| The innovation of the technology industry cannot be separated from the development of new technologies and new fields.With the coming of the information age,the traditional wireless communication and radar imaging technology cannot meet the needs of military equipment update and civilian equipment.In wireless communication,it is urgent to explore new technical means to improve channel capacity and spectrum utilization efficiency.In the aspect of radar target imaging,multi-dimensional target position imaging and target micro feature extraction have become one of the trends of future radar imaging technology,so as to adapt to the requirements of accurate acquisition and identification of radar target position in the future battlefield complex environment.Electromagnetic vortex beam as a new type of electromagnetic beam,its unique beam structure characteristics and beam propagation characteristics have become the focus of more and more experts and scholars.The advantages of beam also have the ability to improve the above industries,which is expected to realize the new technological changes of wireless communication and radar target imaging technology,and drive the whole industry leapfrog.Compared with the conventional planar electromagnetic beam,the electromagnetic vortex beam has a three-dimensional spatial structure.Its particularity is reflected in the helical phase wave front and the existence of phase singularity in the center of the beam.The beam intensity distribution appears as a ring with zero intensity region in the center.In addition,electromagnetic vortex beams carry orbital angular momentum(OAM)and are therefore also known as OAM beams.The helical phase term in the beam electric field expression is denoted as exp(i/φ),where the symbol i is an imaginary number unit.The symbol l represents the angular quantum number or mode number of the orbital angular momentum.The symbol φ represents the azimuth of space.In wireless communication applications,the main advantages of electromagnetic vortex based multiplexing communication are multi-dimension and orthogonality.The multi-dimension of electromagnetic vortex beam is shown in that the angular quantum number l can theoretically be infinite,which can realize the transmission of multi-channel data information in the same space and improve the dimension of data transmission.A large number of angular quantum numbers can also be used as a new encoding way to encode information to achieve "modular division multiplexing" to further improve encoding efficiency and channel capacity.For orthogonality,when several electromagnetic vortex waves with different angular quantum numbers l are transmitted coaxial in space,any two beams are mutually orthogonal.The electromagnetic vortex wave carrying different mode number can be coaxial transmitted by modulating different data information.The orthogonality between different beams ensures that the information will not be crosstalk in the transmission process,which can improve the reliability of data.At the same time,the orthogonality between beams can reduce the complexity of data processing after receiving and improve the efficiency of data transmission.In the field of radar target imaging detection,the helical phase wavefront structure makes it possible to obtain the differential information.The angular quantum number l and the azimuth φ a in the helical phase term can form a natural Fourier transform pair,which creates conditions for radar to obtain the azimuth parameter in target imaging detection.In addition,there is an angle between the direction of Poynting vector of the electromagnetic vortex wave and the beam propagation axis.When the beam irradiates a spinning object and the propagation axis coincides with the rotation axis,the echo frequency will change and the rotation Doppler shift effect will be formed.According to the frequency shift,the angular velocity of the spin object can be obtained,and the fretting features of the object can be extracted.Finally,the object can be accurately recognized.It provides reliable technical means for multi-dimensional and multifunctional radar detection in the future.In order to meet the significant needs of the future development in the field of wireless communication and radar target imaging,this article analyzes and studies the propagation properties of electromagnetic vortex beams from circular antenna arrays to generate electromagnetic vortex beams.The advantages of electromagnetic vortex waves in wireless communication and radar target imaging detection are fully considered and the corresponding application studies are carried out.The aim is to fully explore the characteristics of electromagnetic vortex beam propagation in space and improve the potential of electromagnetic beam in information loading,information transmission and information acquisition.It lays a theoretical foundation for the development of electromagnetic vortex wave in the field of wireless communication and radar target imaging detection and provides corresponding frontier technology exploration.The specific research content and innovation of this article are reflected in the following aspects:1.The characteristics of electromagnetic vortex beam in space transmission are analyzed,and the electromagnetic vortex beam generated by uniform circular array(UCA)is studied.An electric field radiation model is established for the electromagnetic vortex beam generated by a UCA.Considering the deviation between the actual antenna element position in UCA and the optimal position,the deviation is decomposed into radial deviation and angular deviation,and the influence of the array deviation on the intensity and phase of the electromagnetic vortex beam is