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    20 October 2017 Volume 44 Issue 5
      
    Asymmetric spatially-coupled LDPC codes for burst erasure channels
    ZHANG Zhaoji;LI Ying
    Journal of Xidian University. 2017, 44(5):  1-6.  doi:10.3969/j.issn.1001-2400.2017.05.001
    Abstract ( 320 )   PDF (633KB) ( 495 )   Save
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    The outstanding performance of Spatially-Coupled Low Density Parity Check(SC-LDPC) codes deteriorates due to burst erasures over the channels with memory. To improve the performance of SC-LDPC codes over burst erasure channels, an asymmetric spatial coupling structure is proposed where the variable nodes at the same position of the protograph are permitted to couple with check nodes with different coupling widths. According to the structural properties of the Asymmetric Spatially-Coupled LDPC (ASC-LDPC) codes, the characteristics of the stopping sets of the base matrices are analyzed. It is shown that the base matrix of an ASC-LDPC code possesses a longer span and a larger cardinality of the minimal stopping set than that of the conventional SC-LDPC code. Thus a longer single-burst erasure or more multiple-burst erasures can be corrected by ASC-LDPC codes. Simulation results show that the proposed asymmetric spatial coupling structure could improve the performance of SC-LDPC codes over both the single-burst erasure channels and multiple-burst erasure channels.

    Virtual optical network mapping algorithm with load balancing
    XU Zhanqi;WANG Kai;DING Zhe;JIA Wenbin;YANG Fan
    Journal of Xidian University. 2017, 44(5):  7-12.  doi:10.3969/j.issn.1001-2400.2017.05.002
    Abstract ( 321 )   PDF (692KB) ( 238 )   Save
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    Aimed at sharing the elastic optical network (EON) efficiently for multiple virtual optical networks, this paper sets up an integer linear programming model of virtual optical networks (VONs) mapping in EON, and proposes a novel VON mapping algorithm with load balancing (VMLB). In the VMLB, the node computing resource, node closeness centrality and available link bandwidth of the physical network are comprehensively considered to complete node mapping, while both the link distance and occupied link bandwidth are combined to implement link mapping. Simulation results show that the proposed algorithm could reduce spectrum resource usage and effectively improve the uniform performance of the network load.

    MA Hongfei;ZHAO Yuejiao;LIU Ke;LIU Hao
    MA Hongfei;ZHAO Yuejiao;LIU Ke;LIU Hao
    Journal of Xidian University. 2017, 44(5):  13-17.  doi:10.3969/j.issn.1001-2400.2017.05.003
    Abstract ( 287 )   PDF (550KB) ( 231 )   Save
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    In order to improve the accuracy of the voiced/unvoiced/silence classification, a new method based on the Stack Autoencoder (SAE) is proposed. This method is implemented with a deep neural network composed of SAE and Softmax. First, the SAE is trained with the unsupervised method using a speech parameter training sequence which includes the residual signal peak, gains, pitch periods, and line spectrum frequency (LSF), while the Softmax is trained with supervision by the use of the output of the SAE with the speech parameters training sequence as its training input. Then a supervised fine-turning method to the deep neural network is conducted to obtain the final parameters of the networks. Test results have shown that the accuracy of the speech classification of the presented method is better than the traditional methods in different background noise conditions with different signal-to-noise ratios (SNR), especially in the low SNR condition.

    Fast DSSS acquisition algorithm based on the match Fourier transform
    ZHANG Xuan;LI Yongzhao;YE Yinghui;ZHANG Hailin
    Journal of Xidian University. 2017, 44(5):  18-24+113.  doi:10.3969/j.issn.1001-2400.2017.05.004
    Abstract ( 247 )   PDF (936KB) ( 179 )   Save
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    Induced by Chip Effects and Doppler Effects, the accumulating performance of the Direct Sequence Spread Spectrum signal is limited under high dynamic and low signal noise ratio scenarios in near space. To circumvent this problem, a particle swarm fast discrete match Fourier transform based Double Block Zero Padding algorithm is proposed by compensating Chip Effects and Doppler Effects simultaneously. Further, the particle swarm optimization is employed to optimize the iteration of fast discrete match Fourier transforms. Both simulation and theoretical analysis show that the proposed algorithm can not only enhance the acquisition probability and accuracy, but reduce the computational complexity as well. The computation of the proposed algorithm is 46% that of other existing methods when the coherent time is 10ms.

