Research On Generation And Applications Of Radio Orbital Angular Momentum

Due to the popularity of Internet and smart terminals,there will be explosive demands for mobile communications and higher standards of wireless communication system concerning trans-mission rate,delay and throughput.However,the problems of scarce spectrum resource and tra-ditional modulation techniques limit the current communication technologies to meet the develop-ment needs.It is significant to explore potential wireless communication techniques at millimeter-wave band for the new generation of communication systems.Traditional modulation parameters like frequency,amplitude,polarization and phase have been well developed and utilized,so it is difficult to further increase the spectrum efficiency.Orbital angular momentum(OAM),one of the fundamental properties of electromagnetic waves,can be exploited as a novel physical dimension,which brings new opportunities to traditional electromagnetic fields.Most of the work about OAM is focused in optical domain and there are many problems to be solved in microwave domain.In this thesis,the systematic researches on the principles,generations and applications of OAM at micro-and milli-wave range are conducted,which provide reference values to the practical OAM applications.In terms of OAM generation,firstly,a new method to radiate OAM waves is presented based on circular travelling-wave antenna,in which continuous source is used instead of discrete dipoles as in circular antenna array.Hence the problems such as complex feeding network,OAM modes limitations and mutual coupling between elements can be avoided.With comparison to the equiv-alent model of phi-directional circular antenna array,it is theoretically demonstrated that circular travelling-wave antenna can be used for the generation of OAM waves.Under guidance of this theory,a ring resonator based slot OAM antenna is designed with very simply feeding.Accord-ing to the waveguide theory,the resonant modes in the circular resonator is investigated,based on which the design principle concerning on the dimension structure,radiation properties and feeding network are obtained.Moreover,quasi-two-dimensional OAM waves with plane helical structure are obtained by adding circular horn around the resonator.This plane spiral OAM waves have po-tential in expanding the OAM based applications in traditional electromagnetic areas as they solve the unavoidable problems of phase singularity and divergence inconsistency of the conventional OAM waves.Secondly,for miniaturization and integration of OAM antennas,substrate integrated waveg-uide(SIW)is used to radiate OAM waves on the plane structure.With the equivalent model be-tween SIW and metallic waveguide,a half-mode substrate integrated waveguide(HMSIW)antenna which can generate multi OAM modes is proposed.Resonators of different sizes are integrated for OAM mode-multiplexing.The experiment results in near and far field show the performances of l =±2,±6 OAM waves at 10 GHz.Thirdly,metasurface is utilized for generation and focusing of OAM waves.Divergence angle is further decreased by means of flat-lens in comparison to traditional spiral phase plate.The dimensions of multi-layer phase shifting surface structure are analyzed and a series of units are designed,covering phase shift range of 2?r.According to principle of phase superposition,the units are arranged to form the new flat-lens spiral phase plate,and l = 1 OAM wave at 60 GHz is successfully generated and focused.On the basis of OAM generations,new applications on computational imaging and beamform-ing are investigated.Applying compressive sensing theory,the computational imaging turns into a solvable nonlinear minimization problem,in which a measurement matrix is needed.Due to the orthogonality provided by different OAM modes,the reconstruction of a sparse scene can be real-ized with limited measurements.In our experiment,plane spiral OAM waves are used in scheme owing to its special characteristics.The results show that using 12 plane spiral OAM waves,a 2D annular scene can be reconstructed successfully with azimuth resolution of 2° at a single frequency of 10 GHz.In addition,as special phase distributions of different OAM waves can be considered as a set of complete orthogonal bases,plane spiral OAM waves are effectively superposed to realize beam forming and scanning in two-dimensional plane.It is the first time to our knowledge,that OAM waves are demonstrated to realize beam steering by the superposition of multi plane spiral OAM waves.Beamwidth,number and direction of main lobe can be manipulated by superposed OAM modes.This idea provides a new way for the applications in radar and communication systems.