Time-frequency channel modeling and estimation of multi-carrier spread spectrum communication systems

In wireless communications, the channel is typically modeled as a random, linear, time-varying system that spreads the transmitted signal in both time and frequency due to multi-path propagation and Doppler effects. Estimated channel parameters allow system designers to develop coherent receivers that increase the system performance. In this paper, we show how time-frequency analysis can be used to model and estimate the time-varying channel of a multi-carrier spread spectrum (MCSS) system using a complex quadratic sequence as the spreading code. We will show that for this spreading code, the effects of time delays and Doppler frequency shifts, caused by the mobility of environment objects, can be combined and represented effectively as time shifts. The discrete evolutionary transform (DET), as a time-frequency analysis method, enables us to estimate the effective time shifts via a spreading function and to use them to equalize the channel. Using the effective time shifts, the time-varying channel can be represented simply as linear-time invariant system by embedding the Doppler shifts that characterize the time-varying channel into effective time shifts. The channel parameters are used to estimate the data bit sent. To illustrate the performance of the proposed method we perform several simulations with different levels of channel noise, jammer interference, and Doppler frequency shifts.     

“中国科技期刊国际影响力提升计划”1期绩效分析——以SCI收录期刊为例

【目的】 对“中国科技期刊国际影响力提升计划”(以下简称为“影响力计划”)1期实施后的效果进行绩效分析,为今后资助期刊的种类及资金额度提供参考。【方法】 基于《期刊引证报告》的数据指标,对影响力计划1期资助期刊的学术影响力提升效果进行分析。【结果】 影响力计划1期实施3年(2013—2015)以来,受资助与未受资助的中国英文科技期刊学术影响力指标均进步明显,而受资助期刊在学术影响力指标的提升上明显快于未受资助期刊。【结论】 受影响力计划1期资助的期刊在学术影响力指标上提升效果明显,同时也起到了对中国英文科技期刊、学术出版的引领带动作用,推动了中国英文科技期刊整体水平的提升。