一种新型多定子直线型超声电机的研究

直线型超声电机具有可直接驱动、精度高、响应快、适用于真空环境等优点，但驱动力较小。为提高工作台的负载能力，该文提出并研制了一种多定子直线型超声电机。样机由两个定子和一个动子(与工作台台面相联)组成。定子是一个利用1阶纵振和2阶弯振、带双足的兰杰文振子，双足交替驱动。讨论了该电机的驱动机理，对定子的结构进行了优化设计，给出了定子、动子、工作台一体化的设计方案，并对电机的输出性能做了初步的实验研究。     

Performance analysis of a variable‐gain amplify‐and‐forward relayed mixed RF‐FSO system

In this work, an amplify‐and‐forward variable‐gain relayed mixed RF‐FSO system is studied. The considered dual‐hop system consists of a radio frequency (RF) link followed by a free space optical (FSO) channel. The RF link is affected by short‐term multipath fading and long‐term shadowing effects and is assumed to follow the generalized‐K fading distribution that approximates accurately several important distributions often used to model communication channels. The FSO channel experiences fading caused by atmospheric turbulence that is modeled by the gamma‐gamma distribution characterizing moderate and strong turbulence conditions. The FSO channel also suffers path loss and pointing error induced misalignment fading. The performance of the considered system is analyzed under the collective influence of distribution shaping parameters, pointing errors that result in misalignment fading, atmospheric turbulence, and path loss. The moment‐generating function of the Signal power to noise power ratio measured end‐to‐end for this system is derived. The cumulative distribution function for the Signal power to noise power ratio present between the source and destination receiver is also evaluated. Further, we investigate the error and outage performance and the average channel capacity for this system. The analytical expressions in closed form for the outage probability, symbol and bit error rate considering different modulation schemes and channel capacity are also derived. The mathematical expressions obtained are also demonstrated by numerical plots.     

Dual symbol optimization‐based partial transmit sequence technique for PAPR reduction in WOLA‐OFDM waveform

Weighted overlap and add‐orthogonal frequency division multiplexing (WOLA‐OFDM) is a new waveform proposed recently for meeting the requirements of fifth generation (5G) telecommunication standards. In spite of being a serious 5G waveform candidate, WOLA‐OFDM is exposed to the problem of high peak to average power ratio (PAPR) similar to the other waveforms in which multicarrier transmission strategy is employed. Due to the overlapping nature of WOLA‐OFDM waveform, where the extension of the current symbol is overlapped with the extension of the previous symbol, it will not be efficient to apply conventional PTS (C‐PTS) directly to the WOLA‐OFDM waveform. Therefore, in this paper, we propose dual symbol optimization‐based partial transmit sequence (DSO‐PTS) technique for PAPR reduction in WOLA‐OFDM waveform. In our proposed technique, two adjacent symbols are jointly considered when searching for the optimal data block with minimum PAPR unlike the C‐PTS where the adjacent symbols are optimized individually. In the simulations, our proposed DSO‐PTS technique, C‐PTS, and GreenOFDM that is developed recently by modifying the conventional selective mapping (SLM) method are compared with each other with regard to PAPR reduction performance for different search numbers (SNs). In addition, the effects of DSO‐PTS, C‐PTS, and GreenOFDM on the amount of out of band (OOB) radiation in the power spectral density (PSD) graph of WOLA‐OFDM employing solid state power amplifier (SSPA) is measured for different SNs and input back off (IBO) values. According to the simulation results, our proposed DSO‐PTS technique clearly demonstrates a superior PAPR reduction and PSD performance.     

基于ARID控制系统的固定式海洋风机纵摇控制

针对固定式海洋风机在随机风浪载荷及其联合作用下产生的具有倒立摆运动特征的纵摇,研究采用主动转动惯量驱动系统(Active Rotary Inertia Driver, ARID)对海洋风机纵摇进行控制的问题。ARID控制系统通过伺服电机驱动转动惯性质量产生控制结构摆动的最优力矩,从而减小海洋风机的纵摇运动。基于拉格朗日原理建立海洋风机-ARID控制系统的理论分析模型;采用Simulink对海洋风机-ARID控制系统的有效性进行验证,并分析系统参数(控制算法参数、转动惯量比等)对控制效果的影响规律;设计海洋风机-ARID控制系统的振动台试验,设置多种载荷激励形式,验证控制系统的稳定性与广谱有效性。数值模拟和试验结果均验证了所建立的分析模型的正确性,表明ARID控制系统对风机的纵摇运动具有显著的控制作用,为ARID控制系统在具有倒立摆运动规律的工程结构中的应用奠定理论基础。     

