浅海远程水声Turbo编码跳频通信研究

结合Turbo编码和水声跳频通信技术,基于数字信号处理(DSP)成功地研制出了一种高效的抗多途、抗噪声浅海远程水声通信系统.该系统针对浅海远程水声信道的低信噪比、强多途及快速时变衰落特点,采用一种联合有限脉冲响应(FIR)数字窄带滤波和线性调频信号互相关检测的快速、精确的帧同步方法;利用快速频谱分析进行解调,使用复杂度...     

基于有效抽头和进化规划算法的自适应水声信道估计

为了实现稀疏水声信道的快速高效估计,提出一种新的基于有效抽头的信道参数模型降低运算复杂性,并采用进化算法以最小二乘误差为适应度函数对信道响应中有效抽头的阶数、位置和权重同时进行进化操作寻优,避免了有色输入造成的相邻抽头耦合导致的性能下降。仿真实验结果表明:与传统LMS算法及基于有效抽头检测的稀疏信道估计方法相比,该算法在有色输入信号下具有更优越的估计性能和收敛速度,为稀疏水声信道处理提供了一类新方法。     

一种新型的滤波器组在水声应答器中的应用

针对某型水声应答器中处理信号频带变宽,信号中叠加了多个频率分量以及实时性要求高的问题,提出一种基于频域信道化技术的滤波器组方法。首先给出该滤波器组的理论推导,并对滤波器组进行加窗处理,给出了FFT的长度和移动重叠点数等参数对该滤波器组的影响,最后进行了数值仿真,并进行了湖试试验数据处理。结果证明该方法可有效解决信号的信道串漏问题,验证了该滤波器组的正确性和可行性。该方法通过对信号信道化处理,减小了滤波处理的运算量,提高了数据的信噪比,便于后续的目标检测。该滤波器组算法简单、易实现、运算量小,在水声应答器信号处理中有一定的工程借鉴价值。     

基于分数阶功率谱熵的未知水声脉冲信号检测方法

水声侦察的核心问题是在无先验知识条件下捕获其他平台发射的脉冲信号,单频（Continuous Wave,CW）信号和调频（Frequency Modulation,FM）信号是常用的水声探测脉冲。功率谱熵算法能有效检测低信噪比的CW信号,但对FM信号性能不佳,分数阶傅里叶变换（Fractional Fourier Transform,FRFT）则能聚集FM信号能量。利用FRFT的性质,结合功率谱熵算法,设计了分数阶功率谱熵检测器,可在分数阶域实施未知脉冲信号检测。理论分析了FRFT对FM信号的能量聚集作用,优化了FRFT阶数搜索方法。仿真实验以及海试数据处理结果证实检测器对FM信号性能良好,且对CW信号的侦察检测性能无影响。通过门限学习,检测器可实现对未知水声脉冲信号的统一自动检测。     

一种低能耗的水声通信编码方案的研究

研究了一种用多个不同频率的CW窄脉冲与帧同步脉冲的时延差携带信息的水声通信技术。该技术利用多个不同频率的CW矩形窄脉冲与帧同步CW矩形窄脉冲之间的时延值进行信息编码,在接收端采用时延估计和频率检测技术进行解码,从而使码元间距成为可携带信息的有用资源,将码元间距扩展到足够宽,既可有效地克服水声信号的多途影响、提高频带利用率,又可大大减少码元脉冲数量,达到节省能量并提高通信可靠性的目的。仿真结果表明,该水声通信编码体制数据传输速率适中,编码和解码简易,系统稳健。     

DS-SS在水声跟踪定位系统中的性能分析

传统的水声跟踪定位系统使用单频脉冲信号（Continuous Wave,CW）。CW信号系统容易实现,但却存在很多缺点：对多径干扰敏感、无法多址、无法解决距离分辨力与作用距离之间的矛盾、保密性差等。扩频技术（Spread Spectrum,SS）能够克服CW信号系统的缺点,在通信中有着广泛的应用。在浅水区域,直接序列扩频（Direct Sequence Spread Spectrum,DS-SS）是更适合于水声信道的扩频技术。讨论了一种直接序列系统模型,并仿真了这种模型的抗多途干扰性能、多用户性能、抗单频干扰性能。仿真结果显示此模型性能优良,实用性强。     