    New soft morphological parameterized symmetric logarithmic image processing filtering algorithm
    NI Jie;WANG Junping;YANG Guoyu;WU Yao;MA Shuliang
    Journal of Xidian University. 2017, 44(5):  25-31+45.  doi:10.3969/j.issn.1001-2400.2017.05.005
    Abstract ( 241 )   PDF (3971KB) ( 129 )   Save
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    Synthetic Aperture Radar (SAR) imaging has the inherent random speckle noise, which seriously affects the quality of the SAR image, and it is very important to study the suppression of the speckle noise. In view of the fact that the traditional spot noise suppression method could not remove speckle noise while protecting the image texture information, a novel filtering algorithm is proposed based on the soft morphological and parameterized symmetric logarithmic image processing model. The proposed algorithm combines the flexibility of the order-statistic soft morphological operations and the characteristic of the parameterized symmetric logarithmic image processing model which establishes a parameterized symmetric structure processing part of the image, which can suppress speckle noise while protecting the texture information on the image. In order to validate the despecking performance of the proposed algorithm, we compare the effect of the filter with some of the existing filtering algorithms, and use some of the image evaluation parameters of the suppress speckle noise image to evaluate the performance of the SMPSLIP filtering algorithm. Experimental results show the effectiveness of this algorithm.

    ICIC and CoMP based interference management scheme in small cell networks
    FENG Likang;HOU Ronghui
    Journal of Xidian University. 2017, 44(5):  32-38.  doi:10.3969/j.issn.1001-2400.2017.05.006
    Abstract ( 280 )   PDF (791KB) ( 117 )   Save
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    This paper considers the cross-layer and co-layer interference in small cell networks. We propose an interference management scheme by jointly optimizing user association, ABS (Almost Blank Subframes) configuration, and resource allocation. Especially, the proposed interference management smartly utilizes the opportunities of CoMP-JT (Coordinated Multi Point-Joint Transmission) to mitigate the interference from the macro base station to increase the coverage area of small cells. Simulation results show that the proposed interference management improves network throughput and spectrum efficiency as compared with the existing methods.

    Maneuvering target track-before-detect method using dynamic programming
    CHEN Shuailin;LUO Feng;ZHANG Linrang;HU Chong;CHEN Shichao
    Journal of Xidian University. 2017, 44(5):  39-45.  doi:10.3969/j.issn.1001-2400.2017.05.007
    Abstract ( 371 )   PDF (1264KB) ( 166 )   Save
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    The maneuverability coverage of the traditional dynamic programming track-before-detect (DP-TBD) algorithm is limited to the range of the default valid state transitions. To overcome the above limitation, a novel DP-TBD algorithm suitable for maneuvering targets is proposed. By means of state transitions optimizing, the efficiency of the status searching for the maneuvering target is improved, and the searching range is extended. Through transfer speed weighting, the energy accumulation can be carried out according to the target motion tendency. Without altering the input parameters, the proposed method can detect targets with both strong and weak maneuverability. The performance of the proposed algorithm and that of the traditional method are compared through simulation experiments. The applicability of the algorithm is verified by real measured data.

    Distance domain data support region fusion approach for modified Omega-K algorithm
    HUAI Yuanyuan;LIANG Yi;GAO Yuexin;XING Mengdao
    Journal of Xidian University. 2017, 44(5):  46-50+86.  doi:10.3969/j.issn.1001-2400.2017.05.008
    Abstract ( 745 )   PDF (1677KB) ( 117 )   Save
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    It is hard for the conventional Omega-K algorithm to implement the coherent addition of the sub-aperture image to realize the full-aperture resolution. A distance domain data support region (DSR) fusion approach based on the modified Omega-K algorithm is proposed in this paper. By the implementation of the modified sub-aperture wavenumber domain algorithm, the range-azimuth coupling is solved. At the same time, the burden of the zeros padding processing introduced by the short distance DSR of the sub-aperture data is greatly mitigated. Then the coherence processing and distance domain data support region fusion are performed. By adding the sub-aperture data coherently, the full-aperture resolution in azimuth is realized.