倒装焊中图像对准算法的研究

针对倒装焊中常用的十字架对准标志的几何特征,采用形态学边缘检测算法提取图像边缘,通过将简化的Hough变换和最小二乘相结合的直线提取方法,提取出较为精确的边缘直线,实现了一种提取十字架标志的四条长边缘直线的图像处理算法。实验结果表明,该算法与相关算法相比具有更快的对准速度和更高的对准精度,且对有局部残缺的图像不敏感。     

自动识别数字调制方式的一种方法

提出了一种对经符号成形的数字通信信号进行调制方式自动识别的方案,该方案将数字已调信号的瞬时特征与功率谱特性相结合,设计了一组对信号信噪比不敏感的特征参数。通过提取经升余弦滤波成形的2FSK等六种常用的数字调制信号的5个特征参数,分别采用决策树和神经网络分类器进行调制方式自动识别。仿真表明,当信噪比为11dB时,采用决策树分类器对除2ASK外的5种信号的识别率在95%以上,神经网络分类器此时对所有信号的识别率达到了98%以上,证明了该方案的有效性和可行性。     

一种基于二次谱线生成的码速率识别方法

为了提高正交频分复用(OFDM)系统的解调性能,提出了一种用于OFDM系统的码速率识别方法.首先对OFDM信号分段作快速傅里叶变换(FFT)得到信号谱线序列,选择适当的谱线重构为辅助信号序列,再根据该辅助信号序列进行二次谱线生成.通过第二次生成谱线的间隔值与门限值的比较,可实现对码速率的识别.理论分析和仿真试验证明了本方法的有效性.在信噪比大于3 dB时,识别率在90%以上.     

符号速率相关的带通采样速率选取及其仿真

带通采样速率选取是软件无线电技术的关键问题之一。分析了常用带通采样速率选取方法的不足：采样速率一般不是符号速率的整数倍,后续基带数字信号处理需要进行复杂的小数内插或抽取算法运算,实现相当困难且运算量很大。为解决这一问题,提出了一种实用的与符号速率相关的带通采样速率选取方法,在满足带通采样定理基础上,附加频谱间隔最大条件选取符号速率整数倍的采样速率,最后通过软件仿真验证了符号速率相关的带通采样速率选取方法的正确性和可行性。     

针对AES和CLEFIA的改进Cache踪迹驱动攻击

通过分析"Cache失效"踪迹信息和S盒在Cache中不对齐分布特性,提出了一种改进的AES和CLEFIA踪迹驱动攻击方法。现有攻击大都假定S盒在Cache中对齐分布,针对AES和CLEFIA的第1轮踪迹驱动攻击均不能在有限搜索复杂度内获取第1轮扩展密钥。研究表明,在大多数情况下,S盒在Cache中的分布是不对齐的,通过采集加密中的"Cache失效"踪迹信息,200和50个样本分别经AES第1轮和最后1轮分析可将128bit AES主密钥搜索空间降低到216和1,80个样本经CLEFIA第1轮分析可将128bit CLEFIA第1轮扩展密钥搜索空间降低到216,220个样本经前3轮分析可将128bit CLEFIA主密钥搜索空间降低到216,耗时不超过1s。     

Performance evaluation of a new BPSK-OFDM timing estimation algorithm

Previously, the timing estimation in OFDM system employs cyclic prefix has been presented with the assumption of independent identified distributed OFDM symbol. The information used to find the symbol synchronization depends on the length of cyclic prefix. Actually, the data in the BPSK-OFDM modulated symbol, transferred by inverse fast Fourier transform, is with a complex symmetry character. With this character, more information from the whole OFDM symbol could be provided for the symbol timing. In this paper, the proposed algorithm uses the autocorrelation of the received signal to obtain the symbol timing. The simulation of Beek’s algorithm and the proposed scheme are presented. The performances of these algorithms are evaluated based on the missing symbol probability and the estimator mean square error. The simulation results show that the proposed algorithm is suitable to achieve a better symbol synchronization.