Research On Millimeter Wave Orbital Angular Momentum Multiplexing

In recent years,the electromagnetic wave orbital angular momentum(OAM)characterized by helical wavefront phase has attracted much attention because of its great communication potential.As the basic physical quantity of electromagnetic wave,OAM provides extra freedom for wireless information carrying.OAM multiplex communication is expected to improve the frequency efficiency of wireless communication system exponentially.Therefore,it is of great significance to establish an appropriate OAM multiplexing communication system model,analyze various potential factors that affect system performance,and explore the feasibility of combining OAM multiplexing with other communication methods.This paper focuses on OAM multiplexing communication in millimeter wave band,and the specific contents are summarized as follows:(1)Several methods of generating and receiving OAM beams in the radio frequency band are analyzed and compared,and potential problems of OAM communication are pointed out,such as a larger divergence angle,a larger influence of the deviation of the transmitting and receiving antennas,and difficulty in receiving large-mode OAM beams.In view of the above problems,it is proposed that the OAM beam in the 60 GHz millimeter wave band has better performance,and the conclusion is demonstrated from three aspects of beam divergence,transmission loss,and receiver size.(2)In order to analyze the performance of the OAM multiplex communication system and tap the potential factors that affect the system performance,multiple spiral phase plates(SPP)with different modes are proposed to construct an OAM multiplex transmission model.Multiple SPP and beam splitter are used in the transmitter of the system to realize the coaxial transmission of different mode OAM beams,and the receiving end uses the opposite mode SPP to separate each OAM beam in turn.Then,taking the system capacity as the index,the OAM multiplexing communication systems under different channel state information(CSI)conditions are simulated and analyzed.The results show that OAM multiplexing is not suitable for long-distance communication whether the transmitter knows the system CSI or not.In addition,when the CSI of the system is unknown at the transmitter,the performance of the system is seriously limited by the alignment of the transceiver SPP;when the CSI of the system is known at the transmitter,the performance of the system is mainly affected by the transmission OAM modes.Finally,aiming at the problem of mode selection at the transmitter of OAM multiplexing communication system,the optimal mode selection strategy is given to optimize the performance of the system.(3)In order to solve the problems of small transmission gain and difficult reception of large-mode OAM beam in OAM multiplexing communication,a new OAM-MIMO communication system is proposed by combining OAM multiplexing with multi-input multi-output(MIMO)communication system.Then,taking the system capacity as the index,the OAM-MIMO communication system and the MIMO communication system are analyzed and compared.The simulation results show that in short-distance communication,the transmission capacity of OAM-MIMO system is higher than that of MIMO system,and with the increase of transmission distance,the performance of OAM-MIMO communication system is gradually lower than that of MIMO system,which is mainly due to the exponential decrease of the transmission gain of OAM beam with the increase of transmission distance.In addition,aiming at the problem of inter-channel interference in OAM-MIMO system,a blind equalization interference cancellation scheme based on constant modulus algorithm(CMA)is proposed.The simulation results show that some interference cancellation methods in space division multiplexing are also suitable for OAM multiplexing system and OAM-MIMO system.