Abstract:Satellite communication, as an important means of communication, plays a key role in modern society. However, with the development of communication technology, satellite communication system is also facing the increasingly serious interference problem and the challenge of interference recognition technology. In view of this situation, this paper first introduces the common patterns of satellite interference signals, and summarizes the development trends of satellite interference signals. Secondly, the research status of satellite interference signal classification and recognition technology is introduced from three aspects: feature extraction based interference recognition method, traditional machine learning method and deep learning method, and their respective advantages and disadvantages are analyzed. Finally, the future classification and recognition technology of satellite interference signal is prospected.

Abstract:Satellite communication can provide over-the-horizon, large-capacity communication services. Nevertheless, there will be serious line-of-sight fading and significant multipath effect in urban areas, canyons, forests and other areas where the line of sight is seriously obscured, which will result in the degradation of network communication performance. Recently, an emerging technology―the Reconfigurable Intelligence Surface(RIS) is capable of dynamically altering wireless channels by flexibly controlling the electromagnetic properties of passive reflecting elements, providing a novel route for improving the quality of Low Earth Orbit(LEO) satellite communication. Based on the basic principles and advantages of RIS, the key technologies such as channel modeling, channel estimation, and beamforming in the RIS aided LEO satellite communication are discussed. Finally, the main challenges faced of RIS in practical applications are analyzed. This work can provide a reference for research in the field of RIS aided LEO satellite communication.

Abstract:For addressing the issue of unauthorized actions bypassing security mechanisms to attack systems in the integrated network of heaven and earth in the open electromagnetic environments, an improved Genetic Algorithm(GA) is proposed. It uses the Decision Tree(DT) algorithm as the fitness function, and significantly improves the interception rate of network attacks by deleting redundant features in the dataset.Anomaly classification is performed through machine learning, and the feature selection function of the genetic algorithm is employed to enhance the classification efficiency of machine learning. To verify the effectiveness of the proposed algorithm, the UNSW_NB15 and UGRansome1819 datasets are selected for training and testing. Four machine learning classifiers, namely Random Forest(RF), Artificial Neural Network(ANN), K-Nearest Neighbor(KNN), and Support Vector Machine(SVM), are used for evaluation. The performance of the algorithm is evaluated through indicators such as accuracy, F1 score, recall rate, and confusion matrix. The experiment results prove that the genetic algorithm as a feature selection tool can significantly improve the classification accuracy and achieve significant improvement in algorithm performance. Meanwhile, to tackle with the instability of weak classifiers, this paper further proposes an ensemble learning optimization technique, which integrates weak classifiers and strong classifiers for optimization. The experiment confirms the excellent performance of this optimization algorithm in improving the stability of weak classifiers.

Abstract:In response to the problems of resource waste and long propagation time delay in traditional satellite power control methods, this paper proposes a satellite adaptive power control method based on Online Gate Recurrent Unit(Online-GRU) channel prediction, which solves the cumulative error of offline prediction algorithms by updating network parameters through online training. The simulation results show that the proposed online training algorithm improves the prediction accuracy by 38.30% compared to offline algorithms, saves 63.21% of training time compared to Online Long Short Term Memory(Online-LSTM), and saves 55.74% of transmission power compared to the fixed transmission power method. At the same time, the proposed adaptive power control method has better robustness compared to the adaptive power control method based on ground timing feedback channel state.

Abstract:In the ultra-high dynamic and low Signal-to-Noise Ratio(SNR) communication scenarios like deep space communication, traditional signal acquisition methods suffer from inadequate dynamic range support, insufficient accuracy, and the use of a narrowband Doppler model leading to poor approximation accuracy in the received signal. A wideband Doppler model is employed to model the received signal and a chirp signal is used for Doppler estimation and timing estimation. A sampling rate adjustment method based on binary-search is proposed for the receiver to overcome the mismatch between the Doppler-stretched signal and the local signal sampling rate. The simulation results of the synchronization performance show that the proposed method using Up-Down Linear Frequency Modulation(UD-LFM) signal is capable of achieving a Doppler estimation error below 400 Hz and a timing estimation error below 60 ns under a SNR of -49 dB and a maximum Doppler frequency of 2 MHz. Compared to direct acquiring, the proposed method bears a higher acquisition accuracy and a lower SNR threshold.

Abstract:As an important supplement to the terrestrial network, the Low Earth Orbit(LEO) satellite network is an important part of the future earth-earth integrated network. However, due to the high moving speed of LEO satellites and the long propagation distance of satellite-ground communication, large propagation delay is caused, therefore a new calculation strategy for the Timing Advance(TA) of the uplink in the satellite-ground communication background of LEO satellites is needed. Based on the satellite-earth communication scenario of LEO satellite, TA and its provisions in the protocol are introduced firstly. Then, according to the characteristics of LEO satellite, a timing advance calculation method of LEO satellite communication is proposed. The effectiveness of the proposed scheme is verified by simulation analysis, which provides a reference for the design of satellite-earth communication system of LEO satellite.

