Abstract:A design method of low Passive Intermodulation(PIM) coaxial connectors using noncontact inner conductors is proposed in this paper. Coaxial connectors are widely used in satellite communication systems, and their PIMs seriously affect the communication performance. Two coaxial connectors with noncontact inner conductors in S-band and C-band are designed and implemented, respectively. The measured |S₁₁| of the proposed coaxial connectors is less than -30 dB from 2.1 GHz to 2.7 GHz in S band, and the insertion loss is better than 0.3 dB. The measured |S₁₁| of the C band connector is less than -30 dB from 3.5 GHz to 4.2 GHz, and the insertion loss is better than 0.2 dB. The proposed coaxial connector design method with noncontact inner conductors may lower the PIM of a coaxial connector and has bright future applications in satellite communication systems.

Abstract:Principles of Passive Intermodulation(PIM) production and its control method are introduced. After mainly researching and applying the PIM control technologies for the feed chain of multibeam antenna in high throughput satellite, including optimal design of high isolation of feed system, flange face choke of feed unit, PIM source control of feed array mounting plate, low PIM design of Ka-band PIM test system, etc., a Ka-band multibeam antenna achieves the goals that the 7th order PIM performance of the feed unit is controlled to ≤-135 dBm at temperatures between -60 ℃ and +100 ℃，and the performance of the feed array is controlled to ≤-140 dBm at normal temperature. The application of the product demonstrates the validity of the proposed technology for PIM control, which is of benefit to high performance antenna design.

Abstract:The uniform microstrip line is the basic structure of the microstrip circuit, and it is of great significance to establish the Passive Intermodulation(PIM) analytical model of the microstrip line. Based on the controlled source equivalence, in the lumped circuit equivalent model of the microstrip line, the distributed parasitic nonlinear PIM source in the microstrip line is modeled as a second controlled current source or voltage source, to obtain the matrix expression of the relationship between the voltage and current in the stripline PIM. The analytical calculation model of the uniform microstrip line PIM with the parasitic nonlinear mechanism is finally established. The experimental verification is carried out by comparing the transmission intermodulation and reflection intermodulation laws of nickel-plated microstrip lines with different lengths and nickel-plated microstrip lines with different concentrations of phosphorus doping. The third-order relative permeability nonlinear coefficient of nickel coatings is extracted to be 1×10^-10 m2/A2 at 0.71 GHz. The proposed method based on controlled source equivalence provides a new idea for further establishing PIM models of other microstrip circuits.

Abstract:In modern high power communication distortion problems, the nonlinear mixing distortion effect in passive devices is always the most difficult to investigate, and it has attracted great interests in the modern high power and high density wireless communication system. Facing the typical nonlinear effect on passive device, Passive Intermodulation(PIM), this work concludes the common modeling methods and the important steps in PIM mechanism researches in recent years. In particular, targeting to the typical microscopic contact equivalence, PIM numerical transformation, and PIM prediction method in the engineered cases, the available methods are concluded for present, while a statistics based PIM prediction method for engineered device with dynamic range is proposed for the first time. This method can model the specific contact sources by using statistic strategy. The PIM prediction can be expressed as a statistical interval with the probability of each PIM value. This method can provide a big sample data for the engineered PIM prediction, and can guide the optimization of the PIM stability by controlling the detailed parameters.

Abstract:Passive Intermodulation(PIM) is a subject that attracts researchers in many fields, including base station antennas and satellites. In this work, a new setup for non-contact PIM measurement is proposed, which takes a Substrate-Integrated Slotted Waveguide(SISW) designed by field reconstruction as the excitation and reception path. The SISW with field reconstruction enables a solution for the evaluation of the non-linear characteristics of samples without RF ports through far-field suppression and near-field partitioning of the carrier signal. The proposed test platform combines the finite radiation characteristics of near-field antennas and the adaptability of the gradient test area to transceiver systems in the PIM evaluation. The experimental results suggest that the proposed test method presents considerable potential for application in terms of stability and resolution.

Abstract:With the development of multi-beam satellites communication and the wide application of transmit-receive technology, Passive Intermodulation(PIM) has become more seriously in the high power passive components. A PIM measurement method for phase adjustable Multiple-Input Multiple-Output(MIMO) multicarrier is presented based on that for dual carriers test according to the working mode of multi-beam satellites communication. Theories as well as experiments demonstrate the potentials and usefulness of the method. The experiments are conducted by the designed 7-channel test system under -60~60 ℃, and the PIM power level is below -155 dBm. The proposed test system has been applied to the multi-carrier components of satellites.

