Data consistency based rigid motion artifact reduction in fan-beam CT

It is well known that a rigid in-plane motion can be decomposed into a translation and a rotation around an origin. Based on our previous work, we first extend the Helgason-Ludwig consistency condition (HLCC) to cover a general rigid motion in fan-beam geometry. Then, we model the general motion by several parameters, and develop an iterative scheme for estimation of the in-plane motion parameters. This scheme determines the motion parameters by numerically minimizing an objective function constructed based on the HLCC. After the motion parameters are estimated, image reconstruction can be performed to compensate for the motion effects. Finally, we implement the algorithm and evaluate its performance in numerical simulations.     

Synthesis and Thermoelectric Properties of LAST System Bulk Materials: Substitution of Sulfur for Tellurium

Nonstoichiometric lead-antimony-silver-tellurium (LAST) system thermoelectric bulk materials Ag_0.8Pb_22.5SbTe_20?x S _x (x?=?0 to 8.0) were fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS). The electrical and thermal transport properties were investigated in the temperature range of 300?K to 700?K. The x-ray diffraction (XRD) results indicated that sulfur entered the PbTe during MA process, but gradually precipitated in the sintering process in the form of PbS from the PbTe matrix when the sulfur content was changed from x?=?2.0 to x?=?8.0. It was confirmed that the addition of sulfur effectively reduced the lattice thermal conductivity. A low thermal conductivity of 0.83?W?m^?1?K^?1 at 673?K was obtained for the Ag_0.8Pb_22.5SbTe₁₂S₈ sample. Benefiting from the high electrical conductivity, the Ag_0.8Pb_22.5SbTe₁₈S₂ sample reached a maximum ZT value of ??0.97 at 673?K, which is 32% higher than its counterpart without sulfur substitution.     

激光雷达能见度仪数据采集系统的研制

目前的激光雷达能见度仪基本上都是靠通用数据采集卡进行数据采集,使用起来多有不便.针对这一现状,研制了一套适用于激光雷达能见度仪的专用数据采集系统.系统硬件基于FPGA设计,通过硬件逻辑实现计数、触发和数据存储.系统通过USB2.0高速接口与计算机连接,数据传输速度快、连接方便且支持热拔插.采集软件为采用LabWindo...     

Reduction of PAPR of OFDM Signals Using Nonlinear Companding Transform

In this paper, we propose and evaluate a novel nonlinear companding transform (NCT) scheme for peak-to-average power ratio (PAPR) reduction in orthogonal frequency division multiplexing (OFDM) systems. The key idea of the proposed scheme is to transform the original Gaussian-distributed OFDM signals into a specific statistics form, whose target probability density function is defined by a piecewise function with an inflexion point. By properly choosing the transform parameters, this scheme can enable more flexibility and freedom in the companding form so that a favorable tradeoff between PAPR reduction and bit error rate (BER) performance can be achieved. Moreover, compared to existing NCT techniques, this scheme dramatically decreases the impact of companding distortion on the BER performance to reach a given PAPR level. The analytical expressions regarding the achievable transform gain in PAPR, complementary cumulative density function, attenuation coefficient, and selection criteria of transform parameters are derived. Simulation results justify the significance and accuracy of the analytical expressions presented.     

Secondary User Selection Scheme Using Adaptive Genetic Algorithms for Cooperative Spectrum Sensing Under Correlated Shadowing

Cooperative spectrum sensing has been shown to be an effective approach to improve the detection performance by exploiting the spatial diversity among multiple secondary users (or unlicensed users). However, due to correlated shadowing and cooperation overhead in practical cognitive radio networks, it is desired to select an appropriate set of secondary users which have little correlation with each other to participate in cooperation so as to achieve the effective tradeoff between detection performance and cooperation overhead. In this paper, we first study the hypothesis testing model and detection performance of cooperative spectrum sensing under the correlated log-normal shadowing scenario. Afterwards, based on whether the false-alarm and missed-detection probabilities are constrained, three optimization problems are formulated to find the optimal set of secondary users participating in cooperation, which take into account the tradeoff between detection performance and cooperation overhead. Then the solutions using adaptive genetic algorithms are presented for the optimization problems. Finally, simulation experiments demonstrate that our proposed schemes are very effective.     

Seeking powerful information initial spreaders in online social networks: a dense group perspective

The rapid growth of online social networks (OSNs) has ultimately facilitated information spreading and changed the economics of mobile networks. It is important to understand how to spread information as widely as possible. In this paper, we aim to seek powerful information initial spreaders with an efficient manner. We use the mean-field theory to characterize the process of information spreading based on the Susceptible Infected (SI) model and validate that the prevalence of information depends on the network density. Inspired by this result, we seek the initial spreaders from closely integrated groups of nodes, i.e., dense groups (DGs). In OSNs, DGs distribute dispersedly over the network, so our approach can be fulfilled in a distributed way by seeking the spreaders in each DG. We first design a DG Generating Algorithm to detect DGs, where nodes within the DG have more internal connections than external ones. Second, based on the detected DGs, we design a criterion to seek powerful initial spreaders from each DG. We conduct experiments as well as statistical analysis on real OSNs. The results show that our approach provides a satisfactory performance as well as computational efficiency.     

Steady-State Visual Evoked Potential (SSVEP) in a New Paradigm Containing Dynamic Fixation Points

Steady-state visual evoked potential(SSVEP)has become a powerful tool for Brain Computer Interface(BCI)because of its high signal-tonoise ratio,high information transmission rate,and minimal user training.At present,the edge information of each region cannot be identified in spatial coding based on SSVEP-BCI technology,and the user experience is poor.To solve this problem,this paper designed a new paradigm to explore the relationship between the fixation point position of continuous sliding and the correlation coefficient ratio in the dualfrequency case.Firstly,the standard sinusoidal signal was employed to simulate the Electroencephalogram(EEG)signal,which verified the reliability of characterizing the amplitude variation of test signal by correlation coefficient.Then,the relationship between the amplitude response of SSVEP and the distance between the fixation point and the stimulus in the horizontal direction was tested by Canonical Correlation Analysis(CCA)and Filter bank CCA(FBCCA).Finally,the experimental data were offline analyzed under the condition of continuous sliding of the fixation point.It is feasible and reasonable to detect the amplitude change of frequency component in SSVEP by utilizing the spatial coding method in this paper to improve the extraction accuracy of spatial information.     

半速率优化策略研究

论述了使用半速率技术解决突发性话务量时可能碰到的一些问题,北京联通半速率信道的分配原则、使用策略及一些经验设置,以及结合网优支撑系统如何进行半速率策略的优化,同时,给出了北京联通半速率策略优化后的效果。     

服务器能效测试新方法（下）

服务器指的是管理资源并为用户提供服务的计算机,相对于普通PC来说,服务器在稳定性、安全性、性能等方面都要求更高,因此CPU、芯片组、内存、磁盘系统、网络等硬件和普通PC有所不同。服务器的心用范围涉及政府、教育、医疗、企业等部门,从个人PC到工作站,再到大型超级计算机,是互联网产业不可缺少的一个部分,服务器发挥着重要作用。其在数据中心及云存储中的运用尤为重要。因此,在“绿色”、”低碳”成为ICT产业发展主要趋势的今天,如何评价服务器的节能指标,以及如何提高服务器的能源利用率,成为人们关注的重点。     

网络可靠性仿真

讨论了在链路以及节点均可能失效情况下,把节点的失效等效为与之相连链路的失效,运用蒙特卡洛法对网络可靠性进行仿真。并给出了仿真次数对仿真结果的误差的影响。