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21.
《尚书·禹贡》中所载“九州”是中国先民对中国古代宏观地理的最早认知。结合近年来相关研究成果对其中荆梁二州的分布
地域进行界定,可以发现二州之间相互毗邻;同时,以考古资料为依托分别对荆梁二州在公元前2000 年前后所形成的农业类型进
行描述分析发现,两者之间既存在相同之处,又存在差异之处。梁州农业类型的形成既有来自北方的影响,又有来自临近的荆州的
影响,荆梁二州之间早在新石器时代便开始产生零星的农业文化传播与交流,而这一传播交流的实现大体存在两条路径。川东鄂
西地区的山地或是《尚书·禹贡》荆梁二州的农业文化分界线。 相似文献
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研究网络病毒的传播模型,可以深刻理解病毒扩散对网络造成危害的现象,为反病毒技术研究提供理论基础.通过网络环境下病毒传播数学模型的建立和分析,得出了遏制网络病毒迅速扩散的重要环节和策略.对比传统病毒,在网络环境中,网络病毒疫情的蔓延更加迅速、破坏力也更强.建立有效的预防体系,是尽早发现新型网络病毒,并控制其蔓延的有力举措. 相似文献
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Rajamani Ganesh Kuriacose Joseph Newman D. Wilson Dipankar Raychaudhuri 《International Journal of Wireless Information Networks》1994,1(3):199-222
This paper presents a simulation-based study of cellular packet CDMA systems operating in an integrated voice/data traffic scenario. Spread-spectrum CDMA provides a suitable framework for resource-shared packet transport capable of combining isochronous (voice, ISDN) and bursty data services. In this work, a general network model for cellular packet CDMA with mixed voice/data traffic is described and used to evaluate the capacity/performance impact of several key system parameters. First, the effect of spreading factor (N) and forward error correction (FEC) rate are studied, confirming earlier work indicating a weak dependence onN and a well-defined optimum code rate in the range of 0.5–0.7 (with BCH coding). Next, the effect of propagation loss coefficient () on network capacity is investigated over a range of possible assumptions for, including both constant and distance-dependent models. The results show that system capacity depends strongly on, varying by as much as a factor of 2 over the range of parameters considered. For a given distance-dependent assumption, performance results are also obtained for different cell sizes in order to understand the overall spatial reuse efficiency achievable in different cellular and microcellular scenarios. This is followed by an investigation of traffic source model effects: first the capacity improvement from voice activity detection VAD) is presented, showing the expected 21 gains. Results for varying proportions of voice and data traffic intensities indicate that the operating efficiency does not change significantly as the proportion of bursty data relative to voice is varied. 相似文献
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绝缘栅双极型晶体管功率模块失效主要由温度因素诱发。为提高功率模块可靠性,RC热阻抗模型被提出用于实时预测芯片结温。随着功率密度的提高,模块横向扩散愈发明显,多芯片热耦合效应愈加突出,导致传统RC模型会引入较大误差。文中针对RC模型精准度不足进行改进,揭示热扩散角取决于热流密度的物理内涵,结合多层封装结构下的傅里叶级数解析热流模型,建立一维RC热网络与三维热流物理场的本质联系,构造计及多芯片热路耦合的扩散角热网络,较为准确地描述多芯片结温动态特性,揭示热扩散与热耦合效应对芯片温度场形成的规律。与其他传统热扩散角模型相比,所提出方法的结温计算结果准确度最高。最后以型号SEMi X603GB12E4p模块为例,针对提出的物理模型进行验证,仿真与实验结果均表明,该模型能够表征不同工况条件下功率模块的热过程,验证了所提建模方法的有效性与准确性,误差小于4%,且验证了所提模型较不计及热耦合模型精度提高了16.72%。 相似文献
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Identification of Pivotal Causes and Spreaders in the Time‐Varying Fault Propagation Model to Improve the Decision Making under Abnormal Situation 下载免费PDF全文
Under abnormal conditions, timely and effective decisions of system recovery and protective measures are of great significance for safety‐critical systems. The knowledge of the roles that network nodes play in the spreading process is crucial for developing efficient maintenance decisions; for singling out and preferential control, the ‘pivotal spreaders’ may be a way to maximize the chances to timely hinder the fault pervasion. Inspired by the inhomogeneous topological nature of a complex fault propagation network, this study is devoted to exploring the spreading capabilities of nodes regarding both structural connectivity and causal influence strength, so as to provide decisions of preferential recovery actions under specific fault scenarios. Specifically, the dynamic betweenness centrality and nonsymmetrical entropy are incorporated to adaptively measure the system‐wide fault diffusion risk of a set of controllable fault events. In order to model the dynamics and uncertainties involved in the complex fault spreading process, we introduce the model of a dynamic uncertain causality graph, based on which solutions of time‐varying structure decomposition and causality reduction are adopted to improve the reasoning efficiency. Verification experiments consisting of simulated calculation cases and generator faults of a nuclear power plant show empirically the effectiveness and applicability of this method in large‐scale engineering practice. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
28.
分析了我国服装CAD应用的现状,探讨了服装CAD在我国应用的市场潜力和应用前景,并针对当前存在的主要问题,提出了国产服装CAD的应用推广策略 相似文献
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The throughput enhancement of space-time spreading (STS)-based code division multiple access (CDMA) system is investigated in this paper. Variable spreading factor (VSF) is utilized to improve the data throughput of the system in non-frequency selective Rayleigh fading channel. In this contribution, an analytical approach is proposed to compute a new expression for the bit error rate (BER) performance of the STS-based CDMA system against pre-despreading signal-to-noise-ratio (SNR) for different values of spreading factor. The other contribution of the paper is deriving a new analytical expression for the throughput enhancement of the VSF STS-based CDMA system in non-frequency selective Rayleigh fading channel. Simulation and analytical results demonstrate that using VSF method in STS-based CDMA system improves the throughput of the system by keeping the BER performance at a target level. 相似文献