Advanced Materials Research Vols. 846-847

Abstract: The excitation system is an important component element that influences the dynamic behavior of alternators. However, when the two-quadrant H-half bridge converter is used in the exciter field current closed-loop control system, there will be some problems of nonlinearity. To prevent the converter from working in the nonlinear area, the working conditions of linear and nonlinear area of H-half bridge converter is discussed firstly in this paper. And then it is verified by the simulation results.

Abstract: This paper presents a two-quadrant H-half bridge converter used as the field current amplifier of ac exciter in the synchronous generator brushless excitation system (SGBES). Differing from the conventional amplifier, the exciter field winding can be supplied with positive and negative voltage to help increase the response speed of the excitation system. However, there will be some problems of stability when the converter is used in the current closed-loop control system. To prevent the converter from working in the unstable area, the stability of the linear area has been analyzed. Finally, it is verified by the experimental results. These can provide a valuable theoretical and practical basis for the design of parameters for the current closed-loop control system used by the converter.

Abstract: Combining the energy limit theory and the ANSYS finite element simulation analysis, this paper has designed the new integrated quartz crystal resonator and has used oscillator circuits to carry on the experimental comparison on its temperature-frequency characteristic in high-low temperature chamber. The experimental results show that the curve of its temperature-frequency characteristic is cubic in the coordinate system, which is basically consistent with that of the single electrode resonator. The frequency change of each resonator on the same substrate is basically the same when they are in the same circumstance and stimulation. According these similarities, this paper can implement differential processing on output frequencies of different resonators on the same substrate, which can effectively restrain the influence that temperature imposes on frequency.

Abstract: Gearbox affect the normal operation of the wind turbines, to study the fault diagnosis, support vector method was used. Parameters selection is very important and decides the fault diagnosis precision. In order to overcome the blindness of man-made choice of the parameters in least squares support vector machine (LSSVM) and improve the accuracy and efficiency of fault diagnosis, a method based on LSSVM trained by genetic algorithm was proposed. This method searches the optimized parameters in LSSVM by taking advantage of the genetic algorithms powerful global searching ability. The research is provided using this method on the fault diagnosis of wind turbine gearbox and compared with the diagnostic method of LSSVM. The experimental results show that the method achieves a higher diagnostic accuracy.

Abstract: We propose and analyze an unslotted CSMA with hybrid sensing in multi-channel cognitive radio network, which combines random m-sensing and all sensing. We model the system by 3-demensional continuous time Markov process, calculate the steady state probability of the system, and obtain several performance measures such as throughput and forced termination probability of SUs. By comparing the performances of hybrid sensing, random m-sensing and all sensing, the numerical results show that hybrid sensing scheme overcome the defect of low performance in random m-sensing scheme.

Abstract: Relaying is one of the key technological enhancements for the data rate and coverage requirements of wireless networks. The interference alignment (IA) is a promising method for co-channel interference in wireless networks. In this paper, we investigate the quantize-and-forward (QF) relaying strategy with IA in the relay networks. The multi-user transmission aided by the relay is a common case in the wireless networks and can be modeled as multi-user relaying interference channels. We design a method for QF relaying strategy at the relay and propose precoding of IA to process the interference at each user. Then we derive the sum rate of the system and simulate it with the varying transmitted power and the distance between the user and the relay. The simulation results show that, the sum rate of designed QF relaying with IA has a better performance than those of the existing methods or processed algorithms.

Abstract: With the rapid development of wireless network technologies and proliferation of related services such as multimedia applications, demands for wireless spectrum resources keep rising. Cognitive radio (CR) is a novel approach for better utilization of the scarce, already packed but highly underutilized radio spectrum. Meanwhile, exclusive functionalities such as spectrum sensing make energy efficiency (EE) a crucial issue in Cognitive Radios (CRs). In this paper, we focus on the energy-efficient power allocation for OFDM-based CRs with imperfect spectrum sensing. The EE maximization for secondary users (SUs) is formulated as a nonlinear fractional programming problem taking into account imperfect spectrum sensing such as miss detection and false alarm. Then by transforming the original problem into a parameter programming, the optimal power allocation is derived with the bisection search (BS) method and dual decomposition method (DDM). Simulation results illustrate the significant performance improvement of our scheme compared to an existing one with objective of maximizing system throughput rather than EE.

Abstract: This passage indicate the design of the software which can let 5720A multifunction calibrator automatically measure the digital multi-meter, and describe the control processes, hardware structure, software design and other key technologies of this system specifically.

Abstract: In this paper, a two-step propagation delay difference estimation method is proposed for LTE compatible multi-beam satellite systems to ensure the initial random access (RA). For GEO satellite system, there exists a large propagation delay difference, we cannot directly apply the LTE delay estimation method. To deal with this issue, we first divide a satellite beam into some layered small sub-areas according to the different delay difference values. Then, two types of Physical Random Access Channel (PRACH) preamble burst format are given. The detailed PRACH parameters are provided. Finally, simulations are performed and the results verify the availability of our proposed design.

Abstract: In this paper, a hybrid HARQ scheme was proposed by combing forced retransmission and traditional HARQ together, after analyzing the characteristics of satellite channel and the problem encountered when utilizing HARQ scheme in GEO satellite communication system. The forced retransmission can make a packet be correctly decoded more quickly and shorten the waiting delay. Meanwhile, to balance the delay and throughput, the proper parameters were given for the proposed hybrid scheme. Simulation results show that the proposed scheme performs well at decreasing the waiting delay, especially when SNR is low. The conclusion can be drawn that the proposed scheme can improve the HARQ performance in GEO satellite communication systems.