Research On Generation And Convergence Of OAM Beams Using Antenna Arrays

Orbital Angular Momentum(OAM)as an inherent property of electromagnetic waves,provides a new physical dimension for signal modulation.Recently,OAM with an infinite variety of mutually orthogonal modes,which aims to improve the capacity and spectrum efficiency of wireless communications,has attracted widespread attention from many scholars.In the research process of OAM,the generation and convergence of OAM beams are particularly important.Also,the generation of multi-mode and convergent beams also plays a significant role in the application of OAM.In this thesis,based on the characteristics of OAM,the generation of multi-mode OAM beams,side-lobe suppression and beam convergence are studied.Firstly,this thesis introduces the research background and significance of the topic,reviews the research progress of OAM,and summarizes the current methods of generating and converging OAM beams.Then,this thesis introduces the principle and detection method of OAM,combs the characteristics of vortex electromagnetic wave(VEM),and makes theoretical and simulation analysis of OAM beams generation based on phased circular array.This work can provide theoretical support for the subsequent research of this thesis.Secondly,the theoretical model of single-ring time-modulated circular array is established and a design criterion of time-modulated sequence for generating multi-mode OAM beams is proposed.The simulation results show that the method can radiate multiple modes simultaneously on the center frequency and harmonic frequencies.On this basis,a method for generating OAM beams by multi-ring time-modulated circular arrays is proposed.The time sequences of uniform switch-on duration and non-uniform switch-on duration are designed.Furthermore,the pattern,amplitude distribution and phase distribution are compared and analyzed.Subsequently,to solve the problem of high side-lobe level,two side-lobe suppression methods based on directional array elements and multi-ring time-modulated circular arrays time sequence optimization are proposed,which effectively suppress the peak side-lobe level(PSLL)of OAM waves in different modes.Finally,the divergence characteristics of OAM beams under different modes and transmission distances are studied.The effects of center frequency,array radius and number of array elements on beam divergence are analyzed.In addition,two methods for converging OAM beams are proposed.The first method is to use multi-ring phased circular arrays.By designing the radius of each circular array,the divergence angles of the beams in different modes are converged to a specified angle.The second method is to use multi-ring time-modulated circular arrays.Under the constraints of the maximum array radius and the minimum interval between adjacent rings,by setting reducing the beam divergence angle as the optimization objective and using genetic algorithm(GA)to optimize the radius of each circular array,the excitation amplitude and the switch-on duration of elements,the OAM beam convergence of the specified mode is achieved.