基于NS2的802.11效果异常现象仿真研究

在802.11 WLAN(Wireless Local Network)中,当一些无线节点的传输速率明显低于其他节点与AP(Access Point)间的传输速率时,将会使区域内所有节点的传输性能降低,这种现象称为效果异常(Performance Anomaly),Hyogon K等人在文献[6-7]中提出用改变封包容量和竞争窗口大小的方式来解决这个问题.文章通过在NS2网络仿真环境下,设计实验拓扑,编写性能分析脚本,对效果异常现象进行仿真并验证了上述两种方案能够有效缓解该现象并保持系统传输性能稳定.     

Prediction of rank deficiency in partition of unity-based methods with plane triangular or quadrilateral meshes

In the partition of unity (PU)-based methods, the global approximation is built by multiplying a partition of unity by local approximations. Within this framework, high-order approximations are achieved by directly adopting high-order polynomials as local approximations, and therefore nodes along sides or inside elements, which are usually adopted in the conventional finite element methods, are no more required. However, the PU-based approximation constructed in this way may suffer from rank deficiency due to the linear dependence of the global degrees of freedom. In this paper, the origin of the rank deficiency in the PU-based approximation space is first dissected at an element level, and then an approach to predict the rank deficiency for a mesh is proposed together with the principle of the increase of rank deficiency. Finally, examples are investigated to validate the present approach. The current work indicates such a fact that the rank deficiency is an unrelated issue to the nullity of the global matrix. It can be resolved in its own manner.     

中英文文本分类系统异同因素的探讨

论文介绍了文本分类技术的研究进展及发展方向,给出了中英文文本分类系统构建的一般模型和过程,分析总结了中英文文本分类过程中的异同,并介绍了针对这些异同因素常用的处理方法。     

Broadband dielectric microwave microscopy on micron length scales

We demonstrate that a near-field microwave microscope based on a transmission line resonator allows imaging in a substantially wide range of frequencies, so that the microscope properties approach those of a spatially resolved impedance analyzer. In the case of an electric probe, the broadband imaging can be used in a direct fashion to separate contributions from capacitive and resistive properties of a sample at length scales on the order of one micron. Using a microwave near-field microscope based on a transmission line resonator we imaged the local dielectric properties of a focused ion beam milled structure on a high-dielectric-constant Ba(0.6)Sr(0.4)TiO(3) thin film in the frequency range from 1.3 to 17.4 GHz. The electrostatic approximation breaks down already at frequencies above approximately 10 GHz for the probe geometry used, and a full-wave analysis is necessary to obtain qualitative information from the images.     

ISDN功能与性能测试

本文通过对ISDN功能和性能测试的简要阐述,为数据网建设人员提供在网络构建、工程设计、运行和市场服务方面的参考。     

阴极溶出伏安法测定黄原酸盐

提出了一种测定微量有机硫化合物的方法一阴极溶出伏安法测定黄原酸盐，研究了它的溶出伏安性质和测试条件，此化合物在酸性溶液中，在电位-0．45V附近出现溶出峰。溶出峰高度与浓度在10^-6-10^-8mol／L范围内具有良好的线性关系。     

低密度、高强度DG型高变催化剂的研制

本文叙述了一种适合大型氨厂使用的、堆密度低到1.25 kg·L^-1、还原态侧压强度高达350 N·cm^-1以上、本体含硫＜500 ppm(wt)、低温活性好的高温变换催化剂的研制开发过程 还介绍了催化剂制备工艺特点和工业试用效果。     

Stress wave propagation in supporting bolts: A test for bolt support quality

A test method for the non-destructive determination of bolt length, anchoring length, and bolt body force is described. This addresses the problems of low accuracy, limited number of data, and untimely warning signs encountered with existing test methods. Numerical simulations of the bolt, resin, and rock system show that the length accuracy when using the velocity wave is lower than when using the acceleration wave. It is accepted practice to use the acceleration wave for length tests because of improved signal to noise ratios of the waveforms. Laboratory and in situ underground tests showed that the precision of the measurements meets field requirements. Using this method the anchor properties of each single bolt and, thus, the safety of the entire roadway support may be evaluated.     

Identification of Breast Cancer Prognosis Markers via Integrative Analysis

In breast cancer research, it is of great interest to identify genomic markers associated with prognosis. Multiple gene profiling studies have been conducted for such a purpose. Genomic markers identified from the analysis of single datasets often do not have satisfactory reproducibility. Among the multiple possible reasons, the most important one is the small sample sizes of individual studies. A cost-effective solution is to pool data from multiple comparable studies and conduct integrative analysis. In this study, we collect four breast cancer prognosis studies with gene expression measurements. We describe the relationship between prognosis and gene expressions using the accelerated failure time (AFT) models. We adopt a 2-norm group bridge penalization approach for marker identification. This integrative analysis approach can effectively identify markers with consistent effects across multiple datasets and naturally accommodate the heterogeneity among studies. Statistical and simulation studies demonstrate satisfactory performance of this approach. Breast cancer prognosis markers identified using this approach have sound biological implications and satisfactory prediction performance.     

Synthesis of neural tree models by improved breeder genetic programming

Neural tree model has been successfully applied to solving a variety of interesting problems. In most previous studies, optimization of the neural tree model was divided into two steps: first structure optimization, then parameter optimization. One major problem in the evolution of structure without parameter information was noisy fitness evaluation. In this paper, an improved breeder genetic programming algorithm is proposed to the synthesis of neural tree model. The effectiveness and performance of the method are evaluated on time series prediction problems and compared with those of related methods. Simulation results show that the proposed algorithm is a potential method with better performance and effectiveness.