    Active false-target discrimination method based on the amplitude ratio feature
    ZHAO Shanshan;ZHANG Linrang;LIU Nan;ZHANG Juan;ZHOU Yu
    Journal of Xidian University. 2017, 44(5):  51-57+152.  doi:10.3969/j.issn.1001-2400.2017.05.009
    Abstract ( 271 )   PDF (703KB) ( 144 )   Save
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    Cooperation in multiple-radar has become an important trend in radar deception electronic counter-countermeasure. To solve the problem that the existing signal fusion-based methods cannot achieve a constant false alarm rate, this paper proposes a novel target discrimination method based on the difference in amplitude ratio feature. By the analysis of the random distribution of target amplitude ratio, multiple-dimensional test threshold is applied to discriminate the detected targets under a preset false alarm probability. Simulation results verify the feasibility and effectiveness of the proposed method.

    Design method for bandwidth and synthetic aperture time in GEOSAR
    LIU Wenkang;SUN Guangcai;CHEN Jianlai;XING Mengdao
    Journal of Xidian University. 2017, 44(5):  58-63.  doi:10.3969/j.issn.1001-2400.2017.05.010
    Abstract ( 324 )   PDF (863KB) ( 144 )   Save
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    The complex relative motions between the Radar platform and the targets are more complex in geosynchronous Synthetic aperture radar (GEO SAR). The assumption that range and azimuth are uncoupled in Low-Earth-Orbit SAR does not hold in GEO SAR. The common calculation method for bandwidth and synthetic aperture time cannot be used in GEO SAR. In this paper, a method suit able for GEO SAR is proposed to design imaging parameters including the bandwidth and synthetic aperture time. First, a resolution analysis method with analytical expression is deduced based on geometrical mapping, and then based this expression, the bandwidth and the synthetic aperture time are optimized to meet the resolution's needs on value and shape (close to a circle). Finally, a design example in a GEO ellipse orbit is given, and the resolution analysis result corresponds to the image obtained by the Back Projection (BP) method. The designed bandwidth and synthetic aperture time are also provided. Simulation results validate the effectiveness of the proposed method in the GEO SAR system design.

    Accurate optimization algorithm for the antenna phase center
    YUAN Haobo;WU Zhengguo;HOU Jianqiang;DANG Xiaojie
    Journal of Xidian University. 2017, 44(5):  64-68.  doi:10.3969/j.issn.1001-2400.2017.05.011
    Abstract ( 491 )   PDF (1223KB) ( 186 )   Save
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    The antenna phase center (APC) is very important for the positioning of the antenna and the accuracy of the measurement. This paper presents an optimization method to calculate the phase center accurately. Suppose the phase center is at some known point, a so-called projection sphere can be drawn with this point as the center, and all the measured points can be projected onto this sphere. Then the phases of the projected points are obtained by those of the former, and an objective function is defined by the sum of squares of the differences between these phases and their average value. Finally, the conjugate gradient method (CG) or the genetic Algorithm (GA) can be used to find out the coordinate of the phase center. Instead of measuring the antenna phase repeatedly, this new method can determine the coordinates of the phase center accurately in one single operation. Its accuracy is validated by the numerical results of a dipole and a horn antenna.

    Optimized initial aligning algorithm for point cloud registration
    LUO Nan;WANG Quan
    Journal of Xidian University. 2017, 44(5):  69-74.  doi:10.3969/j.issn.1001-2400.2017.05.012
    Abstract ( 248 )   PDF (3887KB) ( 130 )   Save
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    This paper deals with the initial aligning problem of pairwise point cloud registration. A novel Distance Disparity Matrix algorithm derived from Euclidean invariants of rigid motion is proposed to prune the obvious outlier matches while keeping the most inliers.The pruned matches are then sent into a Least-Squares Backward procedure to estimate optimized rigid transformation in fewer iterations.The employed thresholds are automatically determined with respect to the actual resolution of input point clouds. Experimental results show that the proposed strategy effectively eliminates the outliers to achieve better initial alignment at a high speed, and further enhances the performance of point cloud registration.

