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1975年MAZO提出超Nyquist码元速率(FTN)理论,通过理论推导证明,信号在加性高斯白噪声信道下,以超过Nyquist码元速率25%以内的速率进行传输,误码性能可以不下降。但MAZO以及至今都没人能完成FTN的接收,因为当信号以超Nyquist码元速率进行传输所引起的码间串扰长度无限长,若要完全消除码间串扰,使用最大似然检测(MLSE)方法,其复杂度无穷大,是不可实现的。基于MMSE均衡和WCE均衡级联的两级均衡已证明在长码间串扰信道下其误码性能接近于MLSE。文章将两级均衡运用于FTN接收,仿真证明在加性高斯白噪声信道下,信号以超过Nyquist码元速率25%以内的速率进行传输,误码性能不下降。成功实现FTN的接收,验证了MAZO的理论。 相似文献
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Composite solid propellants (CSPs) have widely been used as main energy source for propelling the rockets in both space and military applications. Internal ballistic parameters of rockets like characteristic exhaust velocity, specific impulse, thrust, burning rate etc., are measured to assess and control the performance of rocket motors. The burn rate of solid propellants has been considered as most vital parameter for design of solid rocket motors to meet specific mission requirements. The burning rate of solid propellants can be tailored by using different constituents, extent of oxidizer loading and its particle size and more commonly by incorporating suitable combustion catalysts. Various metal oxides (MOs), complexes, metal powders and metal alloys have shown positive catalytic behaviour during the com-bustion of CSPs. These are usually solid-state catalysts that play multiple roles in combustion of CSPs such as reduction in activation energy, enhancement of rate of reaction, modification of sequences in reaction-phase, influence on condensed-phase combustion and participation in combustion process in gas-phase reactions. The application of nanoscale catalysts in CSPs has increased considerably in recent past due to their superior catalytic properties as compared to their bulk-sized counterparts. A large surface-to-volume ratio and quantum size effect of nanocatalysts are considered to be plausible reasons for improving the combustion characteristics of propellants. Several efforts have been made to produce nanoscale combustion catalysts for advanced propellant formulations to improve their energetics. The work done so far is largely scattered. In this review, an effort has been made to introduce various combustion catalysts having at least a metallic entity. Recent developments of nanoscale combustion catalysts with their specific merits are discussed. The combustion chemistry of a typical CSP is briefly discussed for providing a better understanding on role of combustion catalysts in burning rate enhancement. Available information on different types of combustion nanocatalysts is also presented with critical comments. 相似文献
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传统的Reimer-Tiemann反应产率不高(一般低于35%).通过改进反应条件,控制反应过程,使产率提高到60%,并对反应中的有关问题进行了讨论. 相似文献
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多普勒参数的估计精度直接影响机载合成孔径雷达系统(SAR)成像质量。根据平均后的方位谱来估计多普勒中心频率,通过计算子图像之间的位移值来估计多普勒调频斜率。为了消除在实际应用中各种干扰的影响,采用了对原始回波数据加窗的方法来提高对估计的准确性。该算法的估计结果可以用于补偿载机运动误差引入的相位误差,从而实现高分辨率成像。通过使用实验数据进行分析,验证了该方法的有效性。 相似文献
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Various methods and criteria for comparing coherent systems are discussed. Theoretical results are derived for comparing systems of a given order when components are assumed to have independent and identically distributed lifetimes. All comparisons rely on the representation of a system's lifetime distribution as a function of the system's “signature,” that is, as a function of the vector p= (p1, … , pn), where pi is the probability that the system fails upon the occurrence of the ith component failure. Sufficient conditions are provided for the lifetime of one system to be larger than that of another system in three different senses: stochastic ordering, hazard rate ordering, and likelihood ratio ordering. Further, a new preservation theorem for hazard rate ordering is established. In the final section, the notion of system signature is used to examine a recently published conjecture regarding componentwise and systemwise redundancy. © 1999 John Wiley & Sons, Inc. Naval Research Logistics 46: 507–523, 1999 相似文献
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为了满足组合发动机模态转换的要求,连续稳定调节推进剂流量十分关键。为此,针对气体推进剂,在常规音速喷嘴的基础上设计了一种可调音速喷嘴。通过塞锥改变音速喷嘴的节流面积,进而实现流量的连续调节。采用两次包络线方法设计塞锥型面,使得可调音速喷嘴具有线性的流量特性。采用计算流体动力学数值仿真研究可调音速喷嘴的工作特性。仿真结果表明反压小于临界反压时,可调音速喷嘴流量不受反压影响。可调音速喷嘴保持临界状态的临界反压比随流量的减小呈增大的趋势。线性可调音速喷嘴的流量与塞锥位置有较好的线性关系,其流量系数高且几乎不受塞锥位置的影响。 相似文献