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    20 October 2018 Volume 45 Issue 5
      
    Properties of vortex Lommel beam propagation in anisotropic atmospheric turbulence
    YANG Ruike, ZHOU Jing
    Journal of Xidian University. 2018, 45(5):  1-6+31.  doi:10.3969/j.issn.1001-2400.2018.05.001
    Abstract ( 506 )   PDF (1625KB) ( 597 )   Save
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    In order to reduce effectively the turbulence effects of optical wave propagation in the atmosphere and explore methods to mitigate turbulence effects, based on the vortex optical wave and Markov approximation theory for optical wave propagation in atmospheric turbulence and utilizing the method of decomposing the vortex beam into the superposition of the spiral harmonics in cylindrical coordinates, the models of the receiving power and crosstalk power are derived, for a non-diffracting vortex Lommel beam transmitted in non-Kolmogorov turbulence by considering inner and outer scales. The influences of the beam and turbulence parameters on the receiving power and crosstalk power are analyzed for Lommel beams propagating in anisotropic turbulence with different intensities and different degrees. The results show that with the increase of the anisotropic degree of turbulence, the receiving power of each angular momentum mode increases and the crosstalk power decreases, and they are greatly influenced by the power exponent for the non-Kolmogorov spectrum. Moreover, when the shape of the Lommel beam is closer to the circular symmetry mode, the effect of turbulence can be mitigated more. Theoretical analysis shows that the propagation property of the Lommel beam in anisotropic atmospheric turbulence is better than that of  the multiple Hankel-Bessel and Laguerre-Gaussian vortex beam.

    Topology control with a high reliability and low transmission overhead in DTN
    QI Xiaogang, MA Jiulong, LIU Lifang
    Journal of Xidian University. 2018, 45(5):  7-12.  doi:10.3969/j.issn.1001-2400.2018.05.002
    Abstract ( 822 )   PDF (630KB) ( 348 )   Save
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    Complex environment in delay tolerant networks may lead to node or link failure. In addition, the continuous movement of nodes and intermittent connectivity of the links also bring challenges to the reliable topology control. Effective reliable topology control schemes based on time-space graph are proposed in this paper, which are suitable for delay tolerant networks where nodes periodically move, such as satellite networks, interplanetary networks, other space network, etc. First, the network topology is transformed into a space-time graph. Then, the reliable topology control problem is defined. In the case that the network is connected, the topology control aims at finding the most reliable path for any pair of nodes and minimizing the transmission overhead. Simulation shows that the proposed methods can guarantee the reliability and reduce the transmission overhead. So the proposed topology control schemes are applicable to delay tolerant networks with predictable periodic topology.

    Novel algorithm for DOA estimation based on the sparse reconstruction
    WEI Juan;JI Yongxiang;NIU Junru
    Journal of Xidian University. 2018, 45(5):  13-18.  doi:10.3969/j.issn.1001-2400.2018.05.003
    Abstract ( 578 )   PDF (787KB) ( 294 )   Save
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    In order to improve the DOA estimation accuracy of far-field narrow-band signals with a low signal-to-noise ratio or few snapshots, a new weighted algorithm based on -norm is proposed. First, the covariance matrix of the output data of the array is generated by using the forward-backward spatial smoothing technique. Second, the cepstrum coefficient vector is constructed in the spatial spectrum function of the Modified Capon(MCapon). The weighted matrix in accordance with the weighted-norm is obtained. Finally,the SVD algorithm is used to reduce the dimensionality of the received data to obtain the model with the weighted-norm constraint based on sparse reconstruction. The algorithm makes spurious peaks in spatial spectrum suppressed, increases robustness and improves the DOA estimation performance with a low signal-to-noise ratio (SNR) or few snapshots without decorrelation processing.