    Bit-flipping decoding algorithm for the MLC flash memory
    ZHANG Xuan;MU Jianjun;JIAO Xiaopeng
    Journal of Xidian University. 2017, 44(5):  75-80+146.  doi:10.3969/j.issn.1001-2400.2017.05.013
    Abstract ( 325 )   PDF (887KB) ( 170 )   Save
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    As the bit storage density of the multi-level cell (MLC) for flash memory increases, cell-to-cell interference is the dominant distortion source of the flash memory. By making a thorough analysis of the error characteristics of the intra-cell bit storage channel, a bit-flipping (BF) rule for the MLC flash memory is designed, and an improved BF algorithm is presented for the MLC flash memory. Simulation results show that the decoding performance of the improved bit-flipping algorithm is better than that of the BF decoding algorithm for the MLC flash memory with the same sensing precision, and the improved BF decoding algorithm for the MLC flash memory can effectively reduce the average number of iterations, which can achieve a better tradeoff between decoding complexity and decoding performance.

    Electromagnetic leakage characteristics of the digital video cable
    WANG Sen;QIU Yang;TIN Jin;XU Qinglin
    Journal of Xidian University. 2017, 44(5):  81-86.  doi:10.3969/j.issn.1001-2400.2017.05.014
    Abstract ( 290 )   PDF (1896KB) ( 88 )   Save
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    The information leakage caused by electromagnetic wave emitted by computer cables which contains video information is studied. The digital video interface of computer video information transmission is analyzed. The time and frequency domain model corresponding to serial transmission and parallel transmission of video information are built and then the leakage characteristics of different video signals which are transmitted in the serial and parallel way are obtained. According to the principle of video information interception and reconstruction, we can draw a conclusion that it is easy to intercept when the gray image is transmitted in a serial way because it shows the characteristic of “false parallel and real serial”. The experimental result agrees with it very well.

    Electronic transport properties of cross graphene nano-junction
    LOU Lifei;WANG Dongyang;PAN Qingbiao;ZHANG Junqin
    Journal of Xidian University. 2017, 44(5):  87-91+120.  doi:10.3969/j.issn.1001-2400.2017.05.015
    Abstract ( 245 )   PDF (1303KB) ( 78 )   Save
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    Graphene nanoribbons have special electrical properties, The electron transport properties can be regulated by controlling their geometry or doping. The metallic properties and semiconductor properties of different chiral graphene nanoribbons are used to build the cross graphene nano-junction in this paper. The method of density functional theory and non-equilibrium Green's function are used to research electronic transport properties of the graphene nano-junction with different heights and widths and locations of doping. It is shown that the graphene nano-junction shows metallic properties with increase in height and width of the arm. But the transmission spectral line has a clear oscillation. Since the doped nitrogen atom intensifies electron scattering, the electronic transport properties decrease. And the electron transport properties when the two arms are doped worsens more compared to the case of trunk doping.

    Low-offset high-PSRR bandgap voltage reference
    LI Yani;SUN Yadong;WANG Xu
    Journal of Xidian University. 2017, 44(5):  92-96.  doi:10.3969/j.issn.1001-2400.2017.05.016
    Abstract ( 697 )   PDF (621KB) ( 285 )   Save
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    Based on the problem that the accuracy of the bandgap affects the performance of the integrated circuit, a novel BGR (bandgap voltage reference) is proposed. It utilizes a feedback compensation network to enhance PSRR and reduce the offset voltage, which improves the system stability and precision. Cadence spectre simulation has been done by the SMIC 018μm 1.8V CMOS process for validation. The results show that the achieved temperature coefficient is 34.6×10<sup>-6</sup>/℃ over -30℃ to 100℃ and that the PSRR is -63.5dB at a low frequency. The power assumption is only 1.5μW. The circuit is suitable for a low-voltage low-power energy harvesting system.

