浅海水声信道中QC-LDPC码性能研究

针对大起伏、强多途、窄带宽的浅海水声信道需采用码长短、性能好、易于实时处理的信道纠错码技术才能提高水声通信系统可靠性的问题,提出了准循环低密度奇偶校验( QC-LDPC)码作为浅海水声信道的信道纠错编码方案.建立了QC-LDPC码在水声信道中的仿真模型,采用BP译码算法,在典型3径浅海水声信道中就码率、围长等参数对误码...     

水声DS-CDMA通信中多相位假设码片速率自适应判决反馈均衡算法研究

研究了直接序列扩频码分复用(DS-CDMA)通信中基于多相位假设的码片速率自适应判决反馈均衡算法.水声信道的衰落和多普勒频移会严重影响扩频信号的相关特性,需要在做解扩频之前进行多普勒频移补偿和信道自适应均衡.将空间分集-多普勒频移补偿-自最佳自适应判决反馈均衡算法应用到DS-CDMA通信中,提出一种基于多相位假设的码片速率自适应判决反馈均衡算法,并用实际的千岛湖实验数据和仿真数据对算法性能进行了分析.结果表明算法以增加计算复杂性为代价显著提高了DS-CDMA通信性能,在复杂快变的多途和多普勒频移条件下可以保证低的误符号率,在低信噪比条件下能够稳定工作,总体性能良好.     

纠错编码技术在水声跳频通信中的性能研究

将水声跳频通信系统与纠错编码技术相结合进行了研究,针对复杂多变、强多途和起伏干扰的浅水水声信道,建立了基于BELLHOP射线模型的时变衰落水声信道模型,该信道模型建立起了信道特性与海洋环境参数之间的关系,在海洋环境参数给定的条件下可以精确仿真信道。在此基础上通过系统仿真的方法研究了卷积码、RS码和串行级联码三种纠错编码方案在水声跳频通信系统中的性能,给出了两种典型信道下的仿真结果并进行了对比分析,结果表明串行级联码具有更强的纠错能力,比较适合复杂多变的水声衰落信道。     

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。     

基于MSS-PDS和单阵元PTRM信道均衡的深海远程水声通信方案

为了提高水声通信的性能,提出了一种联合运用M元扩频（MSS）通信和Pattern时延差编码（PDS）水声通信体制,并采用单阵元被动式时间反转镜（PTRM）来实现声信道均衡的深海远程水声通信方案。该方案的M元扩频-Pattern时延差编码通信技术既能胜任远程水声通信,又能提高通信速率;单阵元被动式时间反转镜信道均衡技术既可抑制多途扩展产生的码间干扰,又能提高信噪比。根据深海声道特性分析,该方案将通信固定节点置于声道轴以获取会聚增益,提高通信距离。计算机仿真结果证明,所提出的深海远程水声通信方案具有很好的鲁棒性和可行性。     

一种码分多址水声多用户接收系统

研究了一种水声异步多用户通信非集中式接收机系统,将码分多址和空时自适应滤波、多通道判决反馈均衡相结合,克服多用户系统中的多址干扰（MAI）以及水声信道多途效应引起的码间干扰（ISI）,实现指定用户的正确接收。利用实测多通道浅海水声信道响应函数进行了三个用户的异步通信仿真,分别比较了空时滤波均衡与单通道直接解扩、空间分集合并均衡等接收算法的性能,显示了空时滤波均衡接收机在抑制多址干扰、码间干扰、正确区分不同用户等方面的优点。码分多址机制和空时滤波均衡结合使用的特点使系统适合于低速数据传输且水声信道变化较慢的场合。     

Spinal码在深空通信中的应用研究

为了提高深空通信系统的传输能力,将Spinal码用于深空通信研究。结合深空信道损耗大、时延长、动态时变等特点,研究了适合深空信道的Spinal码编译码参数组合、星座映射及信道工作模式的选择等问题。通过仿真优化了Spinal码的编解码参数,对比了Spinal码在模拟信道和数字信道下的性能,给出了与Spinal码相匹配的调制方式。研究结果表明Spinal码具有极好的深空通信应用前景:采用256、512bit的短码长设计,配合以16APSK调制的Spinal码,在模拟信道工作模式下,性能远优于传统上采用的复杂度极高的低密度奇偶校验码(LDPC),在不同的信噪比条件下均能获得逼近Shannon限的性能。     

