抗坏血酸对核乳胶性能的影响

文章介绍抗坏血酸对核乳胶灵敏度、灰雾密度及潜影衰退性能的影响，并与日本富士ＥＴ－７Ｂ型乳胶的潜影稳定性能在不同温度和相对湿度条件下作相应比较。     

一种计算孕育衰退时间的数学模型

确定孕育衰退时间对于正确制定工艺措施、提高铸件质量具有重要意义。以异质形核理论为基础,建立了孕育剂扩散衰退模型,得到了计算衰退时间的表达式。结果表明:孕育衰退时间与孕育剂中有效形核相的溶质扩散系数成反比,与有效形核相的半径及该相中的溶质浓度等参数有关。最后对该模型进行了验证,结果表明该模型与相关试验结果基本符合。     

A356合金细化处理效果的孕育和衰退

通过考察A356合金在静置过程中细化效果的变化，研究了亚共晶铝硅合金细化处理的孕育期和衰退期。细化处理的孕育期的形成是由于细化初期的溶质过冷度小造成的，随着溶质Ti含量的增加，溶质过冷度增大，从而激活更多的形核质点，限制枝晶生长；长时间静置后，尽管细化元素Ti含量基本不变，但是成分偏析倾向加剧，形核质点粗化并沉淀，有效核心减少，出现细化效果衰退；采用预细化合金锭可以缩短甚至消除细化处理的孕育期。     

移动通信中宽带接收系统信号电平特性的研究

首先建立了一个包括直达波和非直达波的无线电波的传播模型,然后得到宽带接收系统中接收信号的电平表达式,再通过MATLAB仿真得到宽带接收系统中信号的电平变化和分布的特性。根据仿真结果,可知宽带接收系统的信号电平变化约为10dB,信号电平的分布受到传播模式中的接收带宽、达波数、路径差的影响。     

相关瑞利衰落信道MRC接收性能分析

通过一种一般性方法研究两种天线设置时相关瑞利衰落信道上MRC接收的BPSK性能，并利用衰落信道上数字信号传输的归一化性能研究结果，分析MRC接收的MQAM的性能，给出平均比特差错率的解析表达式。     

无线因特网上的数据传输

1.引言 90年代以来,以蜂窝通讯技术为代表的无线通信技术迅速发展,从原来的模拟无线电台、第一代模拟无线电话发展到第二代数字式蜂窝电话、数字式寻呼设备,再到即将进入市场的第三代移动通信技术(3G)以及正在酝酿之中的4G,人们在任何时间、任何地点与任何对象进行通信的愿望正在成为现实。随着无线通讯技术的进一步成熟,人们不满足于仅仅通过无线网络传送语音,还想无线传送数据信息,实现“移动计算”的梦想。而要想真正实现随时随地的“移动计算”,不但要     

Maximization of average number of correctly received symbols over multiple channels in the presence of idle periods

In this study, optimal channel switching (time sharing) strategies are investigated under average power and cost constraints for maximizing the average number of correctly received symbols between a transmitter and a receiver that are connected via multiple flat-fading channels with additive Gaussian noise. The optimal strategy is shown to correspond to channel switching either among at most three different channels with full channel utilization (i.e., no idle periods), or between at most two different channels with partial channel utilization. Also, it is stated that the optimal solution must operate at the maximum average power and the maximum average cost, which facilitates low-complexity approaches for obtaining the optimal strategy. For two-channel strategies, an upper bound is derived, in terms of the parameters of the employed channels, on the ratio between the optimal power levels. In addition, theoretical results are derived for characterizing the optimal solution for channel switching between two channels, and for comparing performance of single channel strategies. Sufficient conditions that depend solely on the systems parameters are obtained for specifying when partial channel utilization cannot be optimal. Furthermore, the proposed optimal channel switching problem is investigated for logarithmic cost functions, and various theoretical results are obtained related to the optimal strategy. Numerical examples are presented to illustrate the validity of the theoretical results.     

A multi-criteria comparative analysis of different Rayleigh fading channel simulators

In this paper, an in-depth comparative analysis of the most accepted and approved Rayleigh fading channel simulators is presented. Our study addresses the three common methods for generating time-correlated Rayleigh fading processes: the inverse discrete Fourier transform (IDFT) method, the filtering white Gaussian noise (FWGN) method and the sum-of-sinusoids (SOS) method. Totally seven models, including the recently proposed models, are analyzed and compared with Clarke's reference model. We use four quantitative measures for judicious assessment of the quality of generated processes in terms of both first-order and second-order statistics. Additionally, these measures cover analysis in time domain as well as in frequency domain. An evaluation of the computational effort is also depicted. The simulation results suggest that the recently published Wang's model is very attractive and it outperforms all SOS-based models. We conclude that Zheng and Xiao's first model remains the best choice among Zheng and Xiao's family models. Several other important remarks and conclusions about the accuracy and the capability of the different Rayleigh fading simulators are driven from this study.     

Asymptotic throughput analysis of multicast transmission schemes

This paper demonstrates the use of extreme value theory in the asymptotic throughput analysis of wireless multicast and unicast for large number of users. Exact analysis of these schemes involves finding the probability distributions of maxima or minima of signal to noise ratios (SNRs). Jointly considering random user distribution and Rayleigh fading, exact distributions of these extreme values become complex and lack insights. On the other hand, asymptotic expressions obtained by using extreme value theory are quite accurate even for moderate number of users and they give insights about the performances without using simulations. The results of this analysis can be used in designing cellular unicast and multicast systems.