任意截面的非平行多导体传输线瞬态分析

以分析多导体传输线的时域有限差分(FDTD)法为基础,在考虑任意截面传输线分布参数无法直接计算的情况下,提出MOM-FDTD 混合方法对不平行多导体传输线进行瞬态分析。首先,利用FDTD建立多导体传输线时域差分模型,然后用MOM 法计算任意截面形状的非平行传输线的分布参数,并且与FDTD法混合进行瞬态分析计算。此算法相对于全波算法,在时间与存储空间消耗上具有很大的优势,并且满足精度要求。最后通过同轴传输线与矩形带状线的例子验证这个方法是正确有效的。     

Layer by layer characterisation of the degradation process in PCDTBT:PC71BM based normal architecture polymer solar cells

This work demonstrates the stability and degradation of OSCs based on poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′ benzothiadiazole)] (PCDTBT): (6,6)-Phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) photoactive blend layers as a function of ageing time in air. Analysis of the stability and degradation process for the OSCs was conducted under ambient air by using current-voltage (I-V) measurements and x-ray photoelectron spectroscopy (XPS). The interface between photoactive layer and HTL (PEDOT:PSS) was also investigated. Device stability was investigated by calculating decay in power conversion efficiency (PCE) as a function of ageing time in the air. The PCE of devices decrease from 5.17 to 3.61% in one week of fabrication, which is attributed to indium and oxygen migration into the PEDOT:PSS and PCDTBT:PC₇₁BM layer. Further, after aging for 1000 h, XPS spectra confirm the significant diffusion of oxygen into the HTL and photoactive layer which increased from 3.0 and 23.3% to 20.4 and 35.7% in photoactive layer and HTL, respectively. Similarly, the indium content reached to 17.9% on PEDOT:PSS surface and 0.4% on PCDTBT:PC₇₁BM surface in 1000 h. Core-level spectra of active layer indicate the oxidation of carbon atoms in the fullerene cage, oxidation of nitrogen present in the polymer matrix and formation of In₂O₃ due to indium diffusion. We also observed a steady fall in the optical absorption of the active layer during ageing in ambient air and it reduced to 76.5% of initial value in 1000 h. On the basis of these experimental results, we discussed key parameters that account for the degradation process and stability of OSCs in order to improve the device performance.     

采用高斯脉冲函数计算电路的冲激响应

电路的单位冲激响应是研究电路暂态性质的重要工具之一。为了建立电路冲激响应的物理图景,本文以高斯冲击函数作为激励源,对储能元件采用伴随模型,对一阶RC电路的响应进行仿真计算。由于考虑了冲激函数的作用过程,电压和电流是时间的连续函数,从而为理解冲激响应的物理过程提供一个正确的物理描述。同时,本文给出的数值算法亦可应用于高阶电路的暂态仿真中。     

Pound-Drever-Hall稳频误差信号线性动态范围及灵敏度分析

在Pound-Drever-Hall激光稳频方法中,误差信号的线性动态范围和灵敏度是影响稳频效果的重要指标。为了对Pound-Drever-Hall激光稳频性能进行优化,本文理论分析了调制频率、FP腔镜反射率和腔长对动态范围及灵敏度的影响,并通过实验仿真进行定量讨论和优化。实验结果表明,调制频率、反射率和腔长都分别与线性动态范围的大小有着紧密的联系,通过对这些参数的正确选取,以及对精度和灵活性要求的综合考虑,可以提升稳频系统的效果;而且对调制深度的优化可以提高误差信号灵敏度。     

基于PTS方法降低OFDM系统的峰均功率比

OFDM最主要缺点之一就是时域的大峰均功率比。为了降低大峰均比,PTS是最常用的方法之一,但传统的PTS方法是对频域数据进行IFFT运算后选择最优传输序列,一般需要多个IFFT,OFDM系统复杂度高。为了解决多IFFT这一问题,本文提出了一种新方法,即在频域里判决最优序列,而大大减少IFFT块的数目。     

