一种改进的航空发动机结构概率风险评估方法

针对航空发动机适航条款FAR33.75中关于发动机限寿件(ELLP)结构失效概率要求,提出了一种基于Kriging和蒙特卡罗半径外重要抽样(MCROIS)混合的结构概率风险评估方法。该方法针对ELLP高维、小失效概率事件以及极限状态函数为隐式、高度非线性的特点,利用Kriging元模型模拟隐式极限状态函数,然后通过主动学习迭代算法,计算最优点(MPP,最接近设计验算点的样本点),更新实验设计(DOE)并提高Kriging元模型的模拟精度。在此基础上,利用Kriging元模型确定最优抽样半径,构造半径外重要抽样密度函数,在最优抽样半径确定区域进行抽样,通过构造主动学习函数,使样本点更多落在抽样半径确定的球区域附近,加速失效概率计算的收敛,并构建了ELLP风险概率模型,解决了高维、小失效概率事件以及隐式、非线性极限状态函数的发动机结构概率风险评估难题,以某型发动机低压压气机轮盘为应用示例,与传统的蒙特卡罗仿真(MCS)方法进行了对比,验证了该方法的高效率、鲁棒性和仿真精度。     

局部概率传播译码的非系统低密度卷积码

低密度卷积码(LDCC)是低密度码(LDPC)经由其产生矩阵的侧滑变换成为带状产生矩阵而得到的,具有LDPC和卷积码的误码性能好等一些共同特征。LDCC码的和积算法译码方法可以进一步改进为局部概率传播译码方法。采用流水线结构的局部概率传播译码方法用于LDCC码得到了接近和积算法的译码性能,而且译码延迟比LDPC大为降低,译码器更为简单,便于实时工作。     

遗传算法在微弱GPS信号捕获方法中的应用

 研究了微弱GPS信号的捕获问题，以捕获时较低的虚警概率和较高的检测概率为目标，以累加时的粗捕获(C/A)码周期数和导航数据数，以及捕获门限等参数为决策变量，应用遗传算法(GA)来解决这一多目标优化问题。仿真结果表明，该方法能够有效地解决所提出的问题，所得解能够优化捕获时欲选取的参数，从而提高捕获性能。     

航空发动机限寿件疲劳可靠度计算新方法

针对目前的航空发动机限寿件（ELLP）疲劳可靠性分析中的小失效概率事件以及其极限状态函数具有较强非线性的特点，提出了一种具有自更新机制的半径外自适应重要抽样（AUMCROAIS）疲劳可靠性分析方法。该方法首先利用蒙特卡罗自适应重要抽样（MCAIS）快速逼近真实设计验算点（MPP）附近，随后以近似设计验算点为中心进行极坐标抽样，并依次构造主动学习函数，对近极限状态函数和抽样半径进行最优选取，从而实现最优抽样半径的更新，通过不断的更新确定出最优抽样半径，加速失效概率计算的收敛。本方法提高了设计验算点的收敛速度同时保证了计算精度，解决了小失效概率事件以及强非线性极限状态函数可靠度计算难题，最后以某型发动机压气机轮盘为对象应用本方法，并与传统的蒙特卡罗仿真（MCS）方法、蒙特卡罗半径外自适应重要抽样法（MCROAIS）和一阶可靠性方法（FORM）进行了对比，验证了本方法的高效率、鲁棒性和仿真精度。     

线性分组码不可检错误概率新的下限

线性分组码的不可检错误概率是评价自动请求重传(ARQ)系统的重要参数。不可检错误概率的计算涉及码的重量分布,而大部分码的重量分布还不知道,因此,研究不可检错误概率的限显得非常重要。本文得到了GF()上(n,k)线性分组码不可检错误概率的两个新的下限,在一定程度上,改进了Wolf等人的结果。由于新的限与码的重量结构无关,因此可以方便地估计任何线性分组码的不可检错误概率。     