基于深度神经网络的水声FBMC通信信号检测方法

针对传统水声滤波器组多载波（Filter Bank Multi-Carrier,FBMC）通信接收端需经过信道估计和均衡才可恢复出发送符号,系统复杂度高且信道估计精度不佳等问题。文章将深度神经网络融入到水声多载波通信当中,提出一种基于深度神经网络的水声FBMC信号检测方法。在训练阶段通过大量的数据迭代、调试超参数和优化算法来改善深度神经网络参数,使其具有预期的估计效果。利用训练完成的深度神经网络模型取代传统FBMC通信系统接收端的信道估计、均衡等模块,自适应地学习水声信道状态信息,同时避免了固有的虚部干扰影响。在测试阶段直接将频域序列作为网络的输入来预测发送的二进制序列,仿真结果表明所提出的基于深度神经网络的FBMC信号检测方法相比传统信道估计算法有更好的误码率性能。     

基于混沌的水声定位信号设计及性能分析

针对水声信道中复杂的噪声环境及各种干扰对定位准确度的影响,利用扩频通信技术的抗干扰、抗噪声能力,提出一种基于混沌的扩频信号产生方法。将该信息作为水声定位信号,通过相关法得到精确时延,进而可以计算出节点间的距离,通过在水声信道下对其性能进行分析。仿真结果表明:该扩频信号相关特性理想,具有很强的抗干扰能力,测距准确度高,适合用作水声定位信号。     

一种浅海水声信道分数间隔自适应均衡算法研究

水声信道是最为复杂的数据通信环境之一,而具有重要军用和商用价值的浅海水声信道,其多径与频散效应更为严重。在浅海水声数字通信中,普遍存在着码间干扰和码内干扰效应,严重影响了水下通信的速度和质量。在判决反馈结构的基础上,采用分数的方法,提出了一种适用于浅海水声信道的自适应均衡算法,并通过计算机仿真实验与整数方法进行了性能对比。传输信号采用正交相移键控调制方式,以有效利用有限通信带宽。仿真结果表明,该算法获得了较快的收敛速度和较小的稳态误差,提高了水声通信的有效性及可靠性,因而具有良好的工程应用价值。     

适用于水声信道的新的盲均衡算法

由于时变水声信道的随机相位失真，常模算法（CMA）的误码性能将严重下降。在文献[1]中双模式盲均衡算法的基础上，提出了一种适用于水声信道的新的盲均衡算法（NCMA）。计算机仿真结果表明在时变水声信道中，该算法性能优于双模式算法，能够达到信号相位失真的恢复．并且收敛性能优于双模式算法。     

Thermophysical properties of some key solids: An update

In 1985, the CODATA Bulletin published a Report of its Task Group on Thermophysical Properties of Solids which analyzed available data on, and gave recommended values for, the heat capacity of Cu, Fe, W, and Al₂O₃, the thermal expansion of Cu, Si, W, and Al₂O₃, the electrical resistivity of Cu, Fe, Pt, and W, the thermal conductivity of Al, Cu, Fe, and W, and the absolute thermopower of Pb, Cu, Pt, and W. The analysts for the different properties were R. B. Castanet, S. J. Collocott, P. D. Desai, C. Y. Ho, J. G. Hust, R. B. Roberts, C. A. Swenson, and G. K. White. The present paper is an updated version of the earlier report and includes more recent data which change some of the recommended values. notably the heat capacity of Cu and W and the thermal expansion of Si and W.     

