钻柱信道内隔声体结构建模与仿真分析

在随钻声传输技术中自钻柱底部上行的井下噪声及回波干扰严重降低声信号接收性能。基于低频纵波在声阻抗突变处及周期性管结构信道内的传输特性,设计具有周期性凹槽的隔声体结构。结合信道内噪声特征及隔声阻带中心频率,比较分析单结构隔声体短节与组合结构隔声体频响特性及隔声量,并将隔声体引入钻柱信道,用钻柱内一维低频纵波传输有限差分算法,时、频域仿真分析隔声体在信道内的隔声性能。结果表明,组合隔声结构可实现较宽的隔声频带,使噪声水平降低20~40 dB,从而改善信噪比、提高传输速率,为钻柱声遥测系统优化设计提供方法基础。     

协作通信中一种非差分分布式空时码

构建了一种循环酉矩阵群的非差分分布式空时码,该码字基于一个简易的三跳通信信道模型,可以应用于信道状态不是特别优良,或者由于通信距离两节点之间不能实现直接通信的情况,也可以应用于Ad hoc网络.在源节点处用循环对角酉矩阵群来构建发送码字,利用分集和的方法得到最优生成矩阵.在中继处利用广义Hadamard矩阵构建中继酉矩阵,对信号进行酉线性处理.并得出三跳通信模型下部分相干分布式空时码的译码公式,根据此公式得到如何选择生成矩阵来提高系统性能.最后对该码字进行仿真对比,得出在低信噪比情况下,两跳通信系统性能比三跳模型的好,但是在高信噪比下三跳通信模型要比两跳的更为优良,并且比差分编码性能提高3dB左右.     

钻柱信道内双声接收器的回波抑制方法分析

在随钻测井中地面噪声与周期性信道结构形成的下行回波导致传输性能降低,为此,结合钻柱信道的噪声分析,利用上、下行信道的瞬态脉冲响应,构建回波噪声抑制模型,提出了双声接收器的检测方式。在钻柱激励端分别施加单位正弦脉冲信号和PSK调制数据,在接收端耦合下行的高斯噪声,应用钻柱内一维低频纵波传输的有限差分算法,在单接收器和双接收器模式下时域和频域仿真分析了钻柱内声信号的传输特性,验证了双声接收器检测方式的有效性。该方法可实现井下回波噪声的抑制,从而改善信噪比,提高传输速率,为钻柱声遥测系统优化设计提供方法基础。     

基于NC-OFDM的未授权频段LTE-U非连续信道绑定方法研究

提出了未授权频段LTE-U和WiFi共存时基于非连续正交频分复用(NC-OFDM)技术的非连续信道绑定方法,使LTE-U系统通过频谱感知识别到空闲的WiFi频段后可进行更加灵活地信道绑定,从而提升频谱利用率。仿真显示基于NC-OFDM的非连续信道绑定的功率谱密度(PSD)可以满足IEEE 802.11a对带外泄露的保护要求。在相同信噪比情况下NC-OFDM相比连续正交频分复用(C-OFDM)具有较低的误码率,同时在发送端的峰值平均功率比(PAPR)也较低。     

基于海面散射的莱斯MIMO信道容量研究

通过相关莱斯信道模型对海面散射水声多入多出(MIMO)信道容量进行了研究。从Helmholtz-Kirchhoff积分和Fresnel近似出发,得到海面散射信号的空间相关性,由于信道的协方差矩阵不可以表示成发射相关矩阵和接收相关矩阵的Kronecker积,通过Monte Carlo仿真研究了莱斯因子、接收信噪比和空间相关性对MIMO信道容量的影响。从对outage capacity和信道容量的CCDF(Complementary Cumulative Distribution Function)的仿真计算结果可以看出,莱斯衰落的MIMO信道容量并不总是小于瑞利衰落MIMO信道容量,当信道的空间相关性较大和接收信噪比较小时,由于信道的衰落作用将起主要作用,对于较小的收发阵元数,以非衰落的直达信号为主的莱斯MIMO信道容量将大于瑞利衰落的MIMO信道容量。     