    Analytic circuit model for radiated susceptibility estimation of the microstrip transmission line
    SHI Chunlei;CHAI Changchun;LIU Yuqian;FAN Qingyang;LIU Yang;YANG Yintang
    Journal of Xidian University. 2017, 44(5):  97-101.  doi:10.3969/j.issn.1001-2400.2017.05.017
    Abstract ( 299 )   PDF (2167KB) ( 79 )   Save
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    According to transmission line theory and electromagnetic theory, a time-domain analytic circuit model for radiated susceptibility estimation of a microstrip transmission line is proposed. In order to obtain the susceptibility threshold of a microstrip line under external electromagnetic interference quickly, a transverse electromagnetic cell (due to IEC standard 62132-2) is adopted as the test environment. A transmission line pulse is chosen as the external electromagnetic interference, since its steep edges can directly demonstrate radiated susceptibility of a microstrip line. This is an application case to identify the availability of the model. Specific parameters of the analytic circuit model are derived and comparisons of calculated and measured results of transmission line pulse responses for the matched microstrip line confirm the accuracy of the model. Furthermore, transmission line pulse responses of a microstrip transmission line with arbitrary loads and contributions of electric and magnetic field couplings are calculated. The effect of terminal load on the response of a microstrip transmission line is obtained, which can be helpful for load-selection to improve radiated susceptibility estimation of a microstrip transmission line.

    Modified BM3D algorithm for image denoising using the directed diffusion equation
    FENG Xiangchu;LI Xiaohui;WANG Weiwei;JIA Xixi
    Journal of Xidian University. 2017, 44(5):  102-108.  doi:10.3969/j.issn.1001-2400.2017.05.018
    Abstract ( 287 )   PDF (2792KB) ( 127 )   Save
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    In order to preserve the edges and details of the image better, a modified block-matching and 3D filtering algorithm based on the directed diffusion is proposed. First, the block-matching and 3D filtering result is divided into three perpendicular subbands by using the wavelet decomposition. Second, by adding the diffusion operator and diffusion coefficients to the directed diffusion equation, the diffusion speed can be accelerated and edges can not be blurred quickly. Then we diffuse the subband of the high-frequency, of which the wavelet coefficients are less than the threshold to the corresponding subband of the noisy image to get a new estimate. Lastly, by replacing the corresponding subband of the block-matching and 3D filtering estimated image with the new one, we obtain the improved denoising result. Experimental results show that the improved algorithm achieves better performance than the original block-matching and 3D filtering algorithm in terms of both removing noise and preserving the edges and details of the image.

    Frequency signal control based on the characteristic of the phase difference group
    JIA Zhaomin;LI Zhiqi;YANG Xuhai;SUN Baoqi;MIAO Miao;DONG Shaofeng
    Journal of Xidian University. 2017, 44(5):  109-113.  doi:10.3969/j.issn.1001-2400.2017.05.019
    Abstract ( 316 )   PDF (545KB) ( 59 )   Save
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    Based on thecontinuous character of the phase difference group between frequency signals of different nominal values, a method for direct digital phase lock between the different nominal value signals is proposed. The A/D converter is utilized to extract digital voltage value of the controlled frequency signal,whichreflects the phaseinformation. The switch point of the gate is generated by extracting the zero-crossing digital voltage value,then the frequency offset is obtained, on the basis of which the control voltage is obtained to control VCOCXO, and the frequency is locked. The experiment shows that the noise brought by the common complex circuit is reduced because of the simplified frequency transformation circuits, so that high precise locking is realized. The short term stability can reach the 10<sup>-12</sup>/s order of magnitude,and the phase noise index is good.