The viscosities and densities of selected organic compounds and mixtures of interest in coal liquefaction studies

Experimental measurements are presented for the density and viscosity of selected organic compounds and mixtures at ambient pressure (0.083 MPa) and at temperatures of 298, 318, 338, and 358 K. The compounds studied were decalin, 1-methylnaphthalene, tetralin, m-xylene, tetrahydrofuran, thiophene, quinoline 2,6-lutidine, and m-cresol. Measurements were also made on three mixtures of the compounds decalin, 1-methylnaphthalene, tetralin, m-xylene, and m-cresol. The experimental results are compared with predictions made using a modified corresponding states procedure called TRAPP. The density predictions for the individual compounds and mixtures are good in all cases. For the viscosity, however, the predictions are in reasonable agreement with experiment only for nonassociating compounds and mixtures at reduced densities less than 3. These results suggest that TRAPP may prove very useful as a screening test to distinguish between nonassociating and highly associating mixtures. Such a test would be extremely useful when dealing with mixtures of unknown composition, such as coal liquids.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Cellular cryobiology: thermodynamic and mechanical effects

Several physical stresses kill cells at low temperatures. Intracellular ice is usually fatal, so survival of freezing temperatures involves combinations of dehydration, freezing point depression, supercooling and intracellular vitrification. Artificial cryopreservation achieves intracellular vitrification with rapid cooling, modest osmotic contraction and, often, added cryoprotectants. High warming rates are required to avoid crystallization during warming. Environmental cooling is much slower and temperatures less cold, but environmental freezing damage is important ecologically and agronomically. For modest sub-freezing temperatures, supercooling sometimes allows survival. At lower temperatures, extracellular water usually freezes and cells may suffer large osmotic contractions. This contraction concentrates solutes and thus assists vitrification, but is not necessarily reversible: the rapid osmotic expansion during thawing may rupture membranes. Further, membranes and other ultrastructural elements may be damaged by the large, anisotropic mechanical stresses produced when their surfaces interact via hydration forces. Solutes reduce these stresses by osmotic, volumetric and other effects.     

Betrachtungen zum Löten in Gegenwart und Zukunft

Consideration on brazing and soldering today and in future In all parts of industries using metals, the application of brazing and soldering is increasing. The origin of those technologies is assumed 6.000 years ago, and for a long time, they were limited to a few industrial fields only. In recent years, they turned out to become a source of innovation. Today, they are essential in different industries as microelectronics, electrical engineering, automotive industry, aviation industry, turbine industry or tool industry. Starting with defining brazing and soldering, especially in comparison to competitive joining techniques, some selected developments of the last decades are presented, as brazing of Aluminium, High Temperature Brazing and brazing of ceramic materials. Finally, an outlook is given concerning ongoing and future developments.     

Viscosity Measurements and Predictions for Natural Gas

A vibrating-wire viscometer of very high precision was used to measure the viscosity of methane and of two natural gases. The experimental data were, in general, taken at temperatures of 260, 280, 300, and 320 K and at pressures up to 20 MPa, and additionally in the case of methane at temperatures of 340 and 360 K and at pressures up to 29 MPa. The estimated uncertainty is ±0.3 and ±0.5% for methane and the natural gases, respectively. The new experimental data for methane were used together with zero-density or low-density viscosity values from this study and from the literature to develop a viscosity equation for natural gas composed of two contributions. The mixing rule of Wilke [J. Chem. Phys. 18: 517 1950] was applied for the zero-density viscosity part which is based on zero-density correlations for twelve components (methane, nitrogen, carbon dioxide, ethane, propane, n- and isobutane, n- and isopentane, n-hexane, n-heptane, and n-octane) and agrees with the values derived from experiment within ±0.3%. The density dependence of the residual viscosity part was correlated with methane data only, neglecting any temperature dependence, whereas the composition dependence is characterized by a pseudo-critical viscosity value. For methane the agreement between the correlated and experimental data is within ±0.5 %. The values predicted with the correlation and the experimental data agree within ±1 % for both the high calorific, H, natural gas and the low calorific, L, natural gas.Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

真空计的发展新趋势

结合真空计的发展历史和使用需求 ,通过对国际上近几年的真空计新产品的分析可知 ,目前常用真空计具有向小型化、一体化、集成化、系统化和智能化方向发展的趋势。小型化是指真空计的体积越来越小 ,一体化是指真空计电路部分与规管集成为一体 ,集成化是指将多只真空计组合为一体 ,系统化是指将真空度的测量与控制相结合 ,智能化是指真空计具有自我诊断、自我保护、自动操作和数据处理等综合处理功能。这些趋势是在电子技术和计算机技术迅速发展的技术推动下 ,为满足应用的需求而产生的结果。     