自适应最优相关间隔选择的多普勒频偏估计算法

在信道自相关特性的多普勒频偏估计算法的基础上,分析了存在的估计性能不受信噪比和频偏变化 影响的最优相关间隔。利用该特性提出一种逐次缩小搜索范围的迭代（CIO,correlation唱interval optimized）算法,实 现对其搜索和最大多普勒频偏的估计。通过仿真证明理论分析的正确及算法的有效性。与传统基于相关估计算 法相比,该算法自适应于信噪比和频偏变化,估计性能基本上独立于信噪比和频偏的变化,实现多普勒频偏的 直接性能估计。     

基于循环延迟分集的差分空时频编码方案

结合正交空时码的分集优势,将正交空时码应用到差分空频系统中,提出了一种基于循环延迟分集的多流差分空时频编码新方法,在差分系统中同时获得了空间和频率分集.该差分编码方法在满足信道的时间和频率选择性前提下,只需要发送很少的参考矩阵用于非相干检测.仿真结果表明,正交空时码的引入,可以有效提高整个系统的分集增益,从而在很大程度上改善整个差分MIMO-OFDM系统的误码性能.     

极大似然判决下大气无线光脉冲位置调制差错性能

大气湍流对无线光通信系统影响较大,基于高斯信道及门限判决方法的大气无线光通信脉冲位置调制(PPM)的误码特性分析存在局限性.针对于此,推导了湍流大气中无线光PPM系统在极大似然判决(MLD)下的差错性能计算模型,并运用蒙特卡洛方法验证了理论计算模型的有效性.仿真分析表明,MLD下PPM具有更好性能,但也较易受湍流影响,为使弱湍流信道中PPM的误比特率不高于10-6,调制阶数每增加1阶,须提高平均接收信噪比0.67 dB以上.对于峰值功率恒定PPM系统,应根据器件性能和具体通信条件,在差错性能和功率利用率之间作出平衡.     

基于线性非冗余预编码的MIMO-OFDM系统信道估计算法

文章针对MIMO-OFDM系统中的信道估计关键技术提出了一种信道估算方法—基于线性非冗余预编码和二阶统计量的信道盲估计算法,该算法利用预编码信号的二阶统计特性消除了调制信号的信息,然后又利用STBC（Space-Time Block Coding空时分组编码）特殊的复正交特性对参考子载波信道矩阵进行变换,将求逆过程转换成简单的线性处理过程,从而实现了低复杂度的信道估计。另外,该算法通过选取合适长度的统计数据,能够较快收敛。     

瑞利衰落信道下改进的AMC/SR-ARQ跨层设计方案

针对瑞利衰落信道提出了一种改进的自适应调制编码(AMC)与选择性自动重传请求(SR-ARQ)相结合的无线网络跨层设计方案.通过在计算不同调制方式对应的信噪比区间门限值时使用信道状态信息的二阶统计特性来计算误比特率(BER),使得选取的调制方式因考虑了无线信道的时变特性而更为合理,从而保证了为用户提供有服务质量(QoS)保证的服务.同时,对系统信道状态和系统吞吐进行了建模,并提出了N-成功/N 1-失败系统吞吐模型.在系统模型的基础上,对比分析了改进的方案和现有方案的性能指标.仿真实验证明,改进的AMC/SR-ARQ方案可以真正确保网络的平均误包率约束条件并提高平均频谱效率.     

