Cache Hit Ratio Maximization in Device-to-Device Communications Overlaying Cellular Networks

This paper investigates the content placement problem to maximize the cache hit ratio in device-to-device(D2D)communications overlaying cellular networks.We consider offloading contents by users themselves,D2D communications and multicast,and we analyze the relationship between these offloading methods and the cache hit ratio.Based on this relationship,we formulate the content placement optimization as a cache hit ratio maximization problem,and propose a heuristic algorithm to solve it.Numerical results demonstrate that the proposed scheme can outperform existing schemes in terms of the cache hit ratio.     

高次泛音比法测定厚度振动机电耦合系数

压电振子低频振动模式的高次泛音易与厚度振动模式的基频相耦合,导致厚度振动的基音串联谐振频率不能精确测量(或无法测量),造成传统泛音比法测定压电振子的机电耦合系数的精度和一致性变差或无法进行测定。为了克服传统泛音比法的这一弊端,提出了测定压电振子厚度振动机电耦合系数的高次泛音比法。此法通过测定3次和3次以上的高次串联谐振频率,用高次泛音比来测定压电振子厚度振动机电耦合系数。实验表明,与传统泛音比法相比,高次泛音比法具有精确度高和一致性好的明显优势。为了便于高次泛音比法的应用,提供了高次泛音比和机电耦合系数的对应表。     

提高高职院校图书馆电子资源利用率的研究

通过对部分高校电子文献资源利用情况调查,从而了解高校图书馆电子文献资源的利用情况。调查发现,高职院校电子资源的利用情况不容乐观,应采取措施尽快解决在使用电子资源的过程中存在的问题,才能进一步提高电子文献资源的利用率。     

电反馈对FM光发射机信噪比性能的改善

本文研究了电反馈半导体激光器作为FM光发射机的信噪比性能。分析表明:电反馈可以改善FM光发射机的信噪比性能。这种改善在适当的反馈条件下可达近30dB。反馈环路存在着最佳增益使信噪比达到最大。     

Growth of zinc- blende GaN on GaAs (100) substrates at high temperature using low-pressure MOVPE with a Low V/lll molar ratio

Zinc-blende GaN films were grown on GaAs (100) substrates by low-pressure metalorganic vapor phase epitaxy using trimethylgallium or triethylgallium and NH₃. Films grown at lower temperatures contained considerable amounts of carbon, but the carbon concentration was reduced in high temperature growth. When the film was grown at 950°C using triethylgallium and NH₃, its carbon concentration was on the order of 10¹⁷ cm⁻³. The crystalline and optical quality of zinc-blende GaN crystal also improved with high-temperature growth at a low V/III ratio using a thin buffer layer. The films exhibited only one sharp photoluminescence peak at 3.20 eV with a full width at half maximum as low as 70 meV at room temperature.     

Theoretical study of InAs/GaAs quantum dots grown on [11k] substrates in the presence of a magnetic field

Eight-band k.p theory including strain and piezoelectric effects are employed to calculate the strain distribution and electron and hole energy levels of InAs/GaAs quantum dots grown on [11k] substrates in the presence of an external magnetic field. Height of the dot determines how the increasing of k influences isotropic part of the strain tensor while biaxial part of the strain tensor is always reduced with increasing k. Because of the reduced symmetry of high index surfaces, influence of piezoelectric effect on the electronic structure becomes more dominant with increasing k. Electron energy levels are influenced by the isotropic part of the strain compared to the hole energy levels, where strong heavy hole–light-hole mixing is observed. For the dots grown on the [11k] surfaces magnetic field has smaller influence on the electron and hole energy levels as compared to the referent case.