Thermophysical properties of ice,snow, and sea ice

The paper reviews and discusses data and information on the thermophysical properties of ice, snow, and sea ice. These properties include thermal conductivity, specific heat, density, thermal diffusivity, latent heat of fusion, thermal expansion, and absorption coefficient. The available data are shown graphically for convenience in conjunction with the recommended correlation equations.Paper presented at the Second U.S.-Japan Joint Seminar on Thermophysical Properties, June 23, 1988, Gaithersburg, Maryland, U.S.A.     

Predictors of driving exposure and avoidance in a field study of older drivers from the state of Maryland

Using structural equation modeling techniques, this study examines causal models of driving avoidance and exposure among older adults. Prior studies have revealed that past incidence of falls, Useful Field of View (UFOV) test performance, and Trails Making test performance are predictive of subsequent motor vehicle crash involvement [Owsley, C., Ball, K., McGwin Jr., G., Sloane, M.E., Roenker, D.L., White, M.F., Overley, E.T., 1998. Visual processing impairment and risk of motor vehicle crash among older adults. J. Am. Med. Assoc. 279 (14), 1083-1088; Sims, R.V., McGwin, G., Pulley, L., Roseman, J.M., 2001. Mobility impairments in crash-involved older drivers. J. Aging Health 13 (3), 430-438; Stutts, J.C., 1998. Do older drivers with visual nd cognitive impairments drive less? J. Am. Geriatr. Soc. 46, 854-861]. Data analyses used these indices, along with age, health, measures of physical functioning, and additional measures of cognitive functioning, to examine driving exposure and avoidance behaviors. A field sample of 4,234 drivers, 55 years of age and older, were recruited from the Maryland Motor Vehicles Administration after renewing their driver's licenses. A performance-based assessment, which included the Gross Impairment Screening battery and task 2 of the UFOV test, was completed by participants. A sub-sample of participants (n=815) were interviewed by telephone about their health and mobility 3-6 months following the initial assessment at a renewal center. In addition to age and gender, latent variables for health status, physical functioning, cognitive functioning, driving exposure, and driving avoidance were created. Direct and indirect causal paths were specified. Age, gender, health status, and cognitive functioning had direct effects on both driving exposure and driving avoidance; physical functioning did not have a direct effect on driving exposure or avoidance. The implications of these findings are discussed as they relate to designing interventions to promote mobility.     

Multi-response optimization of the electrical discharge machining of insulating zirconia

In this study, electrical discharge machining has been used to machine insulating zirconia via the assisting electrode method. The process parameter optimization was investigated by combining the Taguchi method with grey relational analysis. The application of Taguchi–grey relational analysis is proven to effectively improve the performance of electrical discharge machining in drilling insulating zirconia. The results of this analysis indicate that the final optimal process parameters are a peak current of 8 A, a pulse duration of 16?µs, a duty cycle of 0.5, and a flushing pressure of 6?MPa. Additionally, the material removal rate, electrode wear rate, and hole taper ratio increase by 39%, 1.5%, and 1.3%, respectively, which improves the grey relational grade by 6.8%. The electrical resistance test confirms that the conductivity of the conductive layer obtained using the final optimal process parameters is better than that of the conductive layer obtained using the initial optimal process parameters. Energy spectrum analysis reveals that the conductive layer is composed of C, Cu, Zn, Zr, and O. Analysis of variance shows that the most significant component of the multi-responses is the peak current, with a 51.4% contribution.     

低介电常数含氟氧化硅薄膜的研究

综述了近十年来低介电常数含氟氧化硅薄膜的研究状况,详细介绍了该薄膜在化学键结构、热性质、湿稳定性以及介电常数四个方面的特性,同时也简单介绍了薄膜的台阶覆盖度、填隙能力和漏电流特性,指出经硅薄膜是一种可有于集成电路中的极富应用前景的低介电常数材料。