Multicast communications in cognitive radio networks using directional antennas

This paper presents a study on multicast communications in cognitive radio networks (CRNs)using directional antennas. The objective is to maximize the throughput of the CRN. The spectrum is divided into multiple channels and licensed to the primary network. While the CRN is accessing the spectrum, the interference power is carefully controlled to avoid impacting the operation of the primary network. The mathematical model is presented and subsequently formulated as a mixed integer non‐linear programming (MINLP) problem, which is non‐deterministic polynomial‐time hard. Therefore, a greedy algorithm is designed to approximate the optimal performance. The MINLP problem is then relaxed and an upper bound is developed. Simulation results are presented to compare the performance of the greedy algorithm and the upper bound, which demonstrates the efficacy of the greedy algorithm as well as the tightness of the upper bound. Copyright © 2012 John Wiley & Sons, Ltd.     

RIG型磁光薄膜的研究进展及应用

随着光通信技术与光子集成电路的发展,非互易性器件作为光通信系统中重要的组成部分得到了越来越广泛的研究与应用。基于磁光效应制成的磁光隔离器和环行器是目前应用最为广泛的非互易性器件,为了将非互易性器件整块集成在硅片上,需制备性能与块状磁光材料相当的磁光薄膜。在近红外通信波段(1 550 nm),以钇铁石榴石(Y₃Fe₅O₁₂,YIG)为代表的稀土铁石榴石(RIG)具备优良的磁光效应,是最具应用前景的磁光材料之一。研究发现,使用稀土离子对YIG薄膜进行掺杂可以有效改善其磁光性能,尤其是Bi³⁺和Ce³⁺掺杂的YIG表现出巨法拉第效应。本文首先介绍了法拉第效应原理,介绍了三种常见磁光薄膜的生长方法,回顾了近年来的主要研究成果,介绍了磁光薄膜在光隔离器和环行器中的应用,最后对磁光薄膜的未来发展趋势进行了展望。     

Effect of random density irregularities on nonlinear evolution of Langmuir waves in interplanetary medium

The evolution of nonlinear Langmuir waves in the interplanetary medium is investigated by appropriately accounting for the random density irregularities of the medium. A pair of modified Zakharov equations, which describe these waves, is solved numerically as an initial value problem for large scale (≫ 10² km) initial pertubations. For an ion acoustic-Langmuir solitary wave, the random irregularities damp the Langmuir wave by way of scattering and let the ion density perturbation radiate away in a few days. However an initial solitary or shock-like Langmuir wave excites the ion density perturbations within a fraction of a second, and then itself gets damped. These effects will strongly decelerate the collapse of large scale Langmuir waves. The possibility of detecting these processes, by means of interplanetary scintillation, is discussed. The authors felicitate Prof. D S Kothari on his eightieth birthday and dedicate this paper to him on this occasion.     

太阳耀斑环——射电微波及相关波段的研究

与大多以光学波段为主的耀斑研究不同的是,作者拟以射电微波(厘米波段)的频谱和成像观测为主及相应的辐射机制研究太阳活动的基本单元——耀斑环及其物理参数的空间分布和时间演化规律。特别是中国研制的具有国际最高的时间、空间和频率分辨率的射电日像仪(MUSER)已投入运行,文章将为推动该设备即将开展的科学研究提供重要的参考工具。此外,文中也加入了与射电微波辐射密切相关的X射线、紫外和光学波段的研究,符合太阳物理乃至整个天体物理多波段研究的大趋势。文章集中介绍作者最具特色的研究,由于篇幅所限只能给出基本物理思想和概念、主要结论和代表性插图,对相应的理论推导、观测数据处理等细节,读者可根据需要参阅对应的参考文献。     

TD—LTE无线网络利用率评估体系探讨

本文在充分分析TD—LTE物理层资源及其调度方法的基础上,提出评估思路,并结合设备厂家数据采集能力提出了无线网络利用率的评估标准和方法。在此基础上,讨论了以该指标为基础的扩容标准的制定。     

InxGa1−xAs ohmic contacts to n-type GaAs with a tungsten nitride barrier

Significant reduction of the contact resistance of In_0.7Ga_0.3As/Ni/W contacts (which were previously developed by sputtering in our laboratory) was achieved by depositing a W₂N barrier layer between the Ni layer and W layer. The In_0.7Ga_0.3 As/Ni/W₂N/W contact prepared by the radio-frequency sputtering technique showed the lowest contact resistance of 0.2 Ωmm after annealing at 550°C for 10 s. This contact also provided a smooth surface, good reproducibility, and excellent thermal stability at 400°C. The polycrystalline W₂N layer was found to suppress the In diffusion to the contact surface, leading to improvement of the surface morphology and an increase in the total area of the In_xGa−As between metal and the GaAs substrate. These improvements are believed to reduce the contact resistance.     

试验设计技术在确定汽车收音机最大产热的设置条件中的应用

阐述了如何确定汽车收音机在什么设置条件下产生的热量最大的方法。主要采用L8DoE(Design of Exper Jments)方法，确定设置条件后选择合适的DoE因素，可预测汽车音响出现产热的最坏情况的设置条件以及在此设置条件下的热量值。该方法同样可应用于其它多媒体组件的热量特性试验。     

ZnS∶TbF_3薄膜电致发光器件性能的研究

用射频磁控溅射和扫描电子束蒸发方法研究制备了ZnS∶TbF3薄膜的交流电致发光器件。研究了制备ZnS∶TbF3薄膜电致发光器件的几种因素对电致发光器件性能的影响 ,诸如 ,磁控溅射过程中衬底温度 ,发光层厚度 ,退火等。