Modelling And Optimization Of OAM Wireless Communication Systems With Unaligned Antennas In Atmospheric Turbulence

With the commercial application of the Fifth generation(5G)wireless communication system,the performance requirements of wireless communication systems,such as the capacity and the number of access users,have been rapidly improved.However,the limited spectrum resources restrict the further development of wireless communication systems.The future Sixth generation(6G)wireless communication systems put forward the high requirements for performance indicators,such as the capacity.Thus,it is necessary to improve the performance of wireless communication systems with the limited spectrum resources to meet the demand of the future wireless communication systems.The orbital angular momentum(OAM)technique,owing to its natural orthogonality among different OAM modes,has become a research hotspot in wireless communication systems.The OAM technique can provide a new degree of freedom for wireless communication systems.Thus,the OAM technique can increase the capacity of wireless communication systems to meet the needs of massive data transmission in the future 6G wireless communication systems.However,OAM beams are interfered by the atmospheric turbulence and the misalignment between the transmitting and receiving antennas which leads to the diffusion of the OAM modes.The diffusion of the OAM modes will lead to the crosstalk among different OAM channels and OAM modes which further influences the orthogonality among different OAM modes.Motivated by the above reasons,this dissertation concentrates on the effects of both atmospheric turbulence and misalignment between the transmitting and receiving antennas on OAM-based multiple-input multiple-output(OAM-MIMO)wireless communication systems.Moreover,optimization algorithms are designed to improve the performance of OAM-MIMO wireless communication systems.The main contributions of this dissertation are summarized as follows:Firstly,the channel and purity models of OAM-MIMO wireless communication systems with unaligned antennas are derived.The circular traveling-wave antennas equipped with reflectors are used to generate OAM beams.The antennas are placed as a uniform linear array.Based on the established OAM-MIMO wireless communication system,the channel models of OAM-MIMO wireless communication systems considering three different types of misalignment,i.e.,lateral displacement,angular deflection,combination of lateral displacement and angular deflection between the transmitting and receiving antennas are derived.Based on the channel model of the OAM-MIMO wireless communication system with unaligned antennas,considering the effect of non-Kolmogorov turbulence,the purity and crosstalk models of OAM-MIMO wireless communication systems are derived.Secondly,the capacity model of millimeter-wave(mm-wave)OAM-MIMO wireless communication systems with unaligned antennas in non-Kolmogorov turbulence and the optimization algorithm of the capacity model are proposed.Based on the formulated channel and purity models of the OAM-MIMO wireless communication system with unaligned antennas in non-Kolmogorov turbulence,a novel capacity model of transmission systems is derived.Moreover,an OAM mode interval optimization algorithm is designed.Analytical results indicate that,compared to the OAM-MIMO wireless communication system adopting the existing optimization algorithm,the OAM-MIMO wireless communication system adopting the proposed OAM mode interval optimization algorithm achieves an 11.8% increment of the average capacity.Thirdly,the error probability model of mm-wave OAM-MIMO wireless communication systems with unaligned antennas in non-Kolmogorov turbulence and the optimization algorithm of the error probability model are proposed.Based on the formulated channel and purity models of the OAM-MIMO wireless communication system with unaligned antennas in non-Kolmogorov turbulence,the error probability model is derived.Moreover,an OAM mode power allocation optimization algorithm is designed.Analytical results indicate that,compared to the error probability of the OAM-MIMO wireless communication system adopting the water-filling algorithm,the error probability of the OAM-MIMO wireless communication system adopting the proposed OAM mode power allocation optimization algorithm is reduced up to 13.0 d B when the signal-to-noise ratio(SNR)is 15 d B.Fourthly,the effect of Kolmogorov turbulence on the bit error rate(BER)and capacity of OAM-based orthogonal frequency division multiplexing(OAM-OFDM)wireless communication systems is investigated.Utilizing the OAM mode and frequency division multiplexing,the OAM technology can be combined with the OFDM technology as the OAM-OFDM wireless communication system model to enhance the capacity of transmission systems.Considering the effect of Kolmogorov turbulence on OAM and OFDM signals,the effect of Kolmogorov turbulence on the performance of OAM-OFDM wireless communication systems is analyzed.A purity model of the OAM-OFDM wireless communication system is derived to investigate the effect of Kolmogorov turbulence on the phase of OAM-OFDM signals.Furthermore,combining the effect of Kolmogorov turbulence on the intensity of OAM-OFDM signals,a BER model of the OAM-OFDM wireless communication system is derived.Finally,based on the proposed BER model,a capacity model of the OAM-OFDM wireless communication system in Kolmogorov turbulence is derived.Analytical results show that the proposed OAM-OFDM wireless communication system achieves a capacity improvement over the conventional OAM wireless communication system and the average improvement is 751%.In summary,this dissertation makes innovative studies on OAM multiplexing,performance analysis and optimization of OAM wireless communication systems with unaligned antennas in atmospheric turbulence.The studies of this dissertation provide guidelines for practical deployment of the OAM technique in wireless communication systems.