Plane Spiral Orbital Angular Momentum Waves And Its Applications

Wireless communication is closely related to our life.With the rapid development of the moible Intenet and smart terminals,the demand for wireless transmission rates is increasing expo-nentially.To solve the problem of frequency-band congestion and further improve the capacity has become an advanced research hotspot in wireless communication.As a fundamental characteristic of electromagnetic(EM)waves,orbital angular momentum(OAM)has attracted increasing interest in recent years.Due to the infinite number of orthogonal states,OAM-carrying beams is expected to used as a new degree of freedom to encode different information on the same frequency and possible to enhance the channel capacity within a fixed frequency band-width.However,in the radio domain,the unavoidable divergence and on-axis phase singularity of OAM-carrying beam will cause receiving problems in a practical situation.With the constraints of the receiver size,the orthogonality among different OAM states will be compromised,and the channel capacity will not increase.Based on the physics of OAM,this paper focuses on the divergence and singularity problems of the OAM-carrying waves,and proposes a new form of OAM wave termed as plane spiral OAM(PSOAM)wave.Different from general OAM wave which propagates along the axial direction,PSOAM wave has the distinguishing characteristic of transverse propagation.Hence it has no singularity along the the propagation path and divergence problem no longer exists.The generation methods,propagation characteristics and potential appications of the PSOAM waves are detailedly discussed in this paper.Three generation methods of PSOAM waves including circular traveling-wave antenna,coax-ial resonator antenna and whispering gallery mode(WGM)dielectric resonator antenna are first proposed in this paper.The simulated and measured results clearly indicate the azimuth phase shifting and the characteristic of transverse propagation of the PSOAM waves.The potential ap-plications of PSOAM waves are also presented.First,the PSOAM waves are combined with the conventional multiple-in-multiple-out(MIMO)system.The performence of such a PSOAM based MIMO(PSOAM-MIMO)system is studied and experimentally demonstrated in a Line-of-Sight(LoS)channel and a multi-path environment,respectively.It is the first time to prove that a prac-tical OAM utilized multiplexing link has a better performance,in sense of higher capacity gain or lower spatial correlations,than the conventional MIMO methods.Second,a PSOAM based beam-forming can be performed by generating different plane spiral OAM waves with same power and same initial phase.It has a better performance but need a much simpler feeding network than the conventional phased-array beam-forming method.The numerical and experimental results also verify the characteristics of orthogonality and constant beam shape of this beam-forming method.As a new form of OAM waves,PSOAM waves has many superiorities such as simple imple-mentation,same direction angle and same link budget.It could achieve a better perfomance when combinede with the MIMO method.We believe PSOAM wave will have extensive applications in the next generation communication systems and radar systems.