    Joint optimization of the phase coded waveform and mismatched filter
    XU Leilei;ZHOU Shenghua;LIU Hongwei;MA Lin
    Journal of Xidian University. 2018, 45(5):  19-24+63.  doi:10.3969/j.issn.1001-2400.2018.05.004
    Abstract ( 350 )   PDF (696KB) ( 187 )   Save
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    For the high range sidelobes of the output of separately optimizing the phase coded waveform and mismatched filter, a joint optimization method for the  phase coded waveform with the constant mainlobe width (PCW-CMW) and mismatched filter is proposed. First, given the duration time of the transmit waveform and a needed range resolution, a criterion of jointly optimizing the PCW-CMW and mismatched filter is proposed. Then, based on a penalty method and the theory of p-norm, we reformulate the constraint and non-smooth optimization problem as an unconstraint and smooth optimization one. Finally, we use a least-pth algorithm to address it. Numerical results indicate that under the given duration time of the transmit waveform and keeping a constant range resolution, the peak sidelobe level of the joint optimization method can be decreased by 10.72dB and 4.77dB, respectively, compared with the separate design methods of the PCW and mismatched filter.

    Two-step multiple flow table construction algorithm in the software-defined network
    ZHENG Ling;QIU Zhiliang;SUN Shiyong;PAN Weitao;WANG Weina;ZHANG Zhiyi
    Journal of Xidian University. 2018, 45(5):  25-31.  doi:10.3969/j.issn.1001-2400.2018.05.005
    Abstract ( 418 )   PDF (662KB) ( 137 )   Save
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    Aimed at the problem of the rapid growth of the flow table size and the inefficient utilization of the flow table storage resources, this paper proposes a two-step multiple flow table construction algorithm. First, the match fields in the single flow table are split into multiple flow tables according to the flow classification vectors, so the wildcards in the flow table are eliminated. Second, the orthogonal decomposition is performed for each sub-tables, and the redundant entries are compressed. Experimental results indicate that the method can save more than 60% storage space for a given flow table. Compared with existing works, the flow table compression ratio is improved by 21.4% to 51.5%. FPGA verification shows that this method can be implemented in practical hardware and the pipeline processing speed is guaranteed. The data throughput can achieve 197 Million Packets Per Second (MPPS), which supports the 100 Gbit/s line-rate packet processing.

    Fusion method based on multilevel quantization in multistatic radar
    CAO Ding;ZHOU Shenghua;LIU Hongwei;SHAO Zhiqiang
    Journal of Xidian University. 2018, 45(5):  32-37+49.  doi:10.3969/j.issn.1001-2400.2018.05.006
    Abstract ( 613 )   PDF (589KB) ( 163 )   Save
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    In multistatic radar, a confidence region-based least square quantizer is proposed to satisfy the communication constraints. The “non-confidence region” is a continuous interval in the decision space of the generalized likelihood ratio of raw observations, while the rest part is called the “confidence region”. The “non-confidence region” is partitioned through the least square quantizer according to the communication bandwidth. The outputs of the local quantizers, also termed quantization levels, are regarded as the local decisions to be transmitted to the fusion center, where a non-coherent integrator is utilized for further global decision making. Global probability of false alarm and probability of detection are derived through the probability mass functions of the quantization levels. Simulation results indicate that the detection performance with 3bit quantization can approach that of the distributed generalized likelihood ratio.

    Estimation of the symbol period of MPSK signals over fading channels with impulsive noise
    ZHANG Junlin;LIU Mingqian
    Journal of Xidian University. 2018, 45(5):  38-42+95.  doi:10.3969/j.issn.1001-2400.2018.05.007
    Abstract ( 305 )   PDF (488KB) ( 101 )   Save
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    In order to solve the problem of the difficulty in blind estimation of the symbol period for Multiple phase-shift keying (MPSK) signals over fading channels under impulsive noise environment, a novel symbol period blind estimation method based on the fractional low order cyclic correlation coefficient is proposed. First, the analytical expressions for the fractional low-order cyclic stationary characteristic are derived by using the cyclostationarity of MPSK. And then, the relationship between symbol period of MPSK signals and fractional low-order cyclic spectral coherence function is presented. Finally, the process of the symbol period estimation method for MPSK is constructed. Simulation results show that the proposed method has a good estimation performance over fading channels under impulsive noise environment.