不平衡2.5D碳纤维机织预制体偏轴拉伸性能

通过偏轴拉伸实验研究了因纱线细度变化而引起的不平衡2.5D碳纤维机织预制体（2.5D CFWPs）的面内剪切变形性能，实验材料包括4组不平衡2.5D CFWPs试样和1组平衡2.5D CFWPs试样。观察了不平衡2.5D CFWPs试样的典型宏观变形形貌和细观结构变化；对比了不平衡试样和平衡试样的剪切角和拉伸载荷变化；通过归一化处理，详细分析了纱线细度对预制体面内剪切变形的影响。结果表明:不平衡2.5D CFWPs试样典型变形形貌呈左右不对称形态，且纱线呈“S”型或“Z”型抽拔；经纬纱细度组合为792 tex×396 tex （表示经纱细度为792 tex，纬纱细度为396 tex）和396 tex×792 tex的不平衡2.5D CFWPs试样的偏轴拉伸性能基本相同，另两组不平衡试样（经纬纱细度组合为198 tex×792 tex和792 tex×198 tex）的偏轴拉伸性能也基本相同；平衡试样的拉伸载荷明显大于不平衡试样，在达到剪切锁紧后，平衡试样载荷快速上升，不平衡试样载荷上升缓慢；对拉伸载荷归一化处理后，剪切角小于20.81°时，5组2.5D CFWPs试样剪切力基本相同，在20.81°~30.02°时，4组不平衡试样剪切力基本相同。      

Dynamic Response Characteristics of a High-Temperature Superconducting Maglev Vehicle under Laterally Unbalanced Load Conditions

Various inevitable disturbances such as crosswinds, centrifugal forces, and nonuniform distributions of passengers or cargos may well cause the unbalanced distribution of the weight on-board of the high-temperature superconducting (HTS) maglev vehicle, more seriously making the maglev vehicle itself overturned. Therefore, it necessitates the investigation of the dynamic responses of the HTS maglev vehicle under unbalanced condition. In our experiments, the unbalanced/balanced weight distributions were artificially created by applying a laterally unbalanced/balanced loading force to the levitated body. Loads are located in the left side of the model for unbalanced load, and in the center of the model for balanced load for contrast. The dynamic responses of the HTS maglev model under such conditions were evaluated by sampling and analyzing the acceleration and displacement signals via the B&K vibration analyzer and a set of laser displacement sensors. The findings clearly reveal that the weight distribution makes a great impact on the dynamic responses of the HTS maglev model, and especially the laterally unbalanced distribution is indeed detrimental to the stability and load capacity of HTS maglev vehicle.     

Efficient reconfiguration of balanced and unbalanced distribution systems for loss minimisation

A simple and efficient two-stage reconfiguration algorithm for minimisation of active power loss in the balanced and unbalanced distribution systems is presented. In the first stage, the proposed method begins with all candidate switches closed. It uses current information obtained from a power flow, rather than from an optimal flow pattern used in other approaches to decide the switch to be opened, in a sequential manner. In the second stage, branch exchange operation is explored for further loss reduction. Results for four balanced systems and one unbalanced system have been obtained with the proposed method, and compared with those reported in the literature.     

The errors of differential balanced measuring circuits

Conclusions We have estimated the mean-square errors of two forms of balanced differential measuring circuits differing in respect of the positioning of the executive organ.We have determined the optimum values of the parameters of the balanced circuits minimizing the measuring error. We have shown that for incomplete correlation between the random variations of the signals being compared the optimum setting of the circuit differs from that usually employed. We have made a comparison between the minimum errors of unbalanced and balanced measuring circuits and have calculated the gain in accuracy obtained on replacing an unbalanced measuring technique by a balanced one. We have found that, for low modulation indices of the probing signal representing the quantity being measured, the use of a compensated measuring circuit instead of an unbalanced one does not increase the measuring accuracy and is therefore undesirable.Translated from Izmeritel'naya Tekhnika, No. 1, pp. 24–26, January, 1975.     

Control of a converter system for an asymmetrical parameter type two-phase induction motor drive operating in motoring and generating modes

The control of a converter system is presented and discussed for an asymmetrical parameter type two-phase induction machine drive that is operating in motoring and generating modes. The proposed system consists of back-to-back voltage source converters. For a machine side, a three-leg voltage source converter provides both unbalanced and balanced two-phase output voltages with a scalar V/F control based on a carrier space vector pulse width modulation (SVPWM) technique. For a front end, a single-phase AC/DC doubled voltage converter with hysteresis current control is used to keep DC-link voltage constant, thus resulting in a bi-directional power flow operation for the motoring and generating modes. A closed-loop design for the DC-link voltage is fully given and also included is a review of carrier-based SVPWM for two-phase three-leg VSI. The proposed drive system was both simulated using MATLAB/SIMULINK and implemented on digital microcontrollers. The comparative performance evaluation of the whole system between balanced and unbalanced two-phase voltages for the machine is given. The simulation and experimental results show that the unbalanced phase voltage offers better performance for the whole system.     

