Photoconductive antennas are of great scientific and industrial value as the key components for generating and detecting terahertz radiation in terahertz time-domain spectrometers. In this paper, Molecular Beam Epitaxy(MBE) is utilized to prepare InGaAs/InAlAs superlattices as light-absorbing materials for 1 550 nm photoconductive antennas. The high growth quality of the materials is verified by Atomic Force Microscopy(AFM), Photoluminescence(PL), and high-resolution X-ray diffraction. The mesa-structured photoconductive antenna with flat sides is obtained by optimizing the preparation conditions. The fabricated photoconductive terahertz transmitting antenna achieves a spectral width of 4.5 THz in a terahertz time-domain spectroscopy system with a dynamic range of 45 dB.

A metamaterial absorber based on vanadium dioxide(VO2) is presented. This structure consists of three layers including two vanadium dioxide circles, intermediate dielectric layer, and metal substrate from top to bottom. The simulated data shows that the absorber has two strong absorption peaks, at 4.96 THz and 5.64 THz respectively, and the corresponding absorption rates reach 99.1% and 98.5%. The physical mechanism of absorption is clarified by using the impedance matching theory and the electric field distribution. The effect of the structural parameters on the absorption rate is also analyzed. In addition, the proposed absorber can regulate the absorption rate flexibly, which provides a good scheme for the realization of terahertz wave regulation, filtering and other functions. Therefore, this absorber has potential applications in image processing, biological detection, and wireless communication.

Terahertz communication has the advantages of both microwave communication and optical communication, which is one of the most effective technical means to solve the problem of communication capacity shortage. In order to solve the problems of serious absorption loss and poor external disturbance resistance in terahertz band, a new terahertz fiber based on circular Photonic Crystal Fiber(PCF) structure is designed to support high performance Orbital Angular Momentum(OAM) modes transmission. The existing common materials are used as the fiber base material, and the high absorption loss of materials is offset by the innovation of hollow porosity arrangement in the fiber structure. The optimal parameters are selected to realize the stable transmission of six OAM mode groups with high mode quality, low confinement loss and wide bandwidth. The mode purity is above 88.9% and the confinement loss is below 10-7 dB/m in the 0.2~0.9 THz band. Through simulation, the key problem of combining terahertz and OAM technology is solved, which lays a theoretical foundation for the application of Mode Division Multiplexing(MDM) technology in terahertz communication system.参考文献原文> LI Shuhui, WANG Jian. Multi-orbital-angular-momentum multi-ring fiber for high-density space-division multiplexing[J]. IEEE Photonics Journal, 2013,5(5):7101007.

The near-vertical backscatter characteristics of complex ground objects are one of the important factors affecting the echo waveform of radar altimeter. The measured data fitting method is adopted based on the parameter model to acquire the near-vertical backscattering coefficient, but it is only suitable for the ground and sea surface targets with flat terrain and relatively single medium. In this paper, with the mapping between Range-Doppler domain and geospatial domain, aiming to the simulation experiment data and the measured real data for airborne radar altimeter's Range-Doppler image of complex ground objects, a backscatter coefficient inversion algorithm is proposed and the obtained backscatter coefficients are quantitatively analyzed. It is verified that the proposed inversion algorithm has certain applicability and reference value.

As a high-throughput satellite resource allocation method, beam-hopping technology can flexibly configure on-board resources according to ground service requirements, thereby improving onboard resource utilization. In this paper, the beam-hopping technology is applied to the Low Earth Orbit(LEO) constellation scenario. In view of the two situations where the user's traffic is unknown and known a priori, the co-channel interference between beams is considered. Based on the iterative algorithm and convex optimization, a suitable low-orbit constellation is formulated. The two beam-hopping beam resource scheduling strategies can achieve higher system throughput compared with the traditional strategy, and bear better delay performance at the same time. They also adapt to the uneven distribution of user service requirements.

To realize Integrated Sensing And Communication(ISAC) in multi-antenna wireless systems, the design of integrated waveform is studied. A weighted objective function is proposed to reduce the correlation between waveforms while enabling data communication. The gradient projection method is employed to obtain the initial iteration points, and then the maximization-minimization algorithm is applied to get the waveform design results. Simulation results show that the designed integrated waveform achieves the tradeoff between the performance of communication and sensing, and can be applied to actual wireless systems.

As the number of wireless communication users grows, the macro cells are overloaded, and the edge users are gradually intensified by the interference of the neighboring micro-cell, resulting in a decrease in communication service quality of the entire cell. In order to solve these problems, the Particle Swarm Optimization(PSO) algorithm and Cell Range Expansion(CRE) technology are combined to dynamically adjust the service area of Femto Base Station(FBS) and find out the optimal offset value of CRE, so as to relieve the load pressure of high hotspot area of Macro Base Station(MBS) and improve the overage transmission rate of system users for the two-layer wireless heterogeneous network. The simulation results show that the number of macro cell users, who use the dynamic CRE bias selection, is decreased by nearly 10%, that is, uninstalling some macro users to access the micro-cell, realizing the effect of unloading macro users for the macro cell. And the Signal to Interference plus Noise Ratio(SINR) of the micro-cell edge user is increased by about 2 dB. The average transmission rate of system users is increased, and the communication service quality of the whole community is greatly improved.

