Time filter

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Zhang X.-Y.,South China University of Technology | Zhang Y.-B.,South China University of Technology | Wang X.-Y.,China Research Institute of Radio Wave Propagation | Hu B.-J.,South China University of Technology
Journal of Electromagnetic Waves and Applications | Year: 2012

This paper presents a bandpass filter with electricallytunable center frequency and bandwidth. The proposed filter employs a centrally-loaded resonator with three varactors. The odd-mode and even-mode resonant frequencies of the resonator can be flexibly controlled by changing the voltage of the corresponding varactors. The adjustability of center frequency is achieved by adjusting both odd- and even-mode resonant frequencies, and the tuning of bandwidth can be achieved by probably adjusting the discrepancy between odd- and evenmode resonant frequencies. To improve the selectivity, the source-load coupling is utilized to generate transmission zeros. The performance of this electronically tuned filter is promising in terms of continuous center frequency and bandwidth tunings. For validation, both theory and experimental results are provided. It is shown that the filter could operate within the frequency range from 525 MHz to 655 MHz and be applicable to cognitive radio systems. Source

Li G.,CAS Institute of Geology and Geophysics | Ning B.,CAS Institute of Geology and Geophysics | Patra A.K.,National Atmospheric Research Laboratory | Abdu M.A.,National Institute for Space Research | And 3 more authors.
Journal of Geophysical Research: Space Physics | Year: 2013

The daytime 150 km echoes, which are associated with the upper E region field-aligned irregularities, have been observed around the equatorial electrojet region but never before at magnetic latitudes near the northern anomaly crest region. We present first results of daytime 150 km echo over Sanya, China, a station located far away from the dip equator in the Northern Hemisphere. A layer of weak radar echoes with spectral width less than 10 m s-1 was seen in the height range from 145 km at 12:15 LT to 152 km at 13:45 LT on 21 July 2010. The interesting aspect is that the rare observation of daytime 150 km echoes with the small Sanya VHF radar was preceded by the occurrence of an unusual intermediate layer, which is identified as abnormal traces at the upper E region in corresponding ionograms. The abnormal intermediate layer associated with possible gravity wave activity (that implicates the presence of upper E region density gradients) could make a significant contribution to the growth of irregularities responsible for the rarely detected daytime 150 km echoes over Sanya. Key Points The 150 km echo is first of its kind from the northern anomaly crest region The presence of abnormal layer traces preceding the 150 km echo is a new point It provides direct evidence on the linkage of 150 km echo and intermediate layer ©2013. American Geophysical Union. All Rights Reserved. Source

Lin L.K.,Nanjing University of Posts and Telecommunications | Lin L.K.,China Research Institute of Radio Wave Propagation | Zhao Z.W.,China Research Institute of Radio Wave Propagation | Zhang Y.R.,Nanjing University of Posts and Telecommunications | Zhu Q.L.,Xidian University
IET Radar, Sonar and Navigation | Year: 2011

A retrieval method is developed to obtain atmospheric refractivity profiles based upon the zenith delay of single ground-based global positioning system receivers. It is found that the key parameter in exponential wet refractivity model is the equivalent height of wet term Hw, while the errors between the integral of the wet terms of refractivity and the true values are single-peaked functions of Hw. Therefore the golden section method is utilised to retrieve Hw; moreover, the refractivity profiles are obtained. Meanwhile, the classic Hopfield model is used as the hydrostatic refractivity model. The experimental results with radiosonde data in Shanghai, China, and simulations using data from Qingdao, China, show that this method has demonstrated a good agreement with radiosonde profiles. © 2011 The Institution of Engineering and Technology. Source

Zhang J.-P.,Xidian University | Wu Z.-S.,Xidian University | Zhu Q.-L.,China Research Institute of Radio Wave Propagation | Wang B.,China Research Institute of Radio Wave Propagation
Progress in Electromagnetics Research | Year: 2011

A new four-parameter modified refractivity profile (M-profile) model for the evaporation duct is introduced in this paper. In the estimation of radio refractivity structure from sea clutters, a parametric M-profile model is normally employed. The conventional M-profile model for evaporation ducts is the one-parameter log linear model, which has some potential disadvantages in describing the observed M-profiles which would result in rough results of evaporation duct estimation. Based on this model, three new parameters are introduced and a four-parameter M-profile model is proposed here. This model has the ability to (a) more accurately match real-world M-profiles, (b) well replicate the observed clutter field, and (c) show clutter power or path loss sensitivity to each model parameter. All these abilities are necessary for robust refractivity estimations. The performance of this model is tested and validated through the estimation for two truly measured M-profiles. Source

Liu S.-Q.,South China University of Technology | Hu B.-J.,South China University of Technology | Wang X.-Y.,China Research Institute of Radio Wave Propagation
IEEE Communications Letters | Year: 2012

In this letter, we consider hierarchical cooperative spectrum sensing based on two-threshold energy detection in cognitive radio networks. In order to solve the sensing failure problem in conventional two-threshold energy detection, soft combination of the observed energy value from different cognitive radio users is investigated. Encouraged by the performance gain of soft combination of energy falling between the two thresholds, we further propose a new softened hard combination scheme to reduce the communication overhead. Results show that the proposed method achieves a good tradeoff between sensing performance and communication overhead. © 2012 IEEE. Source

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