一种水声通信中基于信道估计的假设反馈自适应均衡器

直接序列码分多址(DS/CDMA)是一种受关注的水下声通信网多址接入方案。提出一种基于信道估计的水声DS/CDMA系统自适应码片级均衡器。水声信道的时变特性要求自适应均衡器系数以码片速率更新,而不是以码元速率更新。在线性均衡之前利用信道估计器抑制由先前发送的码片引起的码片间干扰,仅对少量幅值较大的信道抽头做均衡,有效地降低了运算复杂度。仿真实验验证了均衡器的性能。     

一种抗信道多径的扫频-扩频调制技术

水声信道是复杂的多径快衰落信道,其信道特性严重影响了水声通信的数据率和性能,抗多径干扰是提高水声通信系统性能的重要技术之一。对一种新的调制方法——扫频-扩频载波（Sweep-Spread Carrier,SSC）调制技术进行了深入研究,通过系统在水声信道实际数据的仿真,探究SSC的抗多径性能,简化了抗信道多径算法。SSC的基本思路是采用连续变化的载波频率,通过相关接收,将接收的多径信号在频域上分离,从而克服了码间干扰的影响。分析了在给定系统时不同信道多径时延对SSC性能的影响,以及给定信道时不同SSC参数对SSC性能的影响。仿真结果表明,选择合适的参数,SSC调制具有良好的抗多径性能。     

OFDM水声通信线性最小均方误差算法信道均衡

正交频分复用在高速水声通信中的应用越来越广泛.水声信道是多径、高噪声的衰落信道.由于信道条件差,容易产生码间干扰,信道均衡技术是提高通信系统性能的重要技术.提出了线性最小均方估计在水声通信系统中应用的方案,研究了实际通信中利用调频信号如何确定信号的平均多径时延、最大多径时延,比较了不同调频信号测量信道的性能.该方案基于导频辅助均衡方法,利用调频信号测量信道,获得的信道统计信息,计算信道的自相关矩阵.从而根据线性最小均方误差算法进行信道均衡.通过仿真和湖试进行验证,结果表明该方案效果较好,对系统性能改善比较明显.     

Insertion of triangulated surfaces into a meccano tetrahedral discretization by means of mesh refinement and optimization procedures

In this paper, we present a new method for inserting several triangulated surfaces into an existing tetrahedral mesh generated by the meccano method. The result is a conformal mesh where each inserted surface is approximated by a set of faces of the final tetrahedral mesh. First, the tetrahedral mesh is refined around the inserted surfaces to capture their geometric features. Second, each immersed surface is approximated by a set of faces from the tetrahedral mesh. Third, following a novel approach, the nodes of the approximated surfaces are mapped to the corresponding immersed surface. Fourth, we untangle and smooth the mesh by optimizing a regularized shape distortion measure for tetrahedral elements in which we move all the nodes of the mesh, restricting the movement of the edge and surface nodes along the corresponding entity they belong to. The refining process allows approximating the immersed surface for any initial meccano tetrahedral mesh. Moreover, the proposed projection method avoids computational expensive geometric projections. Finally, the applied simultaneous untangling and smoothing process delivers a high‐quality mesh and ensures that the immersed surfaces are interpolated. Several examples are presented to assess the properties of the proposed method.     

Certification Institutions Expected to Update and Transfer Information on Present Standards

Standards are the basis for production enterprises to organize production, ex-factory inspection, trade (delivery) and technical exchanges, product certification, quality arbitration and supervision.……     

Flow calorimeter for enthalpy increment measurements on condensed gases

A flow calorimeter for enthalpy increment measurements on condensed gases is presented. A better knowledge of the properties of the liquefied natural gas is needed, and therefore a liquid loop has been designed for our flow calorimeter. The fluid loop in the calorimeter is designed in order to avoid the two-phase region, since two phases would give compositional disturbances in the measurements. The avoidance of the two-phase region is made possible by increasing the pressure of the test fluid after the measurement section, then heating the fluid at super-critical pressure past the critical point. Finally, the fluid is throttled to the low-pressure gas state at the inlet condition of the compressor that circulates the fluid. To perform the pressure increase, a new cryogenic pump has been designed. To evaluate the new equipment, measurements were taken on liquid ethane over the temperature range 146–256 K at pressure between 0.9 and 5.1 MPa.     