估计误差下DF协作系统性能分析

信道估计误差是引起无线通信系统性能衰弱的重要因素之一。分析了瑞利信道条件下估计误差对译码前传协作系统性能的影响。推导并给出了估计误差条件下正交幅度调制信号译码前传协作系统码元错误概率的解析表达式和上界表达式,并分析了不同信道的方差对协作系统性能的影响。仿真结果显示,存在估计误差时,码元错误概率性能并未随着信噪比的增大而提高。同时发现,中继-目的链路的较大方差较源-中继链路显著改善码元错误概率性能。     

A subgap density of states modeling for the transient characteristics in oxide-based thin-film transistors

The modeling of the transient subgap density of states (DOS) for the investigation of trap densities in the oxide-based thin-film transistors is proposed. The study is based on both transient measurements and physical modeling. In history, the subgap DOS modeling of trap densities have been studied according to the static-state current–voltage characteristics or the capacitance–voltage curves. However, the subgap DOS modeling for the transient curves is seldom proposed. In this study, the transient model of subgap DOS is discussed for amorphous In–Ga–Zn–O (a-IGZO) thin films. This model suggests the subgap DOS exhibits a transient behavior with an exponential distribution on the band edge and a Gaussian distribution in the deep gap level. This study could be helpful to understand and optimize the transient electrical properties of a-IGZO TFTs.     

On-demand routing and channel assignment in multi-channel mobile ad hoc networks

The capacity of mobile ad hoc networks is constrained by the intra-flow interference introduced by adjacent nodes on the same path, and inter-flow interference generated by nodes from neighboring paths. By assigning orthogonal channels to neighboring nodes, one can minimize both types of interferences and allow concurrent transmissions within the neighborhood, thus improving the throughput and delay performance of the ad hoc network. In this paper, we present three novel distributed channel assignment protocols for multi-channel mobile ad hoc networks. The proposed protocols combine channel assignment with distributed on-demand routing, and only assign channels to active nodes. They are shown to require fewer channels and exhibit lower communication, computation, and storage complexity, compared with existing approaches. Through simulation studies, we show that the proposed protocols can effectively increase throughput and reduce delay, as compared to several existing schemes, thus providing an effective solution to the low capacity problem in multi-hop wireless networks.     

码长连续变化的QC-LDPC码的设计

该文基于有限多项式环的理论,提出了码长连续变化的准循环低密度奇偶校验(Quasi-Cyclic Low Density Parity Check, QC-LDPC)码的设计方法。当有限环基数大于某个门限值时,在此环内通过一定规则选择参数生成移位项,利用它们构造出的校验矩阵均可以达到较大的圈长(girth)值。在设计中,有限环基数为连续的整数,且基数与码长呈线性关系,因此能够在girth值不变的前提下实现码长的连续变化。该文分析并证明了该构造方法大大增加了可用的高性能QC-LDPC码数量,更好地服务于自适应链路系统。     

Gas entrainment in an evaporating spray jet

Gas entrainment induced by a spray jet can be significantly affected by the spray evaporation rate. In this study, we have directly measured the air entrainment induced by a liquid nitrogen spray jet into an unbounded and stagnant room air. It is realized that the air entrainment is proportional to the axial gradient of oxygen mass flow in a pure nitrogen spray jet. Hence, the air entrainment can be determined by a combined measurement of local cross-sectional distributions of oxygen concentration, gas temperature and gas velocity along the jet path. These measurements are directly obtained using an in situ oxygen concentration analyzer, a thermocouple system, and a Laser Doppler Velocimeter. In order to evaluate the effect of evaporation rate, direct measurements and numerical simulations of the air entrainment by a cold gaseous jet of nitrogen (at a temperature slightly above that of liquid nitrogen) into room air are also performed. Measurements of the entrainment rate and flow similarity of the gaseous jets without droplets compared very well against those from the single-phase jet theories and numerical simulation, which validates our experimental approach and analysis method. Our experimental results indicate rough flow similarities exist in evaporating spray jets with round nozzles. Although the air entrainment by the liquid nitrogen spray is found significantly increased, as compared to that by the cold gaseous jet of nitrogen from the same nozzle and at the same jetting velocity, the increased ratio is far less than the equivalent momentum ratio of the liquid nitrogen spray to the gas nitrogen jet. This experimental finding suggests that the evaporation of spray markedly weakens the gas entrainment. In this study, a parametric model is also developed to provide a theoretical basis of the data analysis for the cross-section averaged spray evaporation rate within the spray jet region.