In a series of three papers published in 2024[1-3],physicists in Austria,Germany,and the United States reported the first direct observations with table-top lasers of a new nuclear process,in which the nucleus of a th...In a series of three papers published in 2024[1-3],physicists in Austria,Germany,and the United States reported the first direct observations with table-top lasers of a new nuclear process,in which the nucleus of a thorium atom absorbs a photon and goes into an excited state,then re-emits the photon and returns to its ground state.This“thorescence”phenomenon“is exactly the same process as fluorescence,but it takes place inside the nucleus,”said Ekkehard Peik,professor and head of the department of time and frequency at the Physikalisch-Technische Bundesanstalt(the German national metrology institute)in Braunschweig,Germany.展开更多
正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)雷达采用了OFDM信号,具有大时宽带宽积,且信号编码方式灵活,通过不同的波形设计准则,能够自适应调整信号子载频的系数,具备了认知雷达系统的基本特点。通过简要回顾OFDM...正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)雷达采用了OFDM信号,具有大时宽带宽积,且信号编码方式灵活,通过不同的波形设计准则,能够自适应调整信号子载频的系数,具备了认知雷达系统的基本特点。通过简要回顾OFDM雷达的发展历程,讨论了OFDM雷达信号特点、信号处理及波形设计方法等关键技术,对目前的研究成果进行了分析与总结,指出了存在的问题。讨论了OFDM雷达的未来发展方向。展开更多
文摘In a series of three papers published in 2024[1-3],physicists in Austria,Germany,and the United States reported the first direct observations with table-top lasers of a new nuclear process,in which the nucleus of a thorium atom absorbs a photon and goes into an excited state,then re-emits the photon and returns to its ground state.This“thorescence”phenomenon“is exactly the same process as fluorescence,but it takes place inside the nucleus,”said Ekkehard Peik,professor and head of the department of time and frequency at the Physikalisch-Technische Bundesanstalt(the German national metrology institute)in Braunschweig,Germany.
文摘正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)雷达采用了OFDM信号,具有大时宽带宽积,且信号编码方式灵活,通过不同的波形设计准则,能够自适应调整信号子载频的系数,具备了认知雷达系统的基本特点。通过简要回顾OFDM雷达的发展历程,讨论了OFDM雷达信号特点、信号处理及波形设计方法等关键技术,对目前的研究成果进行了分析与总结,指出了存在的问题。讨论了OFDM雷达的未来发展方向。
