Orbital Angular Momentum Based Wireless Transmission System Modeling And Performance Analysis

The electromagnetic waves carrying orbital angular momentum(OAM)have attract much attention recently due to the envisioned abilities in radio communication.As a basic electro-dynamics quantity,OAM provides an extra degree of freedom to modulate information at the physical layer,which can be utilized to enhance the system spectral efficiency based on the inter-modal orthogonality.Considering the characteristics of OAM radio,how to make full use of the under-utilized OAM property and obtain the achievable benefits are of great importance.This dissertation mainly focus on the modeling and performance analysis of the OAM based wireless communication system.The contents of this work can be listed as follows.An uniform circular array(UCA)based OAM multiplexing communication system is mod-eled and analyzed.As an effective and flexible method to generate OAM radio,UCA plays an important role in OAM multiplexing,however,the same configuration can also work in conven-tional MIMO mode,we aim to make a thorough comparison between the two communication systems and then look into whether OAM could bring some insights or benefits than MIMO.A single UCA at the transmitter side is invoked to radiate concurrent OAM modes with linear superposition of excitations,meanwhile,at the receiver side signals are detected by DFT trans-form or base on MMSE criterion.Use system's sum-rate as a metric,our results reveal that the two communication systems are identical when full channel-state-information(CSI)at the transmitter and the parallel UCAs comprise the same number of elements.The advantage of OAM-based transmission is evident when the UCAs are parallel and no CSI at the transmitter,since it approach the maximum sum-rate the channel itself achievable,otherwise the sum-rate decays severely.Considering the inter-modal orthogonality,in the second part of this dissertation,we intend to exploit this property to isolate the self-interference existing in current full duplex system,and such an OAM based system is modeled.OAM beams with different modes work as the carries of the bidirectional transmission,the OAM antennas are assumed to be placed on a common axis with a tilt angle that induces misaligned reception,a common case in practice.The beam usually contains parasitic harmonic components due to the imperfect generation,and the component,with mode the same as the local receiving mode value,remains as residual interference.Based on this model,we analyze the OAM eigenmode pair for the bidirectional transmission that maximize system sum rate under different mode purities and noise intensities.Results show that the optimal modes have extreme values when self-interference dominate noise or vice verse,and the modes vary between the extreme values gradually when both interference and noise exist.Finally,we build and analyze a communication system that combines OAM and MIMO.The incorporation of OAM into MIMO increases the available dimension when compared to pure MIMO,thus by multiplexing multiple OAM modes in each branch of MIMO can poten-tially improve the system spectral efficiency further.Meanwhile,the received power will decay severer on account of the fast divergence of OAM beam,based on which we build a general OAM-MIMO model to analyze the potential benefits the incorporation could provide under the combined effects of the two contradictory factors.The results reveal that,once the antenna cov-ers the root mean square radius of beam,the increase of multiplexing dimension indeed improves the system sum rate,while at large distance,due to power decay,the sum rate of OAM-MIMO will below that of MIMO.