Refrigerators Enter a New Era of Health Preservation Frestech initiates the establishment of China's first standard for household sterilization refrigerators

On November 30, 2007, the China Association for Standardization (CAS) held a press conference at Beijing Diaoyutai State Guest House. Leaders from the China Household Electric Appliance Research Institute, the China Household Electric Appliance Association, and the China Consumers' Association attended and made speeches.……     

Optimum-Simulation von Ernteprozessen

Zusammenfassung Bei der Gestaltung von Ernteprozessen — beispielsweise des Prozesses Mähdrusch, Körnertransport und -abnahme — als transportverbundene Fließarbeitsverfahren mit mindestens zwei verschiedenen Arbeitsmitteln gibt es einen großen Optimierungsspielraum. Solche Prozesse sind mehrstufige Bedienungsprozesse mit gemischter Anordnung der Bedienungseinrichtungen. Außerdem besitzen sie stark ausgeprägte räumliche Aspekte. Es wird vor allem gezeigt, welche Merkmale solcher Prozesse in Simulationsmodellen abgebildet sein müssen, welche Zielgrößen in Abhängigkeit von welchen Einflußgrößen unter Berücksichtigung welcher Prozeßbedingungen ermittelt werden sollten und wie der Bewertungsprozeß zur Ermittlung der gewünschten Informationen gestaltet werden muß.     

A study of centrifugal cast high boron high speed steel

In the present study a high‐boron high speed steel (HSS) roll material was designed. Many expensive alloy elements have been substituted by cheap boron alloy, and high‐boron high speed steel roll has been manufactured by centrifugal casting method. The microstructures, mechanical properties and wear resistance of centrifugal casting high‐boron high speed steel roll have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), and X‐ray diffraction (XRD) analysis, hardness test, impact test and wear test. The results indicated that the solidification microstructures of high‐boron high speed steel roll consisted of M₂(B,C), (W,Mo)₂(B,C), M₃(B,C), M₂₃(B,C)₆ type borocarbides and martensite, a small amount of retained austenite. Borocarbides were continuously distributed over the grain boundary. After quenching from 1050 °C, local broken network appeared in partial borocarbides, and fine secondary borocarbide precipitated from the matrix. After tempering from 525 °C, the amount of precipitated borocarbide increased significantly. After heat treatment, the hardness of high‐boron high speed steel roll excelled 60 HRC, and its impact toughness excelled 8.0 J/cm². The single groove steel rolling amount of high‐boron high speed steel rolls increases by 500% than that of bainite cast iron roll, when the rolls are used in K₁ mill housing of bar mill.     

Thixotropy of Systems with Shear Thinning and Plastic Flow Behavior

The definition of the thixotropy is a decrease in viscosity with time in shear and a subsequent recovery of viscosity after the shear deformation is removed.We ...     

Brainstorming for ISO Strategy 2016-2020

正Responding to ISO’s appeal for survey for the to-be-developed ISO Strategy Plan for 2016-2020,SAC hosted a special workshop over the needs and comments of various fi elds on June 8 in Beijing.The workshop was attended by ISO President-elect and Ansteel General Manager Zhang Xiaogang,SAC Vice-Administrator Yu Xinli,and 30 experts from     

CNAS Passes APLAC's PTP Review

正The review group of Asia Pacific Laboratory Accreditation Cooperation(APLAC)carried out a 4-day peer review of the proficiency testing provider(PTP)for China National Accreditation Service for Conformity Assessment(CNAS)from July 28 to 31,2014.CNAS passed the PTP review with excellent performance.Two senior peer reviewers of APLAC made a comprehensive,rigorous and detailed review of CNAS’s PTP accreditation activities,and made the on-spot inspection of the review groups in Shanghai and Shenzhen respectively.