Wideband balanced folded dipole antenna with a dual-arm monopole structure for mobile handsets

In this study, a balanced antenna for mobile handset applications with enhanced bandwidth performance, covering the bands from 1.8 to 2.45 GHz, is investigated. The antenna is a slot planar dipole with folded structure and a dual-arm on each half. The antenna impedance was investigated using a two-port S-parameter method. For the purpose of antenna power gain measurement, a wide bandwidth planar balun was employed to support a balanced feed from an unbalanced source. For validation, a prototype of the proposed antenna was fabricated and tested. The performance of the antenna was then verified in terms of return loss, radiation patterns and power gain. The calculated and measured results show good agreement and the results also confirm good wideband characteristics with low induced current in the ground plane, thus minimising performance variations and SAR.     

A generalised instantaneous non-active power theory for STATCOM

A generalised instantaneous non-active power theory is presented. Comprehensive definitions of instantaneous active and non-active currents, as well as instantaneous, average and apparent powers, are proposed. These definitions have flexible forms that are applicable to different power systems, such as single-phase or multi-phase, periodic or non-periodic and balanced or unbalanced systems. By changing the averaging interval and the reference voltage, various non-active power theories can be derived from this theory. The definitions of instantaneous active and non- active currents provide an algorithm for a STATCOM to calculate the non-active current in the load current. The theory is implemented by the STATCOM, and four cases (three-phase balanced RL load, three-phase unbalanced RL load, diode rectifier load and single-phase load) are tested. The experimental results show that the STATCOM can perform instantaneous non-active power compensation, and both the fundamental non-active component and the harmonics are eliminated from the utility so that nearly unity power factor can be achieved. The STATCOM also has a fast dynamic response for transients.     

3x3 Mach-Zehnder interferometer with unbalanced differential detection for full-range swept-source optical coherence tomography

Quadrature interferometry based on 3x3 fiber couplers could be used to double the effective imaging depth in swept-source optical coherence tomography. This is due to its ability to suppress the complex conjugate artifact naturally. We present theoretical and experimental results for a 3x3 Mach-Zehnder interferometer using a new unbalanced differential optical detection method. The new interferometer provides simultaneous access to complementary phase components of the complex interferometric signal. No calculations by trigonometric relationships are needed. We demonstrate a complex conjugate artifact suppression of 27 dB obtained in swept-source optical coherence tomography using our unbalanced differential detection. We show that our unbalanced differential detection has increased the signal-to-noise ratio by at least 4 dB compared to the commonly used balanced detection technique. This is due to better utilization of optical power.     

Estimating the output of a series production system

This paper presents methodology which can be used to estimate the output of a production line with surge (in-process inventory or buffer stocks). This methodology is of use to those concerned with designing new production lines and modifying existing ones. The approach is specifically developed for unbalanced production lines but is applicable to the balanced line as well. A balanced production line is one where all machines operate at an identical rate. It is assumed that uptimes are exponentially distributed; however, the development is valid for general downtime distributions.     

Weighted radial basis collocation method for boundary value problems

This work introduces the weighted radial basis collocation method for boundary value problems. We first show that the employment of least‐squares functional with quadrature rules constitutes an approximation of the direct collocation method. Standard radial basis collocation method, however, yields a larger solution error near boundaries. The residuals in the least‐squares functional associated with domain and boundary can be better balanced if the boundary collocation equations are properly weighted. The error analysis shows unbalanced errors between domain, Neumann boundary, and Dirichlet boundary least‐squares terms. A weighted least‐squares functional and the corresponding weighted radial basis collocation method are then proposed for correction of unbalanced errors. It is shown that the proposed method with properly selected weights significantly enhances the numerical solution accuracy and convergence rates. Copyright © 2006 John Wiley & Sons, Ltd.