Abstract:In ground communication systems, microwave components of the base station are exposed in air for a period of time, usually influenced by temperatures, humidity, pollution and other factors. The original clean surface of the components is oxidized and contaminated, leading to poor contact and Passive Intermodulation(PIM), which interferes with the communication system. Therefore, PIM is inevitable in ground mobile communication system. In the satellite communication system, due to the long communication distance and platform limitation, the common transceiver mode should be adopted. As the receiver sensitivity increases, the requirements on PIM indexes are more strict. PIM is one of the key problems for realizing the high reliability of system. The characteristics and research progress of PIM interference in ground mobile communication and satellite microwave components are introduced, which can provide a reference for PIM research.

Abstract:Passive Intermodulation(PIM) is one of the key reliability problems of the microwave circuits. With the development of microwave system integration and miniaturization, the microstrip circuits are widely used in the industry and studied by the researchers. Therefore, it has great value to investigate the physical mechanisms, the evaluation methods, and the suppression technologies of the PIM interference in the microstrip circuits. The nonlinearities in the microstrip circuits are usually distributed, which are usually different from that of the cavity based microwave devices, and they are also associated with the substrate materials, the fabrication processes and the device structures. All these make it difficult to solve the PIM problems in the microstrip circuits. Though more and more PIM investigations for the mircostrip circuits have been reported in recent years, the practical high-quality circuit-design still faces challenge. In this paper, we review the recent developments about the PIM phenomenon in the microstrip circuits including the nonlinearity mechanisms, the modeling and simulation methods, and the measurement technologies, which could provide comprehensive guidance for the future study of the PIM problems in the microstrip circuits.

Abstract:As an important electrical connector, the reliability of spring electrical connection directly affects the electrical performance of the device. It is important to study the regularity and influencing factors of spring electrical connection for improving equipment performance and saving cost. In this paper, through software simulation and experimental measurements, the rule is tested that the contact resistance decreases with the decrease of the resistivity of the contact metal surface material and decreases with the increase of the applied force when the electrical connection parts represented by the spring are under weak external force(<2N). Through the analysis and calculation of mechanical contact theory, it is verified that the contact resistance is affected by the resistivity and external force of the material. The mechanism of contact resistance of the projectile is clearly explained, and the relationship between contact resistance of the projectile and different contact interfaces can be more accurately judged.

Abstract:A method is proposed to measure electrical contact Passive Intermodulation(PIM) based on a slot waveguide, which is separable and can simultaneously measure contact resistance and PIM. On this basis, the statistical behavior of metal contact PIM is investigated. Firstly, it is determined that the existence of oxide film on the surface of Device Under Test(DUT) with aluminum and silver coated aluminum, is the main reason for the producing of PIM, by characterizing the surface topography and analyzing the components. Secondly, the contact resistance(R_c) and its statistical regularities are obtained by repeating loading-unloading at some certain pressure with four-terminal sensing method. Finally, the relationship among contact resistance, PIM and contact pressure of aluminum alloy and silver coated aluminum alloy are measured by special designed slot waveguide measurement system. The experimental results show that, the contact resistance of the contact joint has the statistical characteristic under the certain pressure and its PIM value is also fluctuant. Both the contact resistance and PIM value decrease as the contact pressure increases and their fluctuant ranges decrease.

Abstract:The Passive Intermodulation(PIM) value of the reflector's metal mesh, an important performance parameter of high-power spaceborne mesh antenna, is tested traditionally by the radiation method which requires a large size of metal mesh sample and has a low test efficiency. A 3-port PIM test method is proposed for the metal mesh, which is carried out by adopting a waveguide testing system based on the 3-port module. A PIM performance test system based on the 3-port module is constructed in order to verify the effectiveness of this test method. The PIM performance test is carried out on two metal mesh samples of the traditional type and the improved type, respectively. The test results show that the PIM performance of the improved metal mesh sample has been improved by 50 dB than that of the traditional mesh sample. This metal mesh's PIM test system bears the characteristics of high integration, stable performance, and high system construction efficiency. The test sample size for the proposed method is reduced to about 1/10 than that of the traditional method. The proposed 3-port PIM test method provides support for the rapid research of novel low-PIM-level metal meshes.