    Method for early termination of ADMM penalized decoding for LDPC codes
    WANG Biao;MU Jianjun;JIAO Xiaopeng;WANG Zhongfei
    Journal of Xidian University. 2018, 45(5):  43-49.  doi:10.3969/j.issn.1001-2400.2018.05.008
    Abstract ( 325 )   PDF (732KB) ( 77 )   Save
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    In order to reduce the average number of iterations for low-density parity-check (LDPC) codes decoding, a method for early termination (ET) of the alternating direction method of multipliers (ADMM) penalized decoding is designed for LDPC codes by making a thorough analysis of the change rule of the number of satisfied parity-check constraints in iterative decoding. The proposed method can detect the error codewords at an early stage of decoding and then the decoding process is stopped, which saves the unnecessary decoding iterations. Compared with the two existing stopping methods of ADMM penalized decoding, the proposed ET method reduces the average number of iterations of ADMM penalized decoding at low signal-to-noise rations with a negligible decoding performance loss.

    Improved CS imaging algorithm for the GEO-LEO BiSAR system
    WANG Yuekun;SUO Zhiyong;LI Zhenfang;ZHANG Jinqiang;ZHANG Qingjun
    Journal of Xidian University. 2018, 45(5):  50-56+196.  doi:10.3969/j.issn.1001-2400.2018.05.009
    Abstract ( 432 )   PDF (3159KB) ( 100 )   Save
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    In the spaceborne bistatic synthetic aperture radar (BiSAR) system with a geosynchronous (GEO) illuminator and a low-earth-orbit (LEO) receiver, due to the long signal propagation time and high receiver velocity, the slant range error caused by “Stop-and-Go” assumption cannot be neglected. In addition, the spatial-variant characteristics of the echo signal caused by the complex bistatic configuration need to be considered. In this paper, an improved Chirp Scaling (CS) algorithm based on the time-domain perturbation is proposed. First, the signal model is derived based on the geometry. Then, the perturbation functions for correcting the spatial-variant characteristics are presented. Last, the phase compensation functions are generated to enhance the capability of the algorithm. Simulated data results show the validity of the presented method. The algorithm proposed in this paper is able to implement wide-swath and high-resolution imaging and produce the phase-preserved SAR image.

    Hybrid method for transient electromagnetic scattering from multiple targets above the rough sea surface
    WANG Qiang;Guo Lixin;LIU Zhongyu
    Journal of Xidian University. 2018, 45(5):  57-63.  doi:10.3969/j.issn.1001-2400.2018.05.010
    Abstract ( 274 )   PDF (633KB) ( 147 )   Save
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    In order to improve the calculation efficiency of the time-domain algorithm, a time-domain hybrid algorithm combining the time-domain integral equation (TDIE) with the time-domain Kirchhoff approximation (TDKA) is proposed to solve composite electromagnetic scattering from multiple targets above the one-dimensional conducting rough sea surface. In the hybrid method, the scattering of every target and the coupling scattering between them are accurately calculated by the TDIE method, and transient scattering from the sea surface can be estimated very well by the TDKA method. Taking into account the interactions between TDKA region and TDIE region, a time-domain matrix equation is given and used to solve the transient composite electromagnetic scattering from multiple targets above the rough sea surface. Compared with the conventional TDIE method, the hybrid method has a high computational efficiency and good calculation accuracy.

    Block codes concatenated polar codes
    ZHOU Tianxin;LI Ying
    Journal of Xidian University. 2018, 45(5):  64-68.  doi:10.3969/j.issn.1001-2400.2018.05.011
    Abstract ( 455 )   PDF (491KB) ( 148 )   Save
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    To improve decoding performance, a concatenated polar codes coding scheme is designed which uses classical block codes as outer codes and polar codes as inner codes. The information bits with low confidence of the sub-channel are first encoded by the outer code. Then, all information bits and the parity check bits generated by the outer code are put into the polar encoder. It is shown that allocating the most trusted sub-channels to the parity check bits generated by the outer code can improve the performance of the proposed scheme, i.e., a lower decoding error probability can be achieved. A modified successive cancellation list decoder is also proposed. This decoding algorithm chooses the one with the highest correct probability of all the checked decoding results as the final output. Simulation results show that when the code length is 128 and the error frame rate is 10<sup>-2</sup>, the proposed scheme has a gain of 0.25dB over the polarization code scheme with cyclic redundancy check.