线性分组码限距离译码器的性能分析

为了提高自动重传请求(ARQ)系统的通过率,在混合ARQ系统中,使用限距离译码器改善系统的性能。在限距离译码时,线性分组码纠正重量t的错误图样。纯纠错译码器和纯检错译码器都是限距离译码器的特例。本文首先介绍了Krautchouk多项式的几个重要性质,然后,用线性分组码及其对偶码的重量分布,分别导出了两种形式的错误译码概率的简化计算公式。由于生成函数方法的使用,成功地得到了重复码、汉明码和扩展汉明码的错误译码概率的解析公式,最后,讨论了Golay码和某些本原BCH码错误译码概率的计算方法。     

一种用于深空自主无线电的码速率估计算法

针对深空自主无线电的码速率盲估计问题,在码元起始位置未知的情况下,通过对估计误差的理论分析,提出一种改进的基于分离符号矩估计(SSME)的码速率估计算法,设计分区估计信噪比(SNR)并进行比较选大的方法,以保证码速率估计的精度。同时对该算法的计算复杂度进行定量分析,并与传统单次延迟相乘法、改进的多次累加延迟相乘法进行比较。仿真结果表明,本文算法能够有效完成低SNR条件下的码速率估计,其误估计概率明显优于单次延迟相乘法,与多次累加延迟相乘法相当,但计算复杂度不到多次累加延迟相乘法的10%。     

不含短环的（n，3，k）LDPC码的几何构造方法

提出了基于不含短环的(n,2,k)规则低密度奇偶校验(LDPC)码的一种最短环长为8的(n,3,k)规则LDPC码的几何构造方法。该方法简单直观而有效。仿真结果显示,在AWGN信道中该码具有明显优于随机构造的规则LDPC码的性能。     

一种限定虚警概率的PN码捕获自适应门限估计算法

提出了一种限定虚警概率的PN码捕获的自适应门限估计算法,首先在对判决变量的统计特性分析的基础上,计算出了判决门限的有偏估计量;然后分析了估计偏差对捕获系统检测概率和虚警概率的影响;最后,计算机仿真表明,在限定虚警概率的前提下,捕获系统在高斯白噪声信道和瑞利衰落信道下具有较高的检测概率,自适应门限的估计方法易于实现,且适合工程应用。     

基于FPGA的BCH(31,21)码译码器的设计

本文介绍了采用FPGA设计BCH(31．21)码的译码器的方法．译码器能对BCH(31．21)码进行译码和纠正低于或等于2位的随机错误．并给出了在MAX PLUSⅡ软件平台下的仿真结果。     

Neural Networks Combined with Importance Sampling Techniques for Reliability Evaluation of Explosive Initiating Device

Concerning the issue of high-dimensions and low-failure probabilities including implicit and highly nonlinear limit state function, reliability analysis based on the directional importance sampling in combination with the radial basis function (RBF) neural network is used, and the RBF neural network based on first-order reliability method (FORM) is to approximate the unknown implicit limit state functions and calculate the most probable point (MPP) with iterative algorithm. For good efficiency, based on the ideas that directional sampling reduces dimensionality and importance sampling focuses on the domain contributing to failure probability, the joint probability density function of importance sampling is constructed, and the sampling center is moved to MPP to ensure that more random sample points draw belong to the failure domain and the simulation efficiency is improved. Then the numerical example of initiating explosive devices for rocket booster explosive bolts demonstrates the applicability, versatility and accuracy of the approach compared with other reliability simulation algorithm.     