Abstract:Adaptive antenna array technology is the most effective method for anti-interference of Global Navigation Satellite Systems(GNSS). However, this anti-interference processing generates carrier phase measurement bias related to the direction of signal incidence, which limits the effective application in high-precision measurement. In order to reduce the bias of anti-interference processing, an analysis method of introducing measurement bias is proposed, a corresponding model is established, and a software receiver is built for simulation experiments. The results show that without adding additional constraints, adopting the space frequency minimum variance distortionless response algorithm to achieve a space frequency anti-interference processor would not introduce carrier phase measurement bias, making it very suitable for applications in fields with strict requirements on anti-interference performance and measurement accuracy.

Abstract:A 260 GHz frequency tripler based on GaN Schottky barrier diode is proposed. Unbalanced structure is adopted with GaN SBD chip to improve the tolerable power and output power of the circuit. The height-reduced and width-reduced output waveguide structure is employed to suppress the second harmonic. The input and output filter of the frequency multiplier is designed with high and low impedance strip line structure. The test results show that the frequency multiplier achieves a maximum output power of 69.1 mW and conversion efficiency of 3.3% at 261 GHz with good harmonic suppression characteristics.

Abstract:Speed and accuracy must both be taken into consideration when using Terahertz Time-Domain Spectroscopy(THz-TDS) for security inspection of flammable and explosive contraband liquids in railway transportation. Due to speed limitation, the exact comparison of the precise fingerprint location between testing results and database, which is the common method adopted in laboratory analysis, cannot be directly utilized for the security inspection of large passenger flow. The reflective terahertz time-domain spectroscopy data for typical liquids in railway transportation is acquired under standard temperature, pressure, and normal humidity level(25 ℃, RH 60%). The categorized time-domain database for non-flammable and non-explosive liquids, flammable and explosive liquids, and strong acid, is established. Based on the database, the typical time-domain pulse patterns can be classified into four categories, namely terahertz time domain spectroscopy with obvious secondary pulse, minor secondary pulse, no secondary pulse and obvious negative secondary pulse. According to terahertz time domain characteristics, rapid detection with better than 80% of accuracy is achieved through classification decision model.

Abstract:The Software Defined Radio(SDR) requires that the Digital to Analog Converter(DAC) must have higher and higher sampling rate, and the bandwidth of the transmitted signal is wider and wider. The traditional method of the Digital Up Conversion(DUC) cannot meet the application requirements because of the limited clock frequency of the Field Programmable Gate Array(FPGA). An optimized design method of high speed DUC is proposed to improve interpolation filtering and digital frequency synthesis. Firstly, the mathematical model of high speed DUC is deduced and the traditional DUC structure is optimized and improved. The implementation structure of the efficient and flexible interpolation filtering, as well as the structure of the multichannel parallel digital frequency synthesis are designed. Secondly, the coefficients of interpolation filters are given as well as the computing method of phase parameters for parallel digital frequency synthesis. The hardware implementation shows that the function of the optimized method is correct , and the method is convenient for engineering application. The data rate of the output digital Intermediate Frequency(IF)signal can reach 960 MS/s. The optimized method can realize the interpolation of different multiples, and can also produce high speed Local Oscillator(LO) signals with different rates. It can meet the requirements of DUC applications which need to transmit wide band and high rate signals in SDR.

Abstract:When the signal is transmitted between systems with different modulation methods, the modulation format needs to be converted. Modulation format conversion is usually carried out on electrical signals and requires a process of electro-optical conversion after optical-electro conversion. Frequent optical-electro or electro-optical conversion usually increases the system cost. To address this problem, the implementation of format conversion from Binary Phase Shift Keying(BPSK) to Quadrature Phase Shift Keying(QPSK) and from QPSK to Quadrature Amplitude Modulation(16QAM) by using coherent superposition methods on the optical domain is proposed. In the experiment, the conversion model is simulated by Optisystem software, Taking 10 Gpbs pseudo-random signal as the test signal, it is successfully converted to QPSK and 16QAM observed by constellation diagrams. The Bit Error Rate(BER)of the conversion model and the direct QPSK and 16QAM transmission models are compared under different Optical Signal Noise Ratios(OSNRs), optical source bandwidths and signal powers. The BERs of the conversion model and the direct transmission remain highly consistent under the same conditions, indicating that this modulation format conversion method has high stability and accuracy, and can be applied to signal transmission between different systems.