    Design and analysis of multi-tuned impedance matching and broadband antennas
    WANG Le;ZHAO Zhipeng;LI Ya'nan;LIU Jinhai;YIN Yingzeng;LI Hui
    Journal of Xidian University. 2018, 45(5):  69-74.  doi:10.3969/j.issn.1001-2400.2018.05.012
    Abstract ( 665 )   PDF (855KB) ( 330 )   Save
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    In order to broaden the impedance bandwidth of an antenna, the multi-tuned impedance matching technology can be utilized. An antenna can be equivalent to a single-port microwave resonant circuit according to the theory of impedance matching. In a practical antenna design, multi-tuned impedance matching can be performed on either the antenna feed network or the radiator. Two examples are given to validate the feasibility and effectiveness of the multi-tuned impedance matching technology. Experimental results show that a folded monopole achieves double-tuned impedance matching by introducing a loop, thus enhancing its fractional impedance bandwidth from 9.70% to 46.80%. In addition, a sleeve antenna obtains a triple-tuned impedance matching with the presence of four metal pillars, thus improving its band of operation from 55.14% to 82.11%.

    In-depth analysis of bounds on the minimum distance of LRCs
    HAO Xiaohui;CHE Shuling;ZHANG Xinyu
    Journal of Xidian University. 2018, 45(5):  75-79+135.  doi:10.3969/j.issn.1001-2400.2018.05.013
    Abstract ( 288 )   PDF (482KB) ( 72 )   Save
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    In order to enhance the precision of the minimum distance and narrow its scope, this paper proposes two new bounds on the minimum distance which apply to Locally Repairable Codes. First, one bound is proposed by theoretical derivation on the basis of the Singleton-like bound. Second, the other is proposed based on existing methods and the new one proposed, and it is smaller in scope. Third, the connections are proposed among minimum distance bounds. Finally, simulation results show that the first new proposed bound is no better than the Singleton-like bound and the other is better than existing bounds on the minimum distance. 

    Cloud platform dynamic risk access control model
    YANG Hongyu;NING Yuguang
    Journal of Xidian University. 2018, 45(5):  80-88.  doi:10.3969/j.issn.1001-2400.2018.05.014
    Abstract ( 319 )   PDF (753KB) ( 113 )   Save
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    As the risk access control model can not match rules dynamically and the risk values are insensitive to access requests, a cloud platform dynamic risk access control model(CDRAC) is proposed. First, the attribute based access control(ABCA) is improved with the event calculus(EC), and the dynamic rule matching module is constructed in the CDRAC. Then, the dynamic risk evaluation index weight distribution sub-module is designed, and the risk assessment module with high sensitivity to access requests is constructed. Experimental results show that the CDRAC has good effectiveness and feasibility, and that it has a better real-time performance and adaptability than other existing models.

    Synchronization of the homogeneity cyberspace operation based on the collective defensive mechanism
    WANG Gang;HU Xin;MA Runnian;LIU Wenbin
    Journal of Xidian University. 2018, 45(5):  89-95.  doi:10.3969/j.issn.1001-2400.2018.05.015
    Abstract ( 291 )   PDF (897KB) ( 93 )   Save
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    According to the collective defensive mechanism of cyberspace security, the problems of the synchronization model and control based on cyberspace operation are researched. Beginning with the cyberspace security collective defensive operation and its influence ingredients, the uncertainty factors are introduced, and then the new synchronization model of cyberspace operation is established. The active synchronization and adaptive synchronization based on homogeneity are researched according to the parameters difference. When the parameters are known, the synchronization of cyberspace operation is realized by active control. When the parameters are unknown, the adaptive controllers and the adaptive laws of parameters are given based on the stability theory of Lyapunov, and the adaptive synchronization of homogeneity cyberspace security collective defensive operation is realized. Simulations show that the synchronization about homogeneity cyberspace collective defensive operation influenced by uncertainty factors can be controlled by adjusting parameters.

