Research On Wireless Communication And Radar Target Imaging Technique Based On OAM

With the rapid development of wireless communication technology,the congestion problem of radio frequency(RF)band is increasingly serious.Therefore,it is of great significance to develop new wireless communication technology which can improve the spectrum efficiency in the case of limited frequency resource.The study found that,orbital angular momentum(OAM),which is one of the fundamental physical quantities of the electromagnetic field,can be applied to wireless communication due to its theoretically infinite number of eigenmode and the orthogonality between different eigenmodes,so that we can improve the transmission capacity of the channel without increasing the bandwidth.Based on this,in this thesis,we investigate the feasibility of the application of OAM to the future wireless communications,and the related technical ideas and methodology,including the method of generating vortex beam which carrying OAM in the RF band,and the analysis of the propagation characteristics of the vortex beam as well as the detection method of OAM mode.Besides,we also investigate the feasibility of the application of OAM to the radar target detection based on the unique spatial phase property of vortex beam.The related research contents are summarized as follows.A circular dipole antenna array is proposed which can be used to generate vortex beam in the RF band.For the problem of vortex beam generation in the practical application scenario,we proposed a uniform circular antenna array(UCA)consisting of half-wave dipole which is a classical array element in the antenna design field.For the four types of arrangement of the dipole in the circular array,simulation results show that only the parallel dipole UCA stimulated with linear phased signals can radiate vortex beam carrying OAM.On this basis,we study in detail the effect of various parameters of the UCA on the vortex beam radiation characteristics,meanwhile,we set the UCA as the vortex beam receiver and verified through simulation that the different OAM eigenmodes are orthogonal to each other and can be exploited in the wireless communication to increase the channel capacity.An off-axis test method of the OAM mode of vortex beam is developed.For the problem that the detection results of the traditional two-antenna phase gradient method are no longer reliable when there is a lateral displacement between the detection array and the beam axis,we develop an OAM mode detection method based on UCA without the need to know the precise location of the beam axis.Simulation results show that,when the transverse offset of the detection array relative to the beam axis is within a given range,we can obtain reliable mode detection results,and the detection performance is irrelative to the offset.On this basis,in view of larger offset of the beam axis,we also develop another two grid antenna array configurations which can be used to detect low order OAM modes under off-axis condition.Compared with the UCA,these two grid arrays can cover even greater area,and as long as the beam axis fall into the area covered by the grid array,the OAM mode can be detected,and at the same time,the grid that the beam axis falls into can be determined.This provides meaningful reference for subsequent offset correction of the transceiver array and other signal processing techniques in OAM communication.Finally,we put forward a method for radar target two-dimension(2D)imaging based on vortex electromagnetic wave.Based on the two kinds of ideal scattering echo model of the vortex beam generated with UCA,we sample the OAM mode of the echo signal and calculate the autocorrelation function,and then we obtain the space spectrum function with the use of multiple signal classification(MUSIC)algorithm,and finally,we search the space spectrum function for the 2D spectrum peak so as to realize the 2D joint detection of the radar target in the elevation and azimuth domain.Compared with the existing radar target azimuth imaging method based on the vortex beam,this proposed method can achieve simultaneously elevation and azimuth joint imaging,in addition,compared with the classical MUSIC imaging algorithm in the traditional plane wave,the proposed method can achieve higher imaging resolution without relative motion and beam scanning,which is of great significance to the design of modern new type of radar system.