一种地形条件下电波传播入射余角估计算法

入射余角是利用抛物方程模型研究地形条件下电波传播问题的重要参数,但该参数估计具有单次快拍、信号相干和实时性要求高的特点,传统的估计算法难以同时满足上述要求.针对上述问题,提出了基于单次快拍的空间平滑旋转不变性的信号参数估计(Estimating Signal Parameters via Rotational Invariance Techniques, ESPRIT)算法.直接采用单次数据的前后向空间平滑子阵构造centro-Hermitian伪协方差矩阵以实现信号解相干,应用酉变换将复数域的奇异值分解转换为实数域求解,利用ESPRIT算法代替多重信号分类算法估计入射余角.仿真结果表明:该算法能够实现电波传播入射余角的快速估计,准确反映地形起伏对入射余角的影响,与现有算法相比提高了实时性和估计精度.     

光纤中孤子的传输

简述光纤孤子通信研究的进程,并讨论在光纤中孤子形成的机理。文中还介绍光孤子方程和孤子光源的基本原理。     

高斯光束通过位相板后的变化

对基模和高阶高斯光束通过位相板后的变化作了详细的计算分析.研究表明,位相板可以改变光场分布和用功率含量百分比定义的Me2 因子,甚至使Me22.     

基于辛几何理论的非均匀媒质中电磁波传播问题的研究

本文提出了一种利用辛几何理论求解非均匀媒质中电磁波传播问题的新方法,首先引入新的波向量空间与原来的物理空间共同构成辛空间,将物理空间中波的传播问题提升为辛空间中的Ｌａｇｒａｎｇｅ子流形的问题,并通过适当的投影变换可以处理电磁波在非均匀媒质中传播的焦散问题。     

填充旋波媒质矩形波导电磁场的解析求解

提出了一种获得填充旋波媒质矩形波导电磁场解析解的新方法.从旋波媒质本构方程和麦克斯韦方程出发,讨论了电磁波在旋波媒质中的传播特性.经过矢量变换,将难以解析求解的波动方程化简为常规的波动方程,并对其进行了求解.利用矢量变换关系和场矢量的边界条件,求得了电磁场分量表达式和求解传播常数的特征方程组.     

Performance Improvement by Microdiversity for Wideband Signals in Frequency Selective Fading Indoor Channels with Co-Channel Interference

A comprehensive analytical bit-error-rate (BER) model is presented to analyse the performance of antenna-microdiversity for wideband BPSK modulated signals in the frequency selective fading multipath channel, specified by its complex impulse response. The model includes the disturbance by intersymbol interference (ISI) and co-channel interference (CCI), as well as the channels' impact on the carrier phase- and clock recovery in the receiver. The channel impulse responses at the antenna elements are determined by taking into account the direction of arrival of the individual paths. Computational BER- and SNIR-gain results (SNIR = signal-to-noise+interference-ratio) show that a substantial performance improvement is achieved with antenna combining for wideband signals which suffer ISI and/or CCI. For the indoor multipath channel with exponentially decaying power delay profile, the performance enhancement is compared for several antenna combining schemes. Quasi-coherent equal gain combining (QCEGC) is proposed as an novel EGC scheme based on a less accurate phase estimation technique. For wideband signals, QCEGC shows a slight performance degradation when compared to maximal ratio combining or minimum mean square error combining (MMSEC), but has a much lower implementation complexity. In the channel with CCI, where the best performance is achieved with MMSEC, QCEGC performs very poor.     

Effect of thermal cycles on the mechanical strength of quad flat pack leads/Sn-3.5Ag-X (X=Bi and Cu) solder joints

Quad Flat Pack (QFP) Leads/Sn-3.5Ag-X (X=Bi and Cu) joint was thermally cycled between 243 K and 403 K or 273 K and 373 K, and both metallographic examination and mechanical pull test were performed to evaluate thermal fatigue damage of the joint. The addition of bismuth drastically degrades the thermal fatigue resistance of Sn-3.5Ag solder. On the other hand, the pull strength of Sn-3.5Ag-Cu solder joints slightly decreased with increasing number of thermal cycles, though it still remains higher in comparison to that for conventional Sn-37Pb or bismuth containing solder joint. The behavior observed here reflects the isothermal fatigue properties of bulk solder, because thermal fatigue crack initiates at the surface of solder fillet and propagates within the fillet in an early stage of fatigue damage. Furthermore, the lead phases lying at the interface between lead-frame and bismuth containing solder joint may promote the crack propagation at the interface, resulting in the extremely low thermal fatigue resistance of the joint.     

偏心椭圆高斯光束

在椭圆高斯光束横坐标中引入复数偏移量,将其推广为偏心椭圆高斯光束。在垂直于ｚ轴截面内强度分布仍然是椭圆高斯函数分布,在传播方向上光束峰值强度沿直线分布,波前也存在横向位移。对偏心椭圆高斯光束经一阶光学系统的变换关系进行了定量推导。对传播方向偏离ｚ轴的椭圆高斯光束经薄透镜传播时的特例进行了讨论。     

基于BP 神经网络的能见度估测研究

利用安徽省高速公路能见度观测站网的分钟能见度及温湿风资料, 在全面分析能见度与各个气象要素相关 性的基础上, 重点探讨了高速公路能见度的短时预测模型。应用 BP 神经网络, 以湿度、温度、平均风速、瞬时风速、 极大风速作为 BP 神经网络输入层, 输出层为能见度, 结果表明整体试验数据偏差在可接受范围内。采用顺序试验样 本时, 相对误差在 20% 以内的占总试验次数的 68.6%; 在随机样本各次试验中, BP 网络模拟输出与检验样本的相关性 较好, 相关系数在 0.6∼0.8 之间; 低能见度随机样本试验结果表明, 模型输出值与样本值均方根误差集中在 700∼850 m 之间, 变化幅度不大, 说明神经网络算法具有较高的稳定性。     

表面安装PCB设计工艺浅谈

表面安装技术在许多电子产品的生产制造中已被大量采用,本文就表面安装ＰＣＢ设计时需考虑的一些制造工艺性问题进行了阐述。