Researches On Isolation Improvement Of Mimo Antenna Based On The Theory Of Characteristic Mode

MIMO antenna is one of the important components of the fifth generation mobile communication system,and how to reduce the electromagnetic coupling between MIMO antenna elements in a limited space has become a widespread and necessary research topic.Traditional design methods for coupling reduction mostly rely on past experience and optimization of parameters,so the Theory of Characteristic Mode(TCM)with wide application potential and higher computational efficiency is proposed to solve the electromagnetic coupling problem.In this thesis,four designs of planar MIMO antenna based on the Theory of Characteristic Mode are proposed,using FR4 substrates with the size no more than 60 mm × 120 mm.The characteristic mode analysis is used to determine the type and the placement of antenna elements and decoupling structure in a more systematic way to achieve the goal of improving the isolation of MIMO antennas.The main research of this thesis is as follows.Based on the Theory of Characteristic Mode,the characteristic mode analysis of the antenna element and the ground plane is carried out using commercial electromagnetic simulation software CST Studio Suite.When the first two most relevant characteristic modes of the designed antenna have similar resonant frequencies with the characteristic mode of the ground plane,these two elements are placed according to the near-field analysis results of the ground plane.Therefore,the orthogonal characteristic modes of the metal ground plane can be selectively excited to achieve the effect of isolation improvement,and the 2-element MIMO antenna design is completed.According to the characteristic mode analysis,the second antenna element with similar resonant frequency to the characteristic mode of the ground plane is designed,and these two different elements are used as electric antenna and magnetic antenna to excite the mutual-orthogonal characteristic mode of the ground plane,respectively.According to the comparison of the 3D radiation patterns of the antenna elements and that of the ground plane,the correspondence between the characteristic mode of the antenna elements and the characteristic mode of the excited ground plane can be determined.Finally,an obvious isolation enhancement effect is achieved,and the design of the 2-element MIMO antenna composed of different array elements is completed.The current patterns of the characteristic modes are applied for the decoupling of the4-element antenna design.The characteristic currents of the designed MIMO antenna are compared with the surface current when the excitation source is present to distinguish the coupled and uncoupled characteristic modes of the MIMO antenna.Afterwards,simple defective ground structures are placed at specific locations on the substrate to reduce the influence of the coupled modes without significantly affecting the uncoupled modes.The adopted method has a significant decoupling effect on 4-element MIMO antennas composed of same elements or different elements.