Cyclic code shift keying: a low probability of intercept communication technique

A low probability of intercept (LPI), or low probability of detection (LPD) communication technique known as cyclic code shift keying (CCSK) is described. We discuss the basic concepts of CCSK and describe a system based on the use of random or pseudorandom codes for biphase modulation. We use simulation to show that the bit error rate (BER) for CCSK can be closely estimated by using existing equations that apply to M-ary orthogonal signaling (MOS). Also, we show that significantly fewer computations are required for CCSK than for MOS when the number of bits per symbol is the same. We show that using biphase modulation results in waveforms that have a large time-bandwidth product and very low input signal-to-noise ratio (SNR) and thus inherently have an LPI by a radiometer. We evaluate detection by a radiometer and show that LPI can be achieved by using codes of lengths greater than about 2/sup 12/ (i.e., by transmitting more than about 12 bits per symbol). Results illustrate the effect that the CCSK symbol length and error probability, and the radiometer integration time and probability of false alarm (PFA), have on detection by a radiometer. We describe a variation of CCSK called truncated CCSK (TCCSK). In this system, the code of length 2/sup k/ is cyclically shifted, then truncated and transmitted. Although shortened, the truncated code still represents k bits of information, thus leading to an increased data rate. We evaluate radiometer detection of TCCSK and it is shown that the probability of detection is increased compared with the detection of CCSK.     

同时考虑基本变量和失效域模糊性的广义失效概率数字计算方法

针对基本变量和失效域均具有模糊性的广义可靠性分析问题,提出了一种基于模拟退火优化的高效数字计算方法。在数字模拟的过程中,由模拟退火优化的Metropolis准则逐渐优化提取样本的重要抽样密度函数。由于基本变量和失效状态均含有模糊不确定性,因此所提算法在构造重要抽样函数时考虑了两个因素的影响,其一是基本变量的等价联合概率密度函数,其二是状态变量对模糊失效域的隶属函数,从而使得对广义失效概率贡献大的样本出现的概率较大,提高了抽样效率和计算精度。所提算法在模拟退火逐渐寻优的过程中,充分利用了过程中的信息,进一步提高了计算的效率,算例的结果也表明本文所提方法是合理可行的。     

Degradation of Probability of Error Due to IF Filtering

The probability of error P(e) is computed in a binary communication system with a single-pole IF filter. The effect of time-bandwidth product foT and IF frequency to band width ratio f1/Ifo on P(e) is shown. The sampling time is optimized and the effect of nonoptimal sampling time on P(e) is calculated.     

On the Acquisition Time for an Apollo Ranging Code

An Apollo ranging system is considered whose phase reference is obtained by a phase-locked loop for bit synchronization. The bit phase reference is noisy, and the error probability for the ranging code is shown to depend on the input signal energy per bit to noise density ratio. The procedure of computing the acquisition time for the ranging code is then presented and the acquisition time for a lunar ranging code is plotted versus the input signal-to-noise density ratio.     

Probability of Error in a Bandlimited Quadriphase Communication System

The probability of error is computed in a quadriphase communication system, with a causal bandlimiting bandpass filter. Intersymbol interference in each channel and intersymbol interference from the neighboring channel is evaluated. The effect of nonoptimal sampling time on the probability of error is presented.     

小失效概率情况下的全局可靠性灵敏度分析的高效方法

针对目前很多算法都无法准确、高效地计算小失效概率（10^-4,甚至更小）情况下的全局可靠性灵敏度问题,本文提出了一种高效求解小失效概率情况下的全局可靠性灵敏度新算法。所提算法通过扩大标准差构造重要抽样密度函数来进行空间分割（SP）,再与无迹变换（UT）结合,利用函数在分割后的子空间内非线性程度的降低和无迹变换方法可以高效计算低非线性程度函数的前二阶矩,来高效准确地计算小失效概率情况下的全局可靠性灵敏度。所提算法的优点有：重要抽样密度函数的选择可以使得空间分割时向重要区域偏移,并且在分割区域内功能函数的复杂性被降低,从而可以利用无迹变换方法高效计算失效概率,进而高效求得全局可靠性灵敏度。与已有的算法相比,算例说明了本文所提方法的优势。     

New results in importance sampling [of false alarm statistics]

Importance sampling is a technique which can significantly reduce the number of Monte Carlos necessary to accurately estimate the probability of low-probability of occurrence events (e.g., the probability of false alarm P_F associated with a given detection scheme). A new technique called the Chernoff Importance Sampling Method is introduced. It is shown that the number of required Monte Carlos can be reduced by a factor of a Chernoff-like bound on P _F. In addition, techniques for choosing the multiplying factor of the distorted variance method (the most common method used in importance sampling) are presented