Abstract:Wireless communication systems are increasingly developing towards high power and high sensitivity. The interference of Passive Intermodulation(PIM) on signal transmission is one of the difficult problems to be solved in the development of communication systems. Based on this, the mechanism of passive intermodulation is summarized in this paper. The research progress of passive intermodulation suppression technology from three aspects, such as reducing contact surface roughness, applying coating materials and optimizing device structure, are mainly reviewed. Moreover, the advantages and disadvantages of different suppression technologies are analyzed. Finally, the development trends of passive intermodulation suppression technology are prospected. This work aims to provide reference and guidance for relevant researchers.

Abstract:In the context of rapid development of terahertz communication technology, the complex scattering property of wireless channel caused by building occlusion, bad weather and other reasons poses new challenges on secure and reliable data transmission. The confidentiality property of terahertz communication contains risks such as leakage and eavesdropping under the wide area of the communication network. Investigations on the feasibility of existing encryption and anti-eavesdropping methods in terahertz communication systems, promoting the innovation and application of physical layer security technology, and achieving stable and secure data transmission have become a valuably urgent topic. In this paper, from the perspective of secure communication, some of the existing physical layer encryption and anti-eavesdropping methods are presented and analyzed; their fusion applications and efficiencies in terahertz communication are summarized; and the trends on future development are expected.

Abstract:The spectral characteristics of solid α-lactose are studied by using the attenuated total reflection Terahertz Time-Domain Spectroscopy(THz-TDS) technique. Firstly, the experiment is based on the traditional transmission spectroscopy technique, and two strong absorption peaks at 0.53 THz and 1.36 THz are obtained. In addition, taking two samples ofα-lactose with the doses of 1.8 mg+2.56 mg and 1.8 mg respectively the absorption lines are obtained by the attenuated total reflection THz-TDS system. Strong absorption peaks appear at 0.53 THz and 1.36 THz. Increasing the sample dose has no effect on the absorption intensity at 0.53 THz and 1.36 THz. Finally, taking two samples of α-lactose with the same doses, one is screened to 75 μm, and the other is not screened, the absorption lines are obtained by the attenuated total reflection THz-TDS system. Strong absorption peaks appear at 0.53 THz and 1.36 THz. When the particle size of the sample is reduced, the absorption intensity increases around 0.53 THz and 1.36 THz.

Abstract:Micro-motion of helicopter blades forms the micro-Doppler effect, which is a kind of important characteristic and is extremely important for identifying different types of helicopters in the battlefield environment. Determining the micro-motion characteristics of helicopter rotor is the precondition for the identification. Because of the short wavelength and significant Doppler effect of terahertz radar, it is urgent to determine the fretting characteristics of the rotor target in terahertz band. Firstly, the propeller targets with even and odd blades are modeled. Then, the targets are simulated by using the radar in microwave band(3 GHz) and terahertz band(120 GHz, 220 GHz), and the Doppler frequency shift information is extracted from the characteristics of the target echo signal. Using Short Time Fourier Transform(STFT) in time-frequency analysis, the influence of the target and the radar parameters on the Doppler effect and the modulation relationship are contrasted. The simulation results show that the micro-Doppler effect in Terahertz band is more significantly enhanced than that in microwave band, and the Doppler curve is clearer and more details of the blades can be observed under the condition of the same rotation speed, angle of view and blade length. Using terahertz radar to extract micro-Doppler information can provide significant features for helicopter target recognition.