Abstract:In modern wireless communication, passive intermodulation is an important factor affecting the transmission quality of communication signals. Dual-tone intermodulation testing can effectively measure the nonlinear characteristics of passive devices, but it is hard to accurately predict the nonlinear distortion of devices excited by broadband signals. A method for analyzing the Passive Intermodulation characteristics of RF connectors with broadband signal excitation is proposed. Through the dual-tone intermodulation test, the passive nonlinear transmission mathematical models of RF connectors are derived. An equivalent broadband signal considering normal distribution is provided as the excitation source to the connectors. Mathematical calculation results of the proposed model show that the broadband Passive Intermodulation power distribution is the same as the broadband signal power distribution. The bandwidth of the nth-order broadband Passive Intermodulation is n times that of the original broadband signal. It is observed that the power of Passive Intermodulation products with broadband signal excitation are higher than that of the dual-tone signal. Thus, the signal to noise ratio decreases, indicating that the PIM interference induced by the broadband signal in the transmission process is more serious compared with that induced by two Continue Wave(CW) signals. Simulations are conducted by importing the proposed broadband signal and nonlinear transfer function to an equivalent circuit. The simulation results show good consistency to the theoretical calculation, which proves the effectiveness of this analysis method.

Abstract:Based on the classical half-wavelength filter theory, a half-wavelength magnetically coupled rectangular waveguide bandpass filter is studied and designed for terahertz communication system. The simulation results show that the center frequency of the filter is 118 GHz; the passband is 114.3~123 GHz; the relative band-width is 7.4%; the attenuation at 131.8 GHz is more than 30 dB; the passband insertion loss is less than 0.8 dB; and the pass-band return loss is more than 20 dB. After the physical test, the test results are basically consistent with the simulation results. This filter bears a simple structure and a low fabrication difficulty.

Abstract:A focus electrode modulation Pierce gun for G band traveling wave tube is designed, with a beam voltage of 20 kV, current of 50 mA, waist radius of 0.056 mm, and a waist position of 10.3 mm. The impact of thermal deformation on the performance of the electron gun is analyzed by using thermal-structure coupling analysis and electron beam trajectory simulation method. In order to eliminate the influence of thermal deformation, an assembly fixer for compensation is designed and verified by experiments. The electron gun has been applied in a variety of G band traveling wave tube, and has solved technical problems of the key component.

Abstract:In the satellite Internet of Things(IoT) scenario, as the number of terminals continues to increase, the spectrum resources are of shortage increasingly, and the traditional random access technology has low spectrum utilization, making the traditional random access protocol unsuitable for the high concurrent service requirements of the future satellite IoT. At the same time, satellite communication links are long and open, making it difficult to guarantee the security of special terminal signals. This paper proposes a hybrid random access scheme suitable for satellite Internet of Things. This scheme introduces the capacity improvement and security advantages of overlapping transmission, utilizes the ability of spreading code to control the instantaneous power spectral density, constructs non-orthogonal access conditions in the power domain, and achieves robust reception through iterative separation at the receiving end. The derivation analysis and computer simulation of the throughput performance of the proposed scheme are carried out. The simulation results show that the proposed scheme can improve the system throughput compared with the traditional random access protocol. At the same time, compared with common signal concealment methods, the proposed method utilizes the power advantage of conventional access data packets to strengthen the waveform concealment effect and improve the security of special information access.

Abstract:In order to estimate the azimuth angle and pitch angle of airborne target effectively, a two-dimensional Direction Of Arrival(DOA) estimation method based on sparse representation is presented. Firstly, it is necessary to construct two spatial compound angles containing the azimuth and pitch information of the target; then, one of the spatial composite angles is solved by sparse representation technology, and the other spatial composite angle can be decoupled into one-dimensional DOA estimation problem, which can be solved by matrix operation; finally, the azimuth angle and pitch angle are solved in pairs according to the solved two spatial compound angles. Compared with the existing algorithms, the proposed method is less affected by the number of snapshots, and has good performance under the condition of high signal-to-noise ratio and large angular interval.

Abstract:The digital modulation is necessary in order to meet the requirement of higher signal transmission rate in aerocraft data link. The better modulating performance and the longer communication distance can be achieved by digital modulation. The principle of 2 Continuous Phase Frequency Shift Keying(CPFSK) modulation with binary continuous phase is introduced. A 2CPFSK quadrature modulation algorithm based on software radio architecture is proposed.By using the phase accumulation property of 2CPFSK, the algorithm realizes the fractional interpolation of baseband data to adapt to the modulation of wide range code rate.A digital filter is also designed to improve spectral purity. The hardware platform of digital modulator based on Field Programmable Gate Array+Digital-to-Analog Converter(FPGA+DAC) architecture is designed and the algorithm is implemented.Compared with traditional analog Frequency Modulation(FM), this design improves the link performance by 1.7 dB and increases the communication distance of the system.