A cross-coupled chaotic signal generator circuit is presented. Chaotic oscillation conditions of the cross-coupled circuit are proved through establishing nonlinear chaotic model. The stability enhancing mechanism of left-and-right complementary property is analyzed by splitting the cross-coupled circuit into two complementary two-stage chaotic circuits. The chaotic attractor and output spectrum are simulated and tested by tuning the port impedance. Results show that, the stable chaotic output impedance of the cross-coupled chaotic circuit is below 80 Ω, the stability is enhanced by 6 times than that of the conventional circuit with 500 Ω output impedance. The output chaotic signal spectrum is divided into three bands covering 1.5~11.4 GHz, which is enhanced by 50% than that of the conventional circuit.

The presence of noise has a great impact on the accurate measurement of electromagnetic radiation. The influence of noise cannot be ignored especially for the measurement of electromagnetic radiation of some weak signals. At present, the way to correct the electromagnetic radiation measurement results is to reasonably set the parameters of the spectrum analyzer, which is not universal because there exist differences in parameters setting for different spectrum analyzers. In this paper, by studying the characteristics of the base noise of the spectrum analyzer and the noise in the measurement environment, a suitable noise model is built and its rationality is proved through experiments. This method can provide a new way for the assessment and measurement of electromagnetic field exposure.

The syntactic structure of event sentences contributes to semantic understanding. A novel event detection model called BERT(Bidirectional Encoder Representations from Transformers) +D (Dependency)-T(Tree)-LSTM(Long Short-Term Memory network)+D-Attention(BDD) is proposed, which aims to learn semantic and syntactic representation of sentences jointly to enhance the event-sentence understanding ability. Taking the word vector based on BERT as the information source, D-T-LSTM model is designed to integrate the learning of syntactic structure and sentence semantics. An attention mechanism based on the dependency vector is added to strengthen the distinction of different syntactic structures at the aim of event detection. Experiment results on the Chinese Emergency Corpus(CEC) prove the effectiveness of BDD. The precision, recall and F1 value of BDD are rather optimum, and the F1 value is 5.4% higher than that of the benchmark model, and the recall rate is 0.4% higher.

In order to improve the ability of polarimetric radar in hydrometeor classification, more parameters like reflectivity(ZH), differential reflectivity(ZDR), differential phase rate(KDP), and correlation coefficient(ρHV), are adopted to build the standard deviation of reflectivity SD

In the era of big data, how to quickly extract effective information from the massive, complex and diverse electromagnetic big data based on artificial intelligence technology represented by machine learning is a research hotspot. However, machine learning algorithms for electromagnetic data are diverse and variable, and it is difficult for the people without relevant professional and programming knowledge to get started. To solve the complex programming problem of Electromagnetic(EM) big data mining, a graphical programming platform for EM big data is proposed. Various algorithms of machine learning are designed into independent components so that users can build machine learning models and workflows to analyze data without writing code. The EM data is analyzed by visual diagrams to help users better understand the data. The platform can bulid workflows quickly and it is easy for users to get started.

The sensitivity threshold of differential signal receiving chip(DS26C32ATM) is studied by Pulse Current Injection(PCI) technology. The experimental results show that the injection current on transmission line increases with the increase of pulsed voltage, and the interface circuit of differential signal receiving chip is damaged by electromagnetic pulse interference. The failure mechanism of the damaged chip is analyzed and the fault position is located. Based on the analysis results of failure mechanism, the protection of differential signal receiving chip is studied. The experimental results provide a reliable basis for the experiment of anti-electromagnetic pulse interference.

An S-band frequency tunable filter chip is designed based on monolithic microwave integrated circuit technology. The channelized structure is adopted in the tunable filter chip to realize wideband frequency agility by selecting different channels. In each channel, a cascaded form of multistage amplifiers and passive filter network is adopted to improve the frequency selectivity of the filter circuit. The varactor diodes are employed in the passive filter network to achieve continuous tuning

The statistical distribution of bipolar transistors' performance will change after ionizing radiation, from a symmetrical normal distribution before radiation to an asymmetric log-normal distribution. This statistical characteristic conversion lacks a clear physical image. In order to explain this transformation process from the microcosmic level, a large sample of ionizing radiation effect experiments for customized transistors are carried out to obtain the statistical characteristics of base currents and interface trapped charges before and after radiation, and the statistical characteristics of the two are found to be consistent. Based on the analytical physical model of the base current, it is found that the transformation of the base current statistical characteristics before and after radiation are originated from the transformation of the interface trapped charges. Based on the central limit theorem, the physical explanation for the statistical characteristics transformation of interface trapped charges before and after radiation is given, which comes from the physical model of interface defects in the form of multiple random variables.