    ESD power-rail clamp circuit with a 5V power in the 3.3V CMOS process
    CHEN Diping;DONG Gang
    Journal of Xidian University. 2018, 45(5):  96-101.  doi:10.3969/j.issn.1001-2400.2018.05.016
    Abstract ( 506 )   PDF (1068KB) ( 129 )   Save
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    Considering the 5V power supply, a novel ESD(electrostatic discharge) circuit with a 5V power rail based on a conventional GG-NMOS (Gate-Ground NMOS) ESD power-rail clamp circuit is designed by the method of level shifters and the low follow current in the 3.3V CMOS process to avoid a higher cost under the high-voltage process. Due to progressively driving and releasing steps of the optimized circuit, the leakage current is decreased in a regular operation. Moreover, the circuit is verified with simulations based on models in the SMIC's 0.18μm CMOS process technology library and the fabricated ESD power-rail clamp circuit has passed the HBM (Human Body Model) ESD test at ±4000V. The circuits can be successfully used for the 5V power rail ESD protection.

    Coupled-deep belief network based method for image recognition
    MA Miao;XU Xidan;WU Jie
    Journal of Xidian University. 2018, 45(5):  102-107.  doi:10.3969/j.issn.1001-2400.2018.05.017
    Abstract ( 378 )   PDF (575KB) ( 164 )   Save
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    Aiming at the gradient vanishing problem caused by the increasing number of network layers, an image recognition method based on the Coupled-Deep Belief Network (C-DBN) is proposed, which introduces the cross-layer linkage to the Deep Belief Network (DBN). The structure and the parameter updating method for the C-DBN are given in detail, while the performance of the DBN and that of the C-DBN are compared with respect to their respective best parameters and the same net-depth on two image datasets. Moreover, the impact of the weights used in the coupling between the primary line and the secondary line is analyzed at the cross-layer linkage. Besides, several classic deep learning based methods are compared with the proposed C-DBN. Experimental results show that the C-DBN is superior to the DBN in terms of convergence speed and accuracy. And, a good performance is achieved by the proposed method in comparison with some classical deep learning methods. This means that the usage of cross-layer linkage can alleviate the gradient vanishing problems effectively in the DBN training, which helps to improve the following recognition performance.

    Simplified and robust algorithm for three-dimensional estimation of nodes in sensor networks
    ZHAO Jihong;XIE Zhiyong;QU Hua;WANG Mingxin;LIU Xi
    Journal of Xidian University. 2018, 45(5):  108-114.  doi:10.3969/j.issn.1001-2400.2018.05.018
    Abstract ( 282 )   PDF (880KB) ( 106 )   Save
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    A node three-dimensional estimation algorithm that combines the weighted centroid localization algorithm and simplified maximum correntropy unscented Kalman filter is proposed for solving the problems that the observation noise, which appears in estimating the three-dimensional states of the nodes, is heavy-tailed or has some sudden change in the wireless sensor network. First, the algorithm obtains the observation distance of beacon nodes and sensor nodes by using the method of signal strength ranging and gets the approximate estimation of nodes with the centroid localization method. And then a simplified maximum correntropy unscented Kalman filter algorithm is deduced by combining the node estimation model and the robustness of the maximum correntropy criterion for non-Gaussian and nonlinear problems. Finally, the accurate estimation is obtained by using it. Simulation results show that the new algorithm has a better performance for the three-dimensional state estimation of nodes than the classic methods in sensor networks with heavy-tailed non-Gaussian noise. It not only reduces the time complexity of the general maximum correntropy unscented Kalman filter, but also improves the accuracy of node estimation.

    Optimization of the rib of the satellite deployable umbrella reflector
    WANG Bo;ZHENG Shikun;LI Zhouyang;XIE Zhuo
    Journal of Xidian University. 2018, 45(5):  115-120.  doi:10.3969/j.issn.1001-2400.2018.05.019
    Abstract ( 486 )   PDF (1260KB) ( 98 )   Save
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    In order to reduce the mass of the satellite deployable umbrella reflector, a structure optimization model of the rib is established by analyzing the structure of the rib and forces on it. The minimum mass of the rib is calculated by optimizing its structure parameters under given rigidity and loads. Optimized structure parameters are obtained by solving the model with the Monte Carlo method and Compound Form method. Comparing the original structure, 50.4% mass is successfully reduced after optimization. It is found that increasing the height of the rib's cross section and using a variable section are two effective ways to make the rib lighter, and that the Compound Form method has a better performance than the Monte Carlo method when solving this model.

