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981.
本文简述了6kV变频器的主电路拓朴结构及控制方式,介绍了变频器比较选择和PowerFlex7000变频器用于扬州四水厂的工程实例。 相似文献
982.
A novel method to generate high-frequency phase-coded RF pulses using all-fiber components is proposed. The system consists of a mode-locked fiber laser (MLFL), a dispersive element, an unbalanced Mach-Zehnder interferometer (UMZI), an optical phase modulator (PM), and a photodetector (PD). The PM is incorporated in one arm of the UMZI. In the system, an ultrashort pulse generated by the MLFL is broadened and chirped after passing through the dispersive element, which is then sent to the UMZI, to get two time-delayed chirped pulses. By beating the time-delayed chirped pulses at the PD, an RF pulse with its frequency dependent on the time delay difference is obtained. The generated RF pulse can be phase coded if an encoding signal is applied to the PM. A theoretical model is presented which is verified by experiments. The generation of RF pulses with binary phase coding is also experimented 相似文献
983.
Hirose T. Saito K. Kojima S. Yao B. Ohsono K. Sato S. Takada K. Ikushima A.J. 《Electronics letters》2007,43(8):443-445
Long-period fibre grating (LPFG) writing by a CO2 laser-annealing using a fibre-drawing process is demonstrated. The fibre in the drawing process was irradiated periodically by a CO2 laser to modify the refractive index. An LPFG with transmission loss of -10 dB and full width at half-maximum of 13 nm has been fabricated. Results show that the refractive index change was induced by stress at the moment of laser annealing 相似文献
984.
In this paper, we study the effect of imperfect channel estimation (ICE) on the performance of M-level quadrature amplitude modulation (M-QAM) with maximum ratio combining (MRC) and pilot-symbol assisted modulation (PSAM) in generalized Rician fading channels. By expressing the bit error rate (BER) of MRC diversity M-QAM in terms of the distribution of new decision variables, we derive novel, exact, and easy-to-evaluate BER expressions for diversity M-QAM with channel estimation errors. Our results include versatile system and fading channel parameters (e.g., arbitrary spatial and temporal correlation patterns among the diversity branches), and are valid for arbitrary linear channel estimators and square and rectangular M -QAM with different constellation sizes. In addition, we evaluate the performance of minimum mean-squared error (MMSE)- and sinc-interpolator-based channel estimators with PSAM, and provide some new insights into the performance of M-QAM with PSAM in generalized fading channels 相似文献
985.
Yang Yubin Lin Hui Zhang Yao 《IEEE transactions on systems, man and cybernetics. Part C, Applications and reviews》2007,37(6):1081-1098
As the number of available 3D models grows, there is an increasing need to index and retrieve them according to their contents. This paper provides a survey of the up-to-date methods for content-based 3D model retrieval. First, the new challenges encountered in 3D model retrieval are discussed. Then, the system framework and some key techniques of content-based 3D model retrieval are identified and explained, including canonical coordinate normalization and preprocessing, feature extraction, similarity match, query representation and user interface, and performance evaluation. In particular, similarity measures using semantic clues and machine learning methods, as well as retrieval approaches using nonshape features, are given adequate recognition as improvements and complements for traditional shape-matching techniques. Typical 3D model retrieval systems and search engines are also listed and compared. Finally, future research directions are indicated, and an extensive bibliography is provided. 相似文献
986.
On Lifetime-Based Node Failure and Stochastic Resilience of Decentralized Peer-to-Peer Networks 总被引:1,自引:0,他引:1
To model P2P networks that are commonly faced with high rates of churn and random departure decisions by end-users, this paper investigates the resilience of random graphs to lifetime-based node failure and derives the expected delay before a user is forcefully isolated from the graph and the probability that this occurs within his/her lifetime. Using these metrics, we show that systems with heavy-tailed lifetime distributions are more resilient than those with light-tailed (e.g., exponential) distributions and that for a given average degree, k-regular graphs exhibit the highest level of fault tolerance. As a practical illustration of our results, each user in a system with n = 100 billion peers, 30-minute average lifetime, and 1-minute node-replacement delay can stay connected to the graph with probability 1 - 1/n using only 9 neighbors. This is in contrast to 37 neighbors required under previous modeling efforts. We finish the paper by observing that many P2P networks are almost surely (i.e., with probability 1 - o(1)) connected if they have no isolated nodes and derive a simple model for the probability that a P2P system partitions under churn. 相似文献
987.
On Probing Signal Design For MIMO Radar 总被引:7,自引:0,他引:7
A multiple-input multiple-output (MIMO) radar system, unlike a standard phased-array radar, can choose freely the probing signals transmitted via its antennas to maximize the power around the locations of the targets of interest, or more generally to approximate a given transmit beampattern, and also to minimize the cross-correlation of the signals reflected back to the radar by the targets of interest. In this paper, we show how the above desirable features can be achieved by designing the covariance matrix of the probing signal vector transmitted by the radar. Moreover, in a numerical study, we show that the proper choice of the probing signals can significantly improve the performance of adaptive MIMO radar techniques. Additionally, we demonstrate the advantages of several MIMO transmit beampattern designs, including a beampattern matching design and a minimum sidelobe beampattern design, over their phased-array counterparts. 相似文献
988.
Yu Yan Jianping Yao 《Photonics Technology Letters, IEEE》2007,19(19):1472-1474
A novel tunable photonic microwave filter with complex coefficients is proposed and experimentally demonstrated. The complex coefficient is generated using a wideband tunable optical RF phase shifter that consists of two electrooptic intensity modulators. The phase of the RF signal is shifted by simply adjusting the bias voltages applied to the two electrooptic intensity modulators, and the phase shift remains constant over the microwave spectral region of interest. A two-tap photonic microwave filter with one tunable complex coefficient, with a wide and continuous tuning range, is experimentally demonstrated. 相似文献
989.
990.