Abstract:The spectral occupancy model of satellite spectrum sensing data in the temporal dimension is analyzed. Because the bottom noises in satellite receiving data are undulated in different time and space, the traditional modeling methods with fixed threshold are defective. Therefore, the adaptive threshold method is introduced to determine the noise threshold and preprocess the satellite spectrum data to obtain the satellite spectrum occupancy length sequence. In order to make an effective statistical analysis on the situation of the satellite spectrum, the probability density curve of the spectral occupation time length series is fitted by using the Poisson and exponential distribution methods, and a probability distribution model suitable for the satellite spectrum occupation time series is obtained. Based on the obtained satellite spectral occupancy state model, the state transfer matrix at a certain frequency point of the satellite channel is calculated by two-state Markov chains to predict the probability of outgoing channel occupancy and idle. In addition, the Back-Propagation(BP) neural network is trained through the data set constructed by satellite spectrum sensing data to predict the occupancy length of a certain frequency point. By calculating the Root Mean Square Error(RMSE) of the BP neural network and the conventional Long Short-Term Memory(LSTM) neural network prediction methods, 0.172 8 and 2.208 1 are obtained respectively. The evaluation results show that the BP neural network bears the advantage of high accuracy.

Abstract:Due to the complex electromagnetic environment, Electromagnetic Compatibility(EMC) problems emerge, and the impact of interference on transmission lines can not be ignored. Based on the classical field-line coupling model―Agrawal model, this paper calculates the electromagnetic coupling effect of each field-line parameter of Twisted-Wire Pair(TWP) with Finite-Difference Time-Domain(FDTD), and gives guidance on the field-line configuration to reduce the disturbance. Finally, the influence of each parameter on the electromagnetic coupling effect of the interference source on the twisted pair is summarized: length will affect the frequency selection characteristic at the terminal load; the effect of pitch is small, and the shorter pitch will decrease the coupling; the height significantly affects the electromagnetic coupling, and the higher the height, the greater the coupling is; the incidence angle, azimuth and polarization angle all influence the electromagnetic coupling effect differently, and it can be seen from the tangential component of electric field's slice diagram that the analysis on one corner needs to take into account the influence of the other two corners. The electromagnetic coupling effect of TWP transmission line is closely related to the field line parameters, and each parameter shows different degrees of influence. Through the calculation of the model in this paper and the analysis on the results, an effective configuration of low-disturbed TWP transmission line can be obtained.

Abstract:A Ku-band dual-channel 3D integrated T/R module for array antenna system is designed and fabricated based on silicon Micro-Electro Mechanical System(MEMS) technology. The module consists of two layers of silicon-based packages stacked by Ball Grid Array(BGA). The upper and lower layers of silicon-based packages are realized by five layers of silicon wafers through Through-Silicon-Via(TSV) and wafer-level bonding. The module integrates the functions such as 6 bit digital control phase shifting, 6 bit digital control attenuation and serial-to-parallel conversion, negative voltage bias, power modulation. The finished module size is only 15 mm×8 mm×3.8 mm. The test results show that in the Ku-band, the saturation output power of the transmitting channel is greater than 24 dBm, the gain of transmitting channel reaches 20 dB, the gain of the receiving channel is greater than 20 dB, and the noise coefficient is less than 3.0 dB. The assembly bears the advantages of high performance, light weight, small volume, high machining precision and high assembly efficiency.

Abstract:The beamforming method of current Orthogonal Frequency Division Multiplexing(OFDM) passive radar cannot effectively distinguish the target signals distributed at the same angle and different distances. A new frequency diverse processing method based on subcarrier extraction of passive radar receiving signal is proposed. The beampattern can be turned into range-angle dependent by extracting and processing different subcarriers independently for each receiving channel, thus the targets distributed at the same angle and different distances can be processed differently. And the effect of parameters in frequency diverse processing is also investigated. The increased degree of freedom in range dimension with frequency diverse processing can help focus the beam energy on the desired point in space, which improves the performance of radar in target detection, location, and clutter suppression. Simulation analysis shows that a dot-shaped beampattern can be obtained with frequency diverse processing for OFDM passive radar receiving signals, and the experimental results also verify the effectiveness and the practicability of the proposed method.

Abstract:A two-level Nested array is employed to construct a sparse L-shaped array. For this array, an angle estimation method based on compressed sensing is proposed. This method calculates the autocorrelation covariance matrix of the received data and quantizes it, and then reorders and removes the redundancy to obtain the incidence angle information of the virtual array. The length of the virtual array is much larger than that of the actual physical array, so compared with the uniform L-shaped array with the same physical array element, the array aperture and degree of freedom have been greatly improved. Finally, the orthogonal matching pursuit technique is adopted to solve the l₁ norm constraint problem of the virtual array. Computer simulation shows that the proposed algorithm has higher source resolution and better estimation performance under the conditions of high signal-to-noise ratio, high snapshot number and large angle interval.