    Quantitative analysis of sliding window attack for the RSA timing channel
    MAO Baolei;MU Dejun;HU Wei;ZHANG Huixiang;TAI Yu;ZHANG Lu
    Journal of Xidian University. 2017, 44(5):  114-120.  doi:10.3969/j.issn.1001-2400.2017.05.020
    Abstract ( 399 )   PDF (918KB) ( 156 )   Save
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    The timing channel residing in RSA hardware implementations imposes a major threat to hardware security. In this work, we propose a sliding window timing attack method with significant improvement in the attack success rate and employ an information theoretic model to quantitatively measure the effectiveness of the proposed attack. Experimental results show that the sliding window attack method increases the attack success rate from 73% to 96% as compared to Kocher's timing attack, which uses a single bit window. Our results also reveal a strong correlation between attack success rates and the mutual information measurements of the key and encryption time. This indicates that information theoretic metrics can serve as an approach for quantifying the effectiveness of the sliding window timing attack method and evaluating timing channel threat of the RSA hardware architecture.

    Range ambiguous clutter suppression for the SAR/GMTI system based on the frequency diverse array
    LIN Chenchen;HUANG Puming;WANG Weiwei;LI Yu
    Journal of Xidian University. 2017, 44(5):  121-126.  doi:10.3969/j.issn.1001-2400.2017.05.021
    Abstract ( 285 )   PDF (1016KB) ( 181 )   Save
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    A range ambiguous clutter suppression approach based on frequency diverse array configuration is proposed to handle the range ambiguity of the space borne wide-swath high-resolution synthetic aperture radar ground moving target indication (SAR/GMTI) system with a high pulse repetition frequency. Since echoes from different ambiguous range regions are separable in the transmit spatial frequency domain, a filter bank is employed to extract echoes from each region, and then the transmit beam forming is implemented to suppress the clutter in each range region. Simulation results demonstrate that, by making full use of the limited channels, the proposed method could weaken the influence of heterogeneous clutter in each region and reconstruct moving target signals without range ambiguity, so as to expand the detection region evidently.

    Method of tracing the photovoltaic maximum power point based on NN with asymmetric basis
    LIU Daohua;HU Xiuyun;LIU Jiangfeng;GUAN Feng
    Journal of Xidian University. 2017, 44(5):  127-133.  doi:10.3969/j.issn.1001-2400.2017.05.022
    Abstract ( 240 )   PDF (633KB) ( 74 )   Save
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    To ensure that the photovoltaic power generation system always works at its maximum power point, a method for photovoltaic maximum power point tracking by the neural network based on asymmetric basis is proposed, and its concrete implementation steps are given. Fuzzy factor membership functions are built according to the influences of photovoltaic power generation factors on the power generation efficiency, and the fuzzy weights of the influencing factors are calculated, with these weights infused into the building of the neural network based on fuzzy asymmetric basis. The network is trained by using methods of fixed basis width RBF, traditional RBF and the method proposed in this paper with four kinds of quantities of samples, and the comparison in terms of the network training time and the standard deviation indicates that the accuracy of the network with 180 samples is the highest, at least an order of magnitude higher than other that of methods. By determining the working parameters of the photovoltaic system in real time by using this neural network, the photovoltaic system can make the internal and external resistances completely match at every moment through the slide rheostat, thus ensuring that the system always works at the maximum power point.

    Blind beamforming algorithm based on fractional lower-order time-frequency decomposition
    CHEN Pei;ZHAO Yongjun;LIU Chengcheng;LI Haiwen
    Journal of Xidian University. 2017, 44(5):  134-139.  doi:10.3969/j.issn.1001-2400.2017.05.023
    Abstract ( 236 )   PDF (807KB) ( 123 )   Save
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    A novel blind beamforming algorithm based on fractional lower-order time-frequency decomposition (TFD) is proposed to improve the performance of existing blind beamforming algorithms in impulsive noise environment. The traditional short-time Fourier transform (STFT) is first improved by utilizing fractional lower-order statistics to realize TFD in impulsive noise environment. Then, by combining the clustering method and the method for the uncertainty set, the TFD results of the receiving data at each sensor are used to realize the estimation of steering vectors (SV). Finally, the optimal weight coefficients of the proposed blind beamformer are achieved by substituting the estimated SV into the minimum variance distortionless response (MVDR) beamformer. Simulation results demonstrate that this algorithm can achieve superior output performance over the existing blind beamforming methods in impulsive noise environment. The proposed algorithm hardly needs any specific property of the receiving signals and can be more widely used.

