GaN buffer layers (thickness ～60nm) grown on GaAs(001) by low-temperature MOCVD are investigated by X-ray diffraction pole figure measurements using synchrotron radiation in order to understand the heteroepitaxial growth features of GaN on GaAs(001) substrates.In addition to the epitaxially aligned crystallites,their corresponding twins of the first and the second order are found in the Xray diffraction pole figures.Moreover,{111} φ scans with χ at 55°reveal the abnormal distribution of Bragg diffractions.The extra intensity maxima in the pole figures shows that the process of twinning plays a dominating role during the growth process.It is suggested that the polarity of {111} facets emerged on (001) surface will affect the growth twin nucleation at the initial stages of GaN growth on GaAs(001) substrates.It is proposed that twinning is prone to occurring on {111}B,Nterminated facets. 相似文献
A design and implementation for a 2.4GHz quadrature output frequency synthesizer intended for bluetooth in 0.35μm CMOS technology are presented.A differentially controlled quadrature voltage-controlled oscillator (QVCO) is employed to generate quadrature (I/Q) signals.A second-order loop filter,with a unit gain transconductance amplifier having the performance of a third-order loop filter,is exploited for low cost.The measured spot phase noise is –106.15dBc/Hz@1MHz.Close-in phase noise is less than -70dBc/Hz.The synthesizer consumes 13.5mA under a 3.3V voltage supply.The core size is 1.3mm×0.8mm. 相似文献
In this paper, a media access control (MAC) protocol is proposed for multimedia traffic in slotted CDMA wireless communication systems. The channel resource is partitioned into four compartments: video, voice and data compartments, and common resource pool (CRP) which can be used by either voice or data. Video is only permitted to use its pre-allocated channels. Besides assigning voice to its own compartment, a threshold type decision policy is introduced to regulate voice call admission control in CRP. Data can use not only data and CRP compartments, but also the idle capacity of both video and voice. The proposed protocol provides a simple and flexible resource allocation decision for the multimedia traffic such that the bandwidth can be utilized efficiently while satisfying the quality of service (QoS) requirement for each traffic category. Two mathematical analysis methods are applied to evaluate the system performance. The first, Traditional Markov Analysis (TMA) is used to calculate the video and the voice blocking probabilities, and the second, Transient Fluid Analysis (TFA) is adopted to evaluate the average data delay and the channel utilization. Computer simulations verify the accuracy of mathematical models.Miao Ma received the B.Eng. and M.Eng. degrees in Electrical Engineering from Harbin Institute of Technology, China in 1992 and 1996, respectively, and the Ph.D degree in Electrical and Electronic Engineering from Nanyang Technological University, Singapore in 2003. She is a research scientist in the Institute for Infocomm Research in Singapore. Her research interests include network security, media access control, wireless communications and computer networking.Erry Gunawan received the B.Sc. degree in electrical and electronic engineering from the University of Leeds, U.K., in 1983 and the M.B.A. and the Ph.D. degrees, both from Bradford University, in 1984 and 1988, respectively. From 1984 to 1988, he was a Satellite Communication System Engineer at Communication Systems Research Ltd., Ilkley, U.K. In 1988, he moved to Space Communication (SAT-TEL) Ltd, Northampton, U.K. He joined the School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore, in 1989, and currently, he is an Associate Professor in the same school. His research interests include the fields of digital communications, mobile and satellite communications, error coding, and spread spectrum. He has published over 60 international research papers and has been a consultant to a local company on the study of DECT system and BLUETOOTH. 相似文献
Maximizing network lifetime is the main goal of designing a wireless sensor network. Clustering and routing can effectively balance network energy consumption and prolong network lifetime. This paper presents a novel cluster-based routing protocol called EECRAIFA. In order to select the optimal cluster heads, Self-Organizing Map neural network is used to perform preliminary clustering on the network nodes, and then the relative reasonable level of the cluster, the cluster head energy, the average distance within the cluster and other factors are introduced into the firefly algorithm (FA) to optimize the network clustering. In addition, the concept of decision domain is introduced into the FA to further disperse cluster heads and form reasonable clusters. In the inter-cluster routing stage, the inter-cluster routing is established by an improved ant colony optimization (ACO). Considering factors such as the angle, distance and energy of the node, the heuristic function is improved to make the selection of the next hop more targeted. In addition, the coefficient of variation in statistics is introduced into the process of updating pheromones, and the path is optimized by combining energy and distance. In order to further improve the network throughput, a polling control mechanism based on busy/idle nodes is introduced during the intra-cluster communication phase. The simulation experiment results prove that under different application scenarios, EECRAIFA can effectively balance the network energy consumption, extend the network lifetime, and improve network throughput.