Abstract:A ±45° dual-polarized base station antenna with a small size and a broad bandwidth for 2G/3G/4G/5G mobile communication is proposed in this paper. The proposed antenna comprises two pairs of dipole radiation patches, two microstrip feedlines and a reflector. The radiation patches and microstrip feedlines are printed on a FR4 substrate with a thickness of 0.8 mm, using a process called double-sided printed circuit board. The reflector is grooved a circular slot and the antenna is fixed above it. The prototype of the proposed antenna is fabricated and measured. The results show that it has a bandwidth of 1.82~3.60 GHz(port1) and 1.64~3.41 GHz(port2) respectively, during which, the reflection coefficient is less than -10 dB, the port isolation is better than 18 dB, the cross polarization discrimination is better than 17 dB in the boresight direction, and better than 15 dB in ±60° directions, the half power beam width is approximately 65° and the front-to-rear ratio is better than 18 dB. With the advantages of wide bandwidth, small size, simple manufacturing process and low cost, the proposed antenna is suitable for mass production and is an excellent candidate for applications in 5G mobile communication base station.

Abstract:A design method of limiter based on PIN diode is proposed in order to improve the electromagnetic protection capability of the aeronautical Very High Frequency(VHF) communication equipment. The influence of different diode stages on the insertion loss and limiting performance of the limiter are simulated and researched by Advanced Design System(ADS). Furthermore, a semi-active limiter is designed, which is composed of PIN pair parallel structure, directional coupler with lumped parameter, rectifier circuit and matching circuit. To reduce the limiting level of the limiter, a bias voltage is provided for the diode by converting part of the interference signal into DC. The measurements show that the insertion loss of the limiter is less than 1 dB, the return loss is more than 20 dB, and the limiting level is less than 7.5 dBm, the power capacity is more than 25 dBm in 118~136 MHz, which realizes the low insertion loss and high limiting performance.

Abstract:The existing crowd counting methods are not suitable for the subway scene. Therefore, a crowd counting model based on convolutional neural network is proposed. The model takes the VGG16 as the front-end network to extract the shallow features, and an M-Inception structure is combined with the dilated convolution to form the back-end network, which can increase the receptive field and adapt to different sizes of pedestrian targets at different monitoring angles. And a weighted loss function combining the head count loss and density map loss is proposed. The proposed algorithm is compared with four existing models. The experimental results show that the Mean Absolute Error(MAE) and Mean Square Error(MSE) of the proposed algorithm are 1.46 and 2.13, better than those of the four comparison models. The proposed model is deployed to Hisilicon embedded chip. The test results show that the proposed model can achieve high computing speed and accuracy on the embedded chip, which can meet the requirements of the actual application scenarios.

Abstract:A large amount of earth observation data with the high spatial and temporal resolution is employed in many earth science applications. The spatiotemporal image fusion method provides a feasible and economical solution for generating high spatiotemporal resolution data. However, some of the existing learning-based methods are poor in extracting deep image features and utilizing the detail features of high-resolution image. A spatiotemporal fusion method is proposed for remote sensing images based on multi-level feature compensation. It uses two branches to perform multi-level feature compensation and proposes a residual module fused with a channel attention mechanism as the basic unit of the network, which can extract and utilize the deep features of high-resolution input images in more detail. An edge loss is proposed based on the Laplacian operator, which saves the computational cost of pre-training and achieves a good fusion effect. The proposed method is experimentally evaluated by using Landsat and Moderate-resolution Imaging Spectroradiometer(MODIS) satellite images collected from two regions in Shandong and Guangdong. Experimental results show that the proposed method bears higher quality in both visual appearance and objective metrics.

Abstract:A high-precision and digital true root mean square measurement method based on Field Programmable Gate Array(FPGA) is presented. Firstly, FPGA is employed to design Finite Impulse Response(FIR) filter to filter the AD sampled data. Furthermore, the improved RMS formula is adopted to calculate the true RMS of the signal. The mean value of the true RMS value of eight consecutive cycles is taken as the final measurement result. By designing the algorithms of serial extraction and division operations, the use of FPGA resources is reduced. The actual test of the prototype shows that the relative error between the measurement results and the true value of the signal is less than 0.5%. The solution has high measurement accuracy, good consistency, and less resources, which has certain reference value for the design of the true RMS digital measurement chip.