of the PS/TS ratio, and transmit the signal-to-noise ratio on secrecy performance.

This work investigates the proactive eavesdropping through a friendly full-duplex (FD) relay and a legitimate monitor over the wireless-powered suspicious communication network, where one suspect source performs energy harvesting (EH) from one dedicated power beacon and then communicates with one suspect destination. Under this setup, a relay-aided proactive eavesdropping scheme is designed to improve the surveillance performance of the system. The closed-form expressions are derived under linear and nonlinear EH models, including the decoding outage probability for the suspicious link, the eavesdropping outage probability for the eavesdropping link, and the average eavesdropping rate (AER). Based on these obtained expressions, an optimization problem to maximize the AER is formulated by jointly optimizing the power and location of the relay. The existence of the optimal solution is carefully analyzed. Moreover, the separate optimization issues of the power and location are first established, and then a bisection-based searching algorithm is presented for the joint optimization problem with the linear EH model. Furthermore, to reduce the complexity of joint optimization, a low-complexity learning-based iteration algorithm is further proposed by employing the well-fitting characteristic of the deep feedforward neural network. Numerical results verify the effectiveness of the proposed algorithm, and show that the optimized FD relay-aided proactive eavesdropping scheme outperforms the reference schemes. Finally, the application scenarios of the proposed proactive eavesdropping scheme are discussed.

Simultaneous wireless information and power transfer (SWIPT) is a promising technique for energy-constrained wireless networks. To improve the security of SWIPT-enabled relay networks, in this paper, a full-duplex (FD) destinationaided jamming strategy is proposed, where a power splitting (PS) scheme is considered and two transmission phases are designed. In the first phase, the energy-limited half-duplex relay receives the signal superposed by confidential information and artificial noise (AN) from the source and destination, respectively. And then the FD destination transmits AN to the passive eavesdropper and receives the signal from the relay concurrently in the second phase. To maximize the secrecy capacity (SC), the individual optimizations of the PS ratio and power allocation (PA) coefficient are investigated, and the approximate optimal closed-form solutions of PS and PA ratios are derived. Besides, an iterative algorithm is also proposed to obtain the global optimal SC by updating the PS and PA ratios alternately. Simulation results confirm that the theoretical PS and PA ratios coincide closely with the exact results, and demonstrate that the optimal SC obtained by the iterative algorithm matches well with that given by the exhaustive search.

This work investigates a relay-aided proactive eavesdropping network, where a legitimate monitor eavesdrops on communication between suspicious users via a friendly amplify-and-forward full-duplex (FD) relay. The closed-form expressions are derived, including the decoding outage probability for the suspicious link, the eavesdropping non-outage probability for the eavesdropping link, and the average eavesdropping rate (AER). To maximize the AER, separate optimization problems for the power and location of the relay are settled, employing the good fitting characteristic of the deep feedforward neural network. Then, a low-complexity learning-based iteration algorithm is proposed to solve the joint optimization problem. Numerical results demonstrate the effectiveness and optimality of the proposed algorithm, and show that the optimized FD relay-aided proactive eavesdropping scheme outperforms the existing benchmark schemes in terms of the AER.

This paper studies a legitimate proactive eavesdropping scenario where a central monitor covertly wiretaps the communications between suspicious users through a full-duplex multi-antenna relay. Specifically, the suspicious information is intercepted by the eavesdropping relay, and then encoded and forwarded to the central monitor. At the same time, the relay sends jamming signals to the suspect destination to reduce the quality of suspicious communication. To minimize the suspicious secrecy rate, the optimal forwarding and jamming beamforming vectors of the eavesdropping relay are first designed. Based on this optimized result, the best power allocation ratio is obtained. To this end, the optimal beamforming vectors and power allocation ratio are derived in closed-form. The simulation results verify the existence of the optimal forwarding power of the relay and show that the proposed scheme achieves better performance than the conventional schemes.

Legitimate wireless surveillance plays an important and preventive role in protecting public safety. To promote the eavesdropping performance of wireless surveillance systems, a proactive eavesdropping scheme with a decode-and-forward relay is proposed, where the relay intercepts suspicious information and interferes with the suspicious receiver simultaneously. The closed-form expressions are derived, including the decoding outage probability, eavesdropping outage probability, and average eavesdropping rate. To maximize the average eavesdropping rate, the transmit power and location deployment at the relay are optimized. Specifically, the closed-form result of approximate optimal transmit power for the relay is obtained, and the bisection-based algorithm is further proposed to verify its correctness. Simulation results validate that the proposed scheme achieves better eavesdropping performance than that without optimization.

Melt-textured large, single-grain REBCO (RE: rare earth element or Y) bulk superconductors have growth sector boundaries (GSBs) and growth sector regions (GSRs). The critical current density is dependent on the GSBs and the GSRs, which will influence the electromagnetic field and the temperature field distributions. REBCO bulk superconductors are brittle materials and during the manufacturing process, cracks and inclusions may exist, therefore it is necessary to analyze the mechanical response during the magnetization process. In this paper, the mechanical response of an inhomogeneous cylinder superconductor under electromagnetic stress and the thermal stress during the pulsed field magnetization process is studied. By solving the coupled magnetic and heat diffusion equations in association with a modified E − J power-law relationship, we can obtain the electromagnetic field and the temperature field distributions; then, the mechanical response of the bulk can be determined. The trend shown by our numerical simulation is consistent with the experimental results. Then we study the mechanical behavior of inhomogeneous high-temperature superconductors with and without a flux jump. The case without or with the SUS 304 ring is also analyzed.

The influence of PASR (Pilot stage Air Split Ratio) on the ignition and LBO (Lean Blow Out) performances is experimentally investigated for an SPP (Stratified Partially Premixed) injector in this paper. The pilot stage of the SPP injector comprises two axial air swirlers as well as an air blast prefilm atomizer for pilot fuel preparation. It is believed that the variation of the air split ratio between the outer swirler and the inner swirler of the pilot stage will transform the flow structure and fuel distribution of the local flame anchoring zone, and consequently improves or deteriorates the stability of the pilot flame. The ignition and LBO characteristics were measured for PASR = 8:2, 7:3 and 6:4, and several inexplicable but interesting results are observed. In order to make out the underlying reasons for the differences of the obtained ignition and LBO data, the velocity field and spray concentration at the meridian plane were acquired experimentally with the help of optical diagnostics at isothermal conditions. It it concluded that two dominant mechanisms of flame stability exist depending on the range of the injector pressure drop (Δ Psw/P3t). At low pressure drop of the injector, the flame stability is mainly affected by the fuel distribution, however, the flow structure will play a more important role at high Δ Psw/P3t in that it can transform the local flow structures around the pilot flame root. The inherent correlations between the combustion stability and the flow structure as well as the fuel distribution are disscussed and conclusions are drawn for this research work in the end of this paper.