Research On Transmission Technology Of Orbital Angular Momentum Wave Based On Uniform Circular Array

With the rise of new Internet of Things(Io T)technologies such as smart transportation and smart home,and the continuous growth of emerging multimedia services such as live broadcasting and virtual reality(VR),wireless communication requires higher and higher data rates to support these applications.To meet this requirement,more and more highfrequency bands,such as mm Wave and THz bands,are licensed for use in wireless systems for 5G and beyond.In addition to using more frequency bandwidth,people also explore such as multiple-input Multiple Output(MIMO),Cognitive Radio,CR),and some advanced coding methods and modulation techniques to improve the utilization of spectrum resources.In essence,however,these techniques are based on plane waves,i.e.,electromagnetic waves whose equiphasic surface is flat.Since 1992,scientists have experimentally verified that light waves with a helical wavefront can carry Orbital Angular Momentum(OAM),OAMbased wireless communication technology has been considered as a new method to achieve high spectral efficiency by multiplexing a set of orthogonal OAM modalities on the same frequency and channel.Due to the widespread application of antenna arrays in 4G and 5G,the use of uniform circular arrays(UCA)to generate OAM beams has been widely studied and applied.The highest data rate of the OAM communication system implemented by Nippon Telegraph and Telephone Corporation(NTT)using concentric UCA can reach200 Gbps.Based on this,the wireless transmission technology of OAM based on uniform circular arrays in the line of sight(Lo S)channel is studied,including multi-mode nonorthogonal OAM beam multiplexing technology and a low-complexity downlink multi-user OAM communication scheme.The specific research contents are as follows:(1)Orbital angular momentum wireless communication technology can multiplex multiple OAM beams of different modes in the same frequency band,time or space,thereby increasing channel capacity and solving the problem of increasing scarcity of spectrum resources.However,traditional orthogonal multi-mode OAM multiplexing systems usually have poor bit error rate(BER)performance due to the inability to perform water-filling power allocation and adaptive modulation for each sub-channel like closed-loop MIMO systems.In particular,the divergence Angle of the OAM beam with the higher-order mode is larger,resulting in a lower Signal to Noise Ratio(SNR)at receiving UCA with a fixed aperture.Therefore,a non-orthogonal OAM(NO-OAM)multimode multiplexing scheme based on uniform circular arrays is proposed in this paper,which can adjust the divergence angles of all non-orthogonal OAM beams to be consistent to avoid the above problems.The results of mathematical analysis and numerical simulation show that the NO-OAM is better than the existing Uniform Concentric Circular Array(UCCA)beam adjustment scheme,and only one UCA is needed to achieve the beam adjustment.In addition,compared with traditional orthogonal OAM multimode multiplexing,the NO-OAM multimode multiplexing scheme proposed in this paper has a lower bit error rate and asymptotically equivalent channel capacity.(2)Since the UCA-based orbital angular momentum communication system requires precise alignment between the receiver and the transmitter to avoid inter-modal interference,the research on OAM communication mostly focuses on the single-user point-to-point scenario.The research on multi-user OAM wireless communication is relatively less.In this paper,a downlink multi-user OAM(MU-OAM)communication scheme based on uniform circular arrays is proposed.By using the precoding matrix constructed by the mu-OAM channel decomposition method designed in this paper,inter-mode interference and inter-user interference can be eliminated at the transmitting end,thus simplifying the signal detection process at each receiving end to amplitude detection.Both mathematical analysis and simulation results verify that the bit error rate and reachability rate performance of the proposed MU-OAM communication scheme are close to the traditional multi-user MIMO(MU-MIMO)scheme,but the computational complexity can be significantly reduced.