Abstract:The evaluation of System of Systems(SoS) contribution rate is an important index for the effectiveness assessment of electronic information equipment SoS. Taking a typical electronic information equipment for example, the essence of electronic information equipment SoS is investigated, and its complexity such as openness and dynamic property are analyzed.It is pointed out that the contribution rate of electronic equipment SoS should be evaluated by systematic theory. Based on the meta-synthesis method, the integrated method process of contribution rate evaluation of typical electronic information equipment SoS is studied by taking radar equipment as an example, and three important steps of the process are given, including the qualitative integrated judgment and understanding of the connotation, essential characteristics and function mode of contribution rate of electronic information equipment SoS, the indexes selection and evaluation combined with qualitative and quantitative evaluation of SoS contribution rate, as well as the comprehensive integration methodology from qualitative to quantitative analysis and verification of system contribution rate evaluation indexes, which provides reference for contribution rate test and evaluation of electronic equipment test system.

Abstract:Aiming at the nonuniform weighting for covariance lags in virtual array interpolation, the covariance matrix reconstruction of the coprime array is modeled as the low-rank matrix completion and atomic norm reconstruction. A novel covariance matrix reconstruction algorithm based on atomic norm for coprime array is proposed. Firstly, the Generalized Augmentation Approach(GAA) is utilized to obtain a partial covariance matrix of the coprime array. Then the partial covariance matrix is completed with the truncated mean singular value threshold method and reconstructed through the atomic norm minimization. A robust positive definite Toeplitz covariance matrix is accomplished. The proposed algorithm makes full use of the information contained in the coprime array to improve the stability of Direction of Arrival(DOA) estimation algorithm and reduce the computational complexity.

Abstract:In All-Digital Transmitter(ADTx) system, Radio Frequency Pulse Width Modulation(RF-PWM) encodes the amplitude and phase information of baseband modulation signal into the width and position of output pulse. Due to the non-ideal characteristics of digital signal processing devices, the rising edge and falling edge of the clock signal have jitter errors, which affects the output signal quality of RF-PWM. Based on three RF-PWM implementation schemes, the distortion term caused by timing jitter is determined by formula derivation, and the mathematical analytical expressions of the noise floor of different schemes' output pulse signals under the influence of timing jitter are given. Finally, Matlab is employed to simulate and verify the fundamental, odd harmonic and noise floor of different schemes under the condition of timing jitter, which proves the correctness of theoretical derivation. At the same time, the Error Vector Magnitude(EVM) and Adjacent Channel Power Ratio(ACPR) of signal are simulated, and the influence of timing jitter on the performance of signal in-band and out-of-band is analyzed. The results show that the nonlinear distortion caused by timing jitter is mainly reflected in the improvement of noise floor. For different RF-PWM implementation schemes, the influence characteristics of timing jitter are different, among which the five-level scheme has a restraining effect on timing jitter, and it increases with the increase of time resolution.

Abstract:A dual-feed circularly polarized patch antenna based on Substrate Integrated Coaxial Line(SICL) feed is proposed. Dual-feed point method is utilized to realize circular polarization. With the backed cavity, the antenna performance, for example, the bandwidth, the back lobe, the mutual coupling and the structure miniaturization, has been improved. Simulation results show that the antenna can achieve S₁₁<-10 dB(relative bandwidth is 28.8%@51 GHz) at 42.42~56.69 GHz, and the axial ratio is less than 3 dB and the maximum gain is 8.9 dBi at 45.26~48.08 GHz.

Abstract:A Ku-band high gain eight-channel Transmitter and Receiver(T/R) module is discussed and fabricated with the advantages of highly integrated design of Low Temperature Co-fire Ceramic(LTCC) technologies. The module improves the transmit and receive gains of the component effectively by using bidirectional amplifier. By combining the silicon aluminum shell and the aluminum alloy heat dissipation teeth, the heat conduction ability of the module under continous wave emission conditions is improved greatly. A transmitting power level of 1 W, an emission gain greater than 45 dB, and a receiving gain greater than 29 dB, a receiving circuit noise less than 3.5 dB of the eight-channel T/R module are realized, with a size of 84 mm×48 mm×6 mm, and a low weight of 60 grams. The eight channels of the module have good consistency and stable performance. The module has a good engineering applicability value.