    Multi-path reliable transmission mechanism in the virtualized integrated fiber-wireless access network
    ZOU Hong;CHEN Xiao
    Journal of Xidian University. 2018, 45(5):  121-127+148.  doi:10.3969/j.issn.1001-2400.2018.05.020
    Abstract ( 355 )   PDF (1011KB) ( 107 )   Save
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    For the problem that the localized overload of the integrated networks and the equipment failure will make the business interruption, a virtualized multi path reliable transmission mechanism for the integrated fiber-wireless access networks is proposed. By establishing the virtualized hierarchical model and adopting the centralized management of resources of the virtualized network and the network overall visual superiority, we use the multi-path parallel transmission mechanism to design a recovery scheme after a breakdown so as to ensure the reliable transmission of the network. The results show that the performance of the proposed algorithm is better than the full backup and shared backup algorithm. In addition, the proposed algorithm can ensure the network reliability while the network load balancing is achieved and the resource utilization rate is improved.

    Method for estimation of covariance rank minimization DOA by exploiting spatial smoothing
    WANG Hongyan;FANG Yunfei;PEI Bingnan
    Journal of Xidian University. 2018, 45(5):  128-135.  doi:10.3969/j.issn.1001-2400.2018.05.021
    Abstract ( 337 )   PDF (868KB) ( 154 )   Save
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    Focusing on the problem of poor accuracy and low resolution acquired by the traditional direction of arrival (DOA) estimation algorithm in the cases of the coherent signals and non-uniform noise and based on the spatial smoothing method, a DOA estimation approach via minimizing the rank of the covariance matrix of the received signal is developed. Following the traditional spatial smoothing approach, the covariance matrix of the received signal is multiplied by a certain switch matrix on the left and right sides in the proposed method, respectively, and then the spatial backward smoothing covariance matrix can be obtained. In what follows, the covariance matrix can be reconstructed into a noise-free one by exploiting the low rank property of the smoothing matrix. Finally, the DOA can be implemented by the traditional MUSIC algorithm. Simulation results demonstrate that, compared to the traditional MUSIC, matrix completion based MUSIC (MC-MUSIC) and rank and trace minimization (RTM) algorithms, the proposed method can suppress the non-uniform noise considerably, and improve the DOA estimation performance significantly in the case of the coherent signals.

    Real-time detection and identification of speed limit traffic signs under the BP neural network
    ZHANG Xingguo;LIU Xiaolei;LI Jing;WANG Huandong
    Journal of Xidian University. 2018, 45(5):  136-142.  doi:10.3969/j.issn.1001-2400.2018.05.022
    Abstract ( 467 )   PDF (4488KB) ( 142 )   Save
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    There are a lot of traffic signs based on the picture system of traffic sign recognition, and based on the video data to detect and identify the speed of traffic signs, However, there is a higher error rate. So this paper realizes the automatic detection and localization of speed limit traffic signs, and uses the BP neural network to identify the road signs. Meanwhile, the CamShift method and the optical flow method are used to speed up video, Experiment shows that the algorithm proposed in this paper can shorten the time by more than 50%, and the detection and recognition accuracy is over 90%, which is suitable for the related field of intelligent recognition.

    Fine-granularity gate level formal verification method for hardware security
    QIN Maoyuan;MU Dejun;HU Wei;MAO Baolei
    Journal of Xidian University. 2018, 45(5):  143-148.  doi:10.3969/j.issn.1001-2400.2018.05.023
    Abstract ( 366 )   PDF (488KB) ( 95 )   Save
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    No effective security verification method for hardware design is available for a long time. To solve this problem, this paper proposes a Fine-granularity Gate Level Formal Verification Method for Hardware Security, which is able to describe the security properties of the circuit and construct the semantic circuit in the customized formal language on Coq. With the help of the Hoare Logic theory, a provable theorem for verifying the security property of the semantic circuit is generated. The process of verification relies on tactics to generate and validate the proof in human-computer interaction. The results show that this method not only verifies the security properties of the semantic model, but also solves the problem of insufficient verification coverage in simulation. In conclusion, this method can raise accuracy and efficiency of verification.