    Global resource allocation mechanism based on software-defined of TWDM-PON
    XIONG Yu;ZHANG Zhenzhen;SHI Jin;WU Dapeng
    Journal of Xidian University. 2017, 44(5):  140-146.  doi:10.3969/j.issn.1001-2400.2017.05.024
    Abstract ( 265 )   PDF (942KB) ( 132 )   Save
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    For lack of global centralized control and flexible allocation of the inter-TWDM-PONs network resource, the architecture of the software-defined optical access network is designed and a novel global resource allocation mechanism with prediction is proposed. Based on the software-defined optical access network, the fuzzy neural network predicting model is utilized to reduce the interactive information delay between controller and OLT. Simultaneously, the adaptive and flexible weighting is utilized to allocate the inter-OLTs resource. Furthermore, an allocation algorithm with filling idle slots is designed to further reduce the network delay for the intra-OLT resource. Simulation results show that the proposed mechanism can optimize allocation of the different layers resource from the global network view. Also, the overall network delay is effectively reduced, and the channel utilization is improved.

    Superpixels-based golden dark channel for single image fog removal
    WANG Yunfei;LIU Huawei;ZHAO Boxin
    Journal of Xidian University. 2017, 44(5):  147-152.  doi:10.3969/j.issn.1001-2400.2017.05.025
    Abstract ( 275 )   PDF (3309KB) ( 107 )   Save
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    Fog and haze can cause serious image degradation. In the light of the limitations of dark channel theory, the fog concentration is supposed to be constant locally, and the effectiveness of dark channel prior decays exponentially as the depth increases. Based on this, we propose a superpixels-based golden dark channel algorithm for single image fog removal. Small regions are obtained by superpixels in which the fog concentration and the depth remain constant. The golden dark channel is computed in these regions. The resulting transmittance remains constant and is finer and more precise. This method can suppress the “Halo effect” which occurs in depth mutation. Moreover, An iterative strategy is employed to gradually reduce the overall density of fog, making the residual amount of fog satisfy the golden section after each iteration. Further, the golden section is used to simplify the tolerance value and deal with the color cast problems in the sky region where the depth is infinity. Experimental results show that the algorithm can effectively improve the image of visibility, and performs even better when the fog concentration is greater.

    Collaborative application strategy of evasive maneuver and chaff jamming in the air combat
    ZHOU Zhongliang;CHENG Yue;RUAN Chengwei;LIU Hongqiang
    Journal of Xidian University. 2017, 44(5):  153-157+164.  doi:10.3969/j.issn.1001-2400.2017.05.026
    Abstract ( 381 )   PDF (1077KB) ( 92 )   Save
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    It is a vital technology for combat aircrafts to apply evasive maneuver and chaff centroid jamming collaboratively to reduce the threat from incoming air-to-air missiles. In order to escape from the radar track beam of missiles as soon as possible, we define the “Maximum normal acceleration of the track beam border” as the principle for evasive maneuver, and the stress analysis is carried out on the aircrafts before the corresponding maneuver strategy is acquired. We establish the aircraft-missile-chaff relative motion model, chaff motion and RCS dynamic change model, construct the chaff flare strategy which conforms with the principle of centroid jamming, and sum up the success of the collaborative application conditions. Simulation result shows that the maneuver is better than the traditional maneuvers and facilitates the pilot operation. When the centroid jamming is applied, the avoid effect is significantly increased with fewer chaff bombs.

    Variable step size multi-antenna diversity equalization algorithm using error feedback
    HOU Xiaojian;XU Hua;ZENG Leya
    Journal of Xidian University. 2017, 44(5):  158-164.  doi:10.3969/j.issn.1001-2400.2017.05.027
    Abstract ( 206 )   PDF (764KB) ( 101 )   Save
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    Owning to the correlation of the error signal in multi-antenna diversity adaptive decision feedback equalization being not same with white noises', the equalization performance is constrained. This paper adds error feedback filters to the traditional multi-antenna diversity adaptive decision feedback equalizer. The proposed structure can decrease the correlation of the error signal to increase the performance under the mean square error criterion. However, the proposed structure lowers the algorithm's convergence speed. Consequently, a nonlinear function between step size and error signal by modifying the sigmoid function in order to make this variable step-size measure be adapted to decision feedback equalizers is proposed. The modified algorithm has a larger step size at beginning. Simulation results show that the proposed approach can increase the speed and lower mean square with only little increase in computation.

