Multilevel design efficiency in educational effectiveness research

In educational effectiveness research, multilevel data analyses are often used because research units (most frequently, pupils or teachers) are studied that are nested in groups (schools and classes). This hierarchical data structure complicates designing the study because the structure has to be taken into account when approximating the accuracy of estimation and the power for statistical testing, which should be sufficient to reach meaningful conclusions. Accuracy and power, both referred to as efficiency, can be optimized by carefully choosing the number of units to sample at each of the levels, taking into account the available resources and costs of sampling at these levels. We complement the findings that are found in the literature with regard to designing multilevel studies and propose a simulation approach that can be used to help making study-specific decisions.     

面向成本的设计中成本估算方法的研究

设计阶段决定了产品成本的60%-70%。日益加剧的市场竞争促使人们愈来愈重视对设计阶段的产品成本估算方法的研究。本文论述了近几十年来国内外在这一领域的发展概况及趋势,从并行设计的角度提出面向成本的设计(design for cost, DFC)的思想,在分析DFC中全生命周期成本的构成及其估算方法的基础上,建立基于CAD/CAPP模型的成本估算框架体系,在产品设计阶段不但考虑产品的技术性能,还考虑产品的经济性能,实现参数化成本估算,从而实现了考虑成本的优化设计,使得设计出的产品更加适应市场的需求,有助于提高企业在市场上的竞争力。     

我国外汇储备影响因素的实证分析——基于主成分回归模型

基于主成分回归的方法,建立了我国外汇储备影响因素的计量模型。实证研究结果表明,实际利用外资额和GDP规模是影响我国外汇储备的主要因素,进口额、汇率、进出口差额也对我国的外汇储备造成一定的影响。     

基于OFDM的LS信道估计仿真的研究

信道估计技术保证了信号传输的可靠性。基于导频的LS信道估计算法,通过导频子信道的响应得到整个信道的响应,能够很好地跟踪信道的变化,在复杂度不高的情况下取得较好的误差性能,具有很好的实用性。仿真结果表明采用LS信道估计后有效的降低了系统的误码率。     

Time-frequency channel modeling and estimation of multi-carrier spread spectrum communication systems

In wireless communications, the channel is typically modeled as a random, linear, time-varying system that spreads the transmitted signal in both time and frequency due to multi-path propagation and Doppler effects. Estimated channel parameters allow system designers to develop coherent receivers that increase the system performance. In this paper, we show how time-frequency analysis can be used to model and estimate the time-varying channel of a multi-carrier spread spectrum (MCSS) system using a complex quadratic sequence as the spreading code. We will show that for this spreading code, the effects of time delays and Doppler frequency shifts, caused by the mobility of environment objects, can be combined and represented effectively as time shifts. The discrete evolutionary transform (DET), as a time-frequency analysis method, enables us to estimate the effective time shifts via a spreading function and to use them to equalize the channel. Using the effective time shifts, the time-varying channel can be represented simply as linear-time invariant system by embedding the Doppler shifts that characterize the time-varying channel into effective time shifts. The channel parameters are used to estimate the data bit sent. To illustrate the performance of the proposed method we perform several simulations with different levels of channel noise, jammer interference, and Doppler frequency shifts.     

新旧动能转换的时空演变与区域差异——基于长三角与大湾区城市群的比较研究

在长三角一体化和粤港澳大湾区建设上升为国家战略的背景下，对比分析两大城市群新旧动能转换水平、时空演变规律与区域差异，对两大城市群加快新旧动能转换步伐和推广先行示范效应具有重要现实意义。从需求侧、供给侧和结构转换侧三个动能维度构建评价指标体系，运用组合赋权法测算2012-2019年两大城市群36个城市新旧动能转换水平，并利用自然断点法、核密度法等探究两大城市群新旧动能转换时空演变规律与区域差异。研究得出：①长三角与大湾区城市群新旧动能转换分别呈现出“类N”型和“类U”型波动增长趋势，不充分性特征明显；②长三角城市群新旧动能转换表现出“西北低，东南高”的空间分布、“多极化”的空间格局以及“沙漏型”的空间层级特点，而大湾区城市群新旧动能转换表现出“由东向西”递减的空间分布、“单极化”的空间格局以及“倒金字塔型”的空间层级特点；③两大城市群核密度曲线延展性均由“右拖尾”转变为“收敛”趋势，表明两大城市群区域中心城市新旧动能转换中心极化现象得到逐年改善，中心城市对周边城市的虹吸效应减弱。