Study On The Turbulence Mitigation Scheme Using Multiple-user Detection For Orbital Angular Momentum Multiplexing Communications

Like other degrees of freedom, such as time, frequency, orthogonal code pieces, orbital angular momentum（OAM） mode provides a new degree of freedom for multiplexing. The OAM-multiplexed communication system greatly increases both the channel capacity and spectral efficiency by exploiting the orthogonality phenomenon among different OAM modes. However, the multiplexed OAM is inevitably influenced by atmospheric turbulence（AT） when it is transmitting through the free space, which resulting in crosstalk between the OAM modes and seriously dagrading the performance of the communication system. Therefore, the study on the AT mitigation method for OAM multiplexed communication system has an important theoretical significance and reference value.In this thesis, an AT mitigation scheme for the OAM-multiplexed FSO communication based on minimum mean square error（MMSE） multiple-user detection（MUD） method was proposedat first. AT destroys the phase structure of OAM mode, makes OAM modes power spread, and results in crosstalks between different OAM modes. The results show that the stronger the turbulence is, the more severe the crosstalk is. Based on the equivalent model of the OAM-multiplexed communications system with a multi-user detection idea, a MMSE MUD method is used to process the received information. The numerical simulation results show that the proposed MMSE MUD method can simultaneously mitigate the crosstalk and the random noise, and in comparison the SUD method, the BER of the system using MMSE MUD method can decline 1 to 2 orders of magnitude in the weak and medium atmospheric turbulence. In addition, MMSE MUD method can also outperforms decorrelating MUD method.Then, the OAM-multiplexed FSO communication system based on minimum mean square（LMS） blind adaptive MUD was proposed. Blind multiple-user detection（BMUD） could make the output of the detector as close as possible to the sequence in the sender by using only the priori information of received signals. The simulation results show that LMS BMUD method can effectively utilize the crosstalk among OAM modes to isolate of the desired user’ information, and suppress the effects of atmospheric turbulence on OAM multiplexing communication system, especially in a weak, medium strength(C_n²＜10^-14m^-2/3).Lastly, an adaptive step LMS（AS-LMS） blind adaptive MUD method was used for the AT mitigation scheme of the OAM-multiplexed FSO communication. It can adjust the step size with the channel variance. AS-LMS method can overcome the disadvantages of large amount of disorder and poor dynamic tracking ability, and it is more suitable for practical communication systems. The simulation results show that AS-LMS BMUD method can greatly improve the upper limit of AT intensity for getting the zero-error, and the BER performance can be significantly enhanced in the condition of weak and medium(10^-15m^-1/3＜10^-14m^-2/3) AT.