    Robust registration based compensation method based on sparse recovery
    LIU Yang;ZHANG Yongshun;LIU Hanwei;GUO Yiduo;LONG Zhenguo
    Journal of Xidian University. 2017, 44(5):  165-170+183.  doi:10.3969/j.issn.1001-2400.2017.05.028
    Abstract ( 237 )   PDF (1402KB) ( 83 )   Save
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    The loss of the degree of system freedom and mismatch of prior information based on the registration based compensation (RBC) method will result in a decline in computational performance. In order to solve above problems, a robust method named SRBC is proposed where the sparse recovery method is applied instead of the sub-aperture smoothing operation. First, the SRBC utilizes the sparse recovery to get the super resolution clutter space-time spectrum. Then the transition covariance matrix is calculated. Finally, the clutter covariance matrix is reconstructed by the Capon spectrum. Compared with the traditional RBC method, the SRBC proposed does not depend on sub-aperture smoothing operation and prior information and maintains the performance of clutter range-dependence. In addition, it is also steadier when the sensor error exists. Experimental simulation demonstrates the correctness and feasibility of this method.

    Broadband and high-efficiency reflective polarization rotator using metamaterials
    REN Yuhui;DING Jun;GUO Chenjiang;XING Tianzhang;WANG Fuwei
    Journal of Xidian University. 2017, 44(5):  171-176.  doi:10.3969/j.issn.1001-2400.2017.05.029
    Abstract ( 292 )   PDF (2971KB) ( 189 )   Save
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    A wideband and high-efficiency 90°polarization rotator based on ultrathin planar metamaterial is presented. It consists of the metallic hexagonal patches on the top layer of the dielectric substrate and a metallic film on the bottom layer. Initially, the explanation of the physical mechanism for the polarization rotator is presented according to the anisotropy media theory. Thereafter, we have discussed the influences of the structure dimension on the performance of the proposed polarization rotator. And finally, the structure is also fabricated. Results of the test show that it exhibits over 90% polarization conversion ratio from 841GHz to 113GHz in the condition of normal incidence.

    Accuracy assessment on refined HF frequency prediction method in long distance communication circuit
    ZHANG Xiuqiang;WANG Jian;SU Donglin;XIE Shuguo;JI Shengyu;ZHANG Jiangming
    Journal of Xidian University. 2017, 44(5):  177-183.  doi:10.3969/j.issn.1001-2400.2017.05.030
    Abstract ( 240 )   PDF (1323KB) ( 170 )   Save
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    In order to refine and fully verify the accuracy and applicability of the high frequency prediction method, a refined method which is more suitable for China and surrounding areas is discussed, which is even extensive by used in the Asia area. The MUF and OWF of two circuits are calculated by using the China refined HF frequency prediction method, and are compared with observations. The result show that (a) the predictions are in great agreement with the monthly median statistical characteristics of observation data, and (b) the accuracy is analogous to that in the China region.

    Robust adaptive beamforming with near-field scatterers
    LIAO Ruiqian;XU Jingwei;LIAO Guisheng
    Journal of Xidian University. 2017, 44(5):  184-188.  doi:10.3969/j.issn.1001-2400.2017.05.031
    Abstract ( 296 )   PDF (676KB) ( 118 )   Save
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    Near-field scattering which causes steering vector mismatch with array received data suffers severe performance degradation with beamforming. A new robust adaptive beamforming algorithm based on the steering vector uncertainty set is proposed to utilize the near-field scattering signal to improve the beamforming output signal-to-noise ratio. The properties of the far-field direct-path signal and near-field signal are incorporated. The large uncertainty set is divided into two small ones describing the far-field and near-field steering vectors, respectively. The far-field steering vectors and the near-field scattering coefficient with a transform matrix are evaluated by convex optimization. Simulation examples demonstrate that the algorithm is correct and effective.