Unmanned aerial vehicle (UAV) relay can effectively improve the coverage and performance of land communications, which makes it prospective to be deployed in various applications. However, there exist some practical challenges we have to face in UAV-enabled communication system designing, such as secrecy and wiretapping threats due to the air-to-ground line-of-sight communication link, and energy consumption issues because of the limitation in energy-constraint. To deal with these two key issues, we develop a novel UAV swarm assisted multi-hop mobile relay system which can be applied in special scenarios with severe blockage or long distance to enhance communication performance. An effective cooperative transmission scheme is proposed to address the security issues in a physical layer security perspective, where several UAVs serve as multi-hop relays forwarding information between ground users and the other UAVs are employed as friendly jammers confusing the ground eavesdroppers. The minimum secrecy energy efficiency (MSEE), namely the minimum achievable secrecy rate per energy consumption unit over legitimate links, is designed as the performance indicator to quantify the secure and energyefficient transmission. Subject to information-causality, transmit power, and mobility constraints, the UAV swarm assisted MSEE maximization transmission scheme is formulated into a complicated non-convex problem. To solve the non-convex fractional optimization problem, we decouple the original problem into sub-problems and further propose an efficient algorithm by applying the block coordinate descent method, successive convex approximation (SCA) techniques, and Dinkelbach method. Numerical results demonstrate that the proposed scheme achieves considerable performance enhancement on MSEE compared with benchmark schemes.

Due to the air-to-ground line-of-sight communication links, it is challenging to deal with the security threats in the wireless system with unmanned aerial vehicles (UAVs) integrated. In a UAV-enabled mobile relay system, the paper proposes a novel cooperative communication scheme from a physical layer security perspective to address the security issue. Specifically, in a mobile relay system where a UAV relay is employed to forward confidential information between two ground users, an eavesdropper exists on the ground wiretapping the relay UAV. To enhance the security performance of the system, we introduce a friendly UAV jammer transmitting interference signals and confusing the eavesdropper. A secrecy rate maximization problem is then formulated, subject to mobility constraints, power constraints on both UAVs and the information-causality constraint on the relay UAV. To solve the non-convex optimization problem with closely coupled variables, we decouple the problem into more tractable subproblems. An iterative algorithm is thus proposed to optimize the transmit power and flight trajectory of both UAVs alternately via the update-rate-assisted block coordinate descent and successive convex approximation techniques. Simulation results demonstrate that proposed cooperative design can significantly improve the secrecy rate of the UAV-enabled mobile relay system compared to benchmark schemes.

With the rapid development of railway traffic, traffic safety has become a focus. The ZPW-2000A jointless track circuit is an important part of train control systems. Currently, the fault detection of the ZPW-2000A jointless track circuit still relies on the experience of maintenance personnel, which can introduce several problems, such as a low fault detection efficiency and large amounts of required labor. Although some artificial intelligence fault detection algorithms for the ZPW-2000A track circuit have been developed, their detection accuracy is not high enough to meet the needs of large-scale applications, and due to security requirements, the actual ZPW-2000A track circuit fault data cannot be directly obtained in large quantities. To solve these problems, an equivalent theoretical model of the Chinese ZPW-2000A jointless track circuit is proposed by using four-terminal network theory. Through this equivalent theoretical model, the original fault data were collected. Considering that the relationship between fault data and fault types of the ZPW-2000A jointless track circuit is not obvious, a deep belief network was designed to detect the fault modes of the ZPW-2000A jointless track circuit. In order to optimize the deep belief network performance, the particle swarm optimization algorithm optimized by the genetic algorithm (GAPSO) was selected to optimize the deep belief network. The simulation experiments indicated that the optimized deep belief network could achieve a 98.5% fault detection accuracy and a 98.6% F1 Score rate, which showed that the deep belief network optimization by the particle swarm optimization algorithm which was optimized by the genetic algorithm (GAPSO-DBN) model proposed in this paper, had high accuracy and robustness. The results show that it had higher accuracy and robustness than other fault detection methods, and it can greatly improve the level of ZPW-2000A track circuit fault detection in the future. INDEX TERMS ZPW-2000A jointless track circuit, deep belief network, fault detection, particle swarm optimization, four-terminal network, genetic algorithm.

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Polarizer is a key component of the liquid crystal display panel, and the optical properties mainly include transmittance, degree of polarization and chromaticity. Polarizer is made of a multilayer optical film, in which the PVA polarizing film is the core structure for realizing the polarization of the whole polarizer. PVA polarizing film is commonly manufactured through a multi-step craft, including rinsing, dyeing, stretching and correcting. The correction process has a significant impact on the final apparent color and optical properties of the polarizer. In this study, the KI concentration in the correcting tank, ranging from 1% to 3%, was systematically investigated. With the increase in KI concentration, the Raman vibration peak at 160 cm−1 representing I5− ions gradually weakened, while the Raman vibration peak at 110 cm−1 representing I3− ions gradually increased, indicating that the KI in the correcting tank changed the chemical equilibrium of polyiodide ions in PVA. Then abundant chromophore I5− ions were consumed and I3− ions generated, so that the apparent color of PVA polarizing film gradually changed from dark blue to dark gray, and the chromaticity a-value and b-value gradually increased. The change in the concentration of dichroic species (I5− and I3−) in PVA polarizing film had directly affected its transmittance in the visible range. From the UV-Vis transmittance spectrum of PVA polarizing film, when the I5− ions were consumed and I3− ions generated, the transmittance of PVA polarizing film in the region of 675–525 nm wavelength increased gradually while the polarization degree also increased.