analyzed.Considering the deviation between the actual antenna element position and the optimal position in UCA,the deviation is decomposed into radial deviation and angular deviation.The influence of array deviation on electromagnetic vortex beam is analyzed.Furthermore,an analysis method based on OAM spectrum is proposed to quantify the influence of two kinds of deviation on the mode purity of the electromagnetic vortex beam generated by UCA.The verification results show that the angular deviation of antenna element has more influence on the mode purity of beam than the radial deviation.The result also provides the corresponding support for the manufacturing of UCA.2.In order to solve the problem of slow switching speed and low purity of OAM mode caused by imprecise phase shifting in the process of switching between different OAM modes by UCA,a uniform circular frequency diversity array(UC-FDA)scheme was proposed to generate electromagnetic vortex beam with mode number evolving with transmission time and range.In a UC-FDA,signals with different frequency offsets are fed into different antenna elements,so that the signals have different phase relations in the propagation process,and then the electromagnetic vortex waves with different mode numbers are generated.The results show that the beam can realize fast and automatic OAM mode switching.The switching rate depends on the frequency difference between antenna units.The characteristics of beam evolution over time and space and the evolution law of OAM mode are analyzed and summarized.Furthermore,the preliminary radar target imaging application is carried out based on the spatiotemporal evolution of the mode number of the beam.The results show that the target azimuth data can be accurately measured,which provides a new technical means for radar multi-dimensional imaging.Finally,the secure transmission of the target mode number is also explored using the relationship between frequency offset and mode number.The pre-set target mode number is able to maintain high mode purity at legal distance formation,but cannot complete effective reception in illegal receiving region,which provides a new approach for secure transmission of sensitive information.3.The diffusion effect in electromagnetic vortex beam propagation is studied.Based on the UCA electric field radiation model,the parameters of electromagnetic beam self-diffusion effect are analyzed,and the variation law of the cross-section size of electromagnetic vortex beam with the number of modes is summarized.In order to reduce the problem of increasing annular strength structure in the process of beam propagation,a concentric UCA(CUCA)structure and a multi-ring radial phase modulation plate(RPMP)structure are proposed based on beam focusing theory and collimation theory respectively.The results show that the cross-section size of the electromagnetic vortex beam is controlled and the diffusion effect of the beam is effectively alleviated.This research is helpful to realize the application scenario of electromagnetic vortex beam propagation over long distance,and provides guarantee for full beam reception and effective detection of restricted target location region.Meanwhile,the radial phase modulation does not affect the helical phase wavefront structure of the electromagnetic vortex wave,and the mode purity of the modulated beam can still maintain a high level.4.In the free space transmission of electromagnetic vortex beam,atmospheric turbulence is one of the important external factors affecting the beam propagation effect.The randomness and inhomogeneous distribution of atmospheric turbulence can lead to distortion of the spiral phase wavefront of the electromagnetic vortex beam and disruption of the circular ring with uniform intensity distribution,which will lead to the spectrum broadening of OAM mode and other problems.To analyze the effect of atmospheric turbulence on the spatial structure of electromagnetic vortex waves,a random phase screen is produced to simulate atmospheric turbulence using the power spectrum inversion method.It is proposed to use the plane electromagnetic wave as a probe to obtain the data of the influence of atmospheric turbulence and propagation distance on the beam phase,so as to make a compensation phase screen at the receiver.Further,the distorted phase is corrected by using the compensation phase screen to restore the initial spatial structure of the electromagnetic vortex wave and improve the purity of the mode.5.A dynamic mode decomposition algorithm(DMD)is proposed to detect OAM mode number effectively.The helical phase term is obtained through the algorithm processing of the electromagnetic vortex wave electric field data collected by the receiver.Based on the dual relationship between the OAM mode and the azimuth in the spiral phase term,the number of periods of change of the spiral phase term information in the range of azimuth angle change of 2π is analyzed.According to the relationship between the number of change cycles and OAM mode,the absolute value of OAM mode can be obtained.The sign of the number of OAM modes to be detected is further obtained according to the change direction of the spiral phase term curve near 0°.Meanwhile,the scheme is not only suitable for single-mode detection,but also for mode detection in multi-mode multiplexing scenarios.In addition,when the complete reception of the beam cannot be achieved at the receiver side,this scheme can also detect the transmitted mode by processing part of the received data,which greatly improves the efficiency of mode detection and enriches the practical application scenarios of electromagnetic vortex waves. |
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