Abstract:To address the problems of poor robustness and poor detection accuracy of multimodal remote sensing image saliency detection, this paper proposes a method based on the novel and efficient Multi-modal Edge aware Guidance Network(MEGNet), which mainly consists of a salient detection backbone network for multi-modal remote sensing images, a cross-modal feature sharing module and an edge aware guidance network. First of all, a Cross-modal Feature Sharing Module(CFSM) is used during feature extraction for remote sensing image pairs, which encourages different modalities to complement each other in the feature extraction process and suppresses the influence of defective feature data from different modalities. Secondly, based on the Edge Aware Guidance Network(EAGN), the effectiveness of edge features is detected through the edge map supervision module and the final salient detection map will have clear boundaries. Finally, experiments are carried out on three kinds of saliency objects detection remote sensing image datasets. The average Fβ, Mean Absolute Error(MAE) and Sm scores are 0.917 6, 0.009 5 and 0.919 9, respectively. The experimental results show that the proposed MEGNet is suitable for saliency detection in multi-modal scenes.

Abstract:Multi-channel isolation transmission based on fragmentation of information data is one of the common methods to ensure the security of information transmission, but it lacks the theory of provable security and evaluation methods. A multi-channel fragmentation transmission system is formally defined firstly, including data encryption, fragmentation multi-channel transmission and recombination functions. Then from the data leakage probability analysis in the process of multi-channel transmission, the provable security level of multi-channel fragmentation transmission technology is defined. A set of security level evaluation methods for multi-channel data fragmentation transmission is established to verify the effectiveness of the security level evaluation method from the theoretical point of view. This paper provides theoretical guidance for the realization of multi-channel data fragmentation transmission technology and guidelines for the security evaluation of multi-channel data fragmentation transmission system. The proposed method can also be applied to the security analysis of data fragmentation storage and transmission.

Abstract:Micro-blog text data is high-dimensional, bearing the obvious features of synonymy and polysemy. Traditional topic detection method based on Vector Space Model(VSM) combined with K-means has some problems such as low accuracy, complex calculation, and being difficult to determine the center of clustering. A Relevance Vector Machine(RVM) optimized VSM method is proposed to realize the text vectorization. Firstly, the dimension of VSM feature vector is reduced automatically by using the adaptive feature selection ability of RVM, and then Principal Component Analysis(PCA) is applied to determine the cluster center of K-means clustering algorithm. K-means algorithm is employed to get the clustering results. Finally, according to the number of micro-blog forwarding and comments, the topic with the largest heat index is the current hot topic. The results show that compared with two traditional methods, the accuracy of the proposed method is improved by 7.3% and 1.1%, and the real-time performance is improved by 45% and 53%, respectively.

Abstract:It is difficult to distinguish the spaceborne single-photon laser echo signal mixed with the noise. A two-step method for denoising spaceborne single-photon data based on statistical histogram is proposed. In order to eliminate noisy photons in the spaceborne single-photon echo data, a small window histogram method along the track is used for coarse denoising, and then a distance square statistical histogram method is used for fine denoising. The echo photon data of the Advanced Topographic Laser Altimeter System(ATLAS) spaceborne single photon lidar under three typical conditions of strong and weak beam, day and night, flat ground and mountain are selected as experimental data. Combined with the official results of ATLAS and based on the confusion matrix, the de-noising accuracy is calculated. Experimental results show that the denoising accuracy of strong beam data is 98.86%, and that of weak beam data is 96.94%; the denoising accuracy of night data is 99.02%, and that of daytime data is 98.86%; the denoising accuracy of mountain data is 96.28%, and that of flat data is 96.94%. The results show that the proposed method is suitable for spaceborne single photon data denoising under above three typical conditions.