Studies On The Orbital Angular Momentum Based Wireless Optical Communication Technology

Orbital angular momentum(OAM) of optics has the excellent characteristics of countless eigen modes, multi-dimensional quantum tangle and orthogonality of different eigen modes. OAM can provide new freedom in the domain of modes when used for information modulation, and is expected to sharply promote the capacity of communication system. In this dissertation, we study the basic theory, principle technology and system design of the most potential wireless communication based on OAM.Firstly, the research progress of the OAM based wireless communication both foreign and domestic is analysed. The basic theories of information carrying by OAM and the multiplexing/demultiplexing of OAM are expounded.Secondly, the characteristics of OAM propogation through atmospheric turbulence are studied. The propogation of OAM beam is numerically simulated using the method of adding random phase screens on the path. The results show that the atmospheric turbulence brings distortion to OAM beam and destructs the orthogonality of different OAM modes. Additionally, the channel model of the OAM based wireless communication under the condition of atmospheric turbulence is proposed. It is showed by simulation and theoretic deduction that the self-attenution of one OAM mode caused by atmospheric turbulence becomes more extensive when the atmospheric turbulence intesifies or the absolute value of topological charge increases. Meanwhile, the crosstalk between different OAM modes becomes larger as the atmospheric turbulence intensifies while it decreases as the interval of two modes increases.Thirdly, the modulation and demodulation technologies of the OAM based wireless communication are studied. As for the system adopting the manner of intensity modulation, the optimal constellation of transmitting signal as a super cube is proposed. When the system adopts the manner of intensity and phase combined modulation, two-dimensional signals can be loaded to single OAM mode. The optimization of transmitting signals' constellation further improves the system's performance. In addition, a so called OAM-time coding which introduces correlation both in the domain of OAM modes and time is proposed. The simulation shows that OAM-time coding obtains significant multiple modes diversity gain compared with the traditional coding methods.Last but not least, two kinds of OAM based wireless communication system are designed, for which adopt the method of intensity modulation/direct detection and the method of intensity and phase combined modulation/coherent detection respectively, according to the practical application invironments and the performance demands. A strategy of choosing the optimal OAM modes sequence is proposed based on the principle of maximizing the average signal-to-noise-ratio. The LDPC codes are applied to the OAM based wireless communication. The simulation results show that the performance loss caused by atmospheric turbulence can be reduced to some extent by LDPC codes.