    Malicious collusion detection data forwarding mechanism in opportunistic sensing
    YANG Jing;LI Pengcheng;LI Wuyou;YAN Junjie
    Journal of Xidian University. 2018, 45(5):  149-156.  doi:10.3969/j.issn.1001-2400.2018.05.024
    Abstract ( 303 )   PDF (624KB) ( 81 )   Save
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    Since the existing trust mechanism fails to tackle the problem that malicious nodes cooperate with each other, malicious collusion detection data forwarding in Opportunistic Sensing is proposed. First, the transient connection subnets are constructed according to the state of connection between nodes. At the same time, the anomalous interaction subsets with the maximum aggregation density are searched on the basis of analyzing the abnormal interaction behavior, and the malicious consonants are detected. Second, the transient trust relationship between nodes is evaluated through connection strength and collaboration willingness by establishing the transient trust subnet, so as to select the best relay node for completing data forwarding. Results show that the proposed mechanism can reduce adverse effects of collusion attacks effectively, and can achieve the purpose of accurate and efficient data forwarding.

    Spatial selectivity of network interference in the three-dimensional angular domain
    DU Derong;ZENG Xiaoping;JIAN Xin;YANG Fan;CHEN Li
    Journal of Xidian University. 2018, 45(5):  157-162.  doi:10.3969/j.issn.1001-2400.2018.05.025
    Abstract ( 292 )   PDF (903KB) ( 61 )   Save
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    A three-dimensional (3-D) Gaussian interference distribution model and the corresponding interference angular power density (IAPD) are proposed to investigate the spatial selectivity of network interference in the angular domain for the large-scale wireless communication networks comprising many spatially scattered nodes. Then, based on the IAPD and the theory of 3-D shape factors, the closed-form expressions for some important spatial statistics, such as the 3-D unnormalized complex spherical harmonic coefficients, interference shape factors, fading rate variance, spatial correlation, and coherence distance are derived. Simulation results show that the 3-D spatial angular directions have a significant effect on the IAPD and these spatial statistics, and the 15° elevation angle is the proper pointing direction for 3-D smart antennas and adaptive beamforming design.

    Face recognition algorithm for the deep hash combined with global and local pooling
    ZENG Yan;CHEN Yuelin;CAI Xiaodong
    Journal of Xidian University. 2018, 45(5):  163-169.  doi:10.3969/j.issn.1001-2400.2018.05.026
    Abstract ( 393 )   PDF (2573KB) ( 144 )   Save
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    To reduce the memory occupancy rate and computational resources in face recognition with high-dimension features extracted from large convolutional neural networks, an efficient Fully Convolutional Network (FCN) of the deep hash combined with global and local pooling. First, an FCN based on Global Average Pooling (GAP) is proposed to reduce network parameters and compress the model size. Second, a fusion method for learning approximate hash coding with multiple classification properties is used with Quantization Loss and Softmax Loss. Experimental results show that the method proposed can improve the efficiency up to 68% and that the Rank-1 accuracy is increased slightly with the Visual Geometry Group (VGG) framework. The fusion loss method can improve the efficiency up to 23.7% and the Rank-1 accuracy is maintained with the Face Residual Network (Face-ResNet) framework. The results indicate that the proposed method can improve the efficiency both from feature extraction and reduction. It also can be applied to other frameworks.

