Metasurfaces And Their Applications For Isolation Enhancement In MIMO Antennas

Multiple-input multiple-output(MIMO)antenna system is of considerable significance in the modern wireless communication system since it has the potential for improving the channel capacity and increasing the data throughput.The trend of miniaturization and high integration becomes much more popular in the communication devices,leading to the fact that the distance between MIMO antennas has to be reduced in some application scenarios.However,the closer the antenna elements are arranged in the limited space,the stronger the mutual coupling effect will be produced in the antenna array,which results in the poor MIMO antenna performance.Therefore,it is important that mutual coupling reduction is realized in the MIMO antennas.Besides,according to some researches metasurfaces(MSs)can be used to enhance the antenna performance.They have some special characteristics,such as polarization conversion as well as surface-wave suppression and cancellation.Thus,applying MSs in the MIMO decoupling design can achieve the isolation enhancement between antenna elements.The dissertation primarily focuses on the topic of applying MSs in the MIMO antenna,with the mutual coupling between antennas been reduced in each design.The principle of MS decoupling and its corresponded achievable method are discussed in the dissertation.Polarization conversion and surface-wave suppression effect are used to reduce the mutual coupling between antennas.The main content is stated as follows:First,decoupling method of polarization conversion is proposed.With the help of the characteristic of polarization conversion provided by rectangular rings,the polarization state of the non-excited antenna is changed to its perpendicular direction compared to the antenna without MSs by changing the operation mode on it.The simulated results show that the polarization conversion efficiency is about 100%at 5.8GHz.Then the decoupling structure regarding loading three rectangular rings between two-element E-plane arrangement MIMO antennas is designed.The distance between antennas is 0.5 λ0,with the result of mutual coupling reduction of 32dB has been realized.Besides,envelope correlation coefficient(ECC)is lower than 0.001 at 5.8GHz.The prototype of the designed antenna is manufactured and tested.The measured and simulated results regarding the antenna performan ce are almost the same,which shows the effectiveness of the proposed decoupling method.Second,to decrease the distance between antennas,the decoupling unit is changed to capacitively-loaded-loop spiral-shaped split rectangular ring(CLL)and then they are loaded between the E-plane arrangement MIMO antennas.The distance between antennas is decreased from 0.5λ0 to 0.34 λ0.The substrate thickness is decreased from 3mm to 1.6mm.With the suppression effect in surface wave coupling provided by the decoupling structure,the most part of coupling wave is concentrated on the excited antenna,leading to the reduction of mutual coupling between antennas.The simulation results present that the isolation between antennas is improved by 24dB at 5.7GHz.The ECC is lower than 0.005 from 5.58GHz to 5.79GHz.The gain of the designed antenna with and without decoupling structure in E-/Hplane is 5.4/7.3dBi,respectively.The simulated radiation patterns do not have noticeable variation after loading the decoupling units.Last,to expand the application scenario of the H-plane arrangement MIMO antennas,the decoupling unit is changed to split ring resonator(SRR).The distance between antennas is 0.42λ0 and the thickness of substrate is 1.6mm.With the suppression effect in surface wave coupling provided by the decoupling unit,the cancellation effect between the original coupling waves and the coupling waves produced by the decoupling units is made,leading to the suppression of mutual coupling between antennas.The simulation results show that the maximum isolation enhancement is achieved by 34dB at 5.3 GHz and the calculated ECC satisfies the design requirement.The gain of the designed antenna with and without decoupling structure in E-/H-plane is 5.39/6.63dBi,respectively.The simulated radiation patterns do not have noticeable variation after loading the decoupling units.