    Improved (k, n) tagged visual cryptography scheme
    GUO Songge;LV Donghui;REN Yanli
    Journal of Xidian University. 2018, 45(5):  170-176+183.  doi:10.3969/j.issn.1001-2400.2018.05.027
    Abstract ( 944 )   PDF (2209KB) ( 77 )   Save
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    Decrypted images of the existing tagged visual cryptography has a poor visual quality and the problems of pixel expansion and design of encryption matrices. So this paper exploits the properties of (2, 2) random grid-based visual cryptography to modify the embedding method and location of the tagged images, and improves the visual quality of decrypted images by reducing the impact of the tagged information on secret information. The scheme uses a random grid to avoid pixel expansion and design of encryption matrices, and it can flexibly adjust the visual quality of decrypted images. Compared with the existing schemes, the secret images recovered in this scheme have a better visual quality when the same proportion of tagged information is embedded in shares.

    Improved algorithm for SAR target recognition based on the convolutional neural network
    XU Qiang;LI Wei;ZHAN Ronghui;ZOU Kun
    Journal of Xidian University. 2018, 45(5):  177-183.  doi:10.3969/j.issn.1001-2400.2018.05.028
    Abstract ( 419 )   PDF (3830KB) ( 139 )   Save
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    To prevent the over fitting phenomenon of the convolutional neural network( CNN ) under the condition of insufficient labeled data, and aim at the SAR target recognition under noisy condition, a novel target recognition method is proposed. First, the data augmentation method is used to augment the data set to improve the generalization ability of the model. Second, the feature extraction is carried out by zero phase component analysis( ZCA ), and a set of feature sets is used to pre-train the convolutional neural network. In order to optimize the network structure and prevent the over-fitting phenomenon, the rectified linear unit( ReLU ), Dropout, regularization, unit convolution kernel and other sparse technology are used. Experiments demonstrate that the new algorithm is effective for target recognition, which has a high recognition capability for targets and their deformation sub-classes, and is robust to noise.

    Multi-user detection algorithm in the LEO satellite random access system
    LU Dawei;WANG Qiwei;REN Guangliang
    Journal of Xidian University. 2018, 45(5):  184-189.  doi:10.3969/j.issn.1001-2400.2018.05.029
    Abstract ( 371 )   PDF (602KB) ( 100 )   Save
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    The performance of multi-user detection in the low earth orbit satellite random access system deteriorates seriously over rapidly time-varying channels. To solve this problem, a multi-user detection algorithm is proposed to overcome the large Doppler frequency shift. The algorithm uses the basis expansion model for modeling the channel of each user and the basis coefficients of each user are jointly solved by the maximum likelihood method. Then, the channel of each user is fitted by the basis coefficients. After that, all interference packets are reconstructed and eliminated. Finally, the residual signals after interference cancellation are decoded. To improve the detection probability of the user's packets, the channel estimation and interference cancellation are repeated using the decoded bits, each user's training blocks and the detected user data. Simulation results show that, compared with the existing algorithm, the algorithm has significantly improved the performance of the packet error rate, channel estimation mean square error and throughput. The algorithm can work efficiently in low earth orbit satellite multi-user scenarios, which have a high signal noise ratio and a large Doppler frequency shift.

    Airport object detection combining transfer learning and hard example mining
    XU Yuelei;ZHU Mingming;MA Shiping;TANG Hong;MA Hongqiang
    Journal of Xidian University. 2018, 45(5):  190-196.  doi:10.3969/j.issn.1001-2400.2018.05.030
    Abstract ( 458 )   PDF (7305KB) ( 112 )   Save
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    In order to improve the accuracy and speed of airport detection of remote sensing images, an airport detection method combining transfer learning and hard example mining is proposed. First, the region-based convolutional neural network is used as the basic framework instead of the sliding windows plus artificial features in the traditional methods. Second, based on the common low-level and intermediate-level visual features between natural images and remote sensing images, the pre-training network trained on natural images is transferred to deal with airport detection with limited data after modifying and improving the network. Then, the idea of hard example mining is introduced to improve the ability to discriminate objects and make the training more efficient. Finally, the alternating optimization strategy allows for sharing convolution layers between regional proposal network and detection network, thus greatly reducing the time. Experimental results show that the proposed method can detect different types of airports accurately under complex backgrounds. The experimental results with a detection rate of 93.6%, a false alarm rate of 11.6%, and the time of 0.2s are superior to the results by other comparison methods.