| Multiple Input Multiple Output(MIMO)communication systems generally require multiple antennas to transmit and receive together for higher transmission rates,larger transmission capacity,and more reliable information exchange.However,the multi-antenna system has coupling problems in space,especially on handheld mobile devices.Due to space constraints,the distance between each unit is small,and the same ground makes the unit more susceptible to coupling interference and reduces the efficiency of the unit.Therefore,how to reduce the coupling between multi-unit becomes a key problem to be considered in the 5G mobile phone MIMO antenna system.Advanced decoupling technology research has great significance for the development of 5G communication.At present,there is much research on the corresponding decoupling methods,but there is still room for further improvement from the decoupling effect.For this reason,according to the current development trend of 5G mobile phone MIMO antenna system and the existing decoupling technology problems,this thesis conducts the decoupling technology research of MIMO antenna system,the specific contents are as follows:Firstly,the decoupling method loaded with lossy lumps not only reduces the efficiency but also changes the structure of the unit to degrade the coupling.A magnetic material ferrite film loaded on the surface of the mobile phone MIMO antenna is proposed,which achieves decoupling at a 1.4mm distance while protecting the integrity of the original unit.By selectively loading the ferrite film on the dual-IFA-unit,it is found that the characteristics of the ferrite films loaded under the action of an alternating electromagnetic field on the antenna surface have a significant effect on the antenna coupling.Afterward,the working characteristics of the ferrite film with imaginary permeability under the alternating magnetic field and its influence on the antenna selfimpedance and mutual-impedance are analyzed in detail,and the effect of the ferrite film on the antenna is equivalent to that of the lossy component circuit.By establishing a clear physical model,the theoretical basis for loading ferrite film decoupling method to replace traditional lossy element decoupling is provided.The results show that the ferrite film loaded on the grounded branch of the IFA unit can increase isolation from-6.64 d B to-10.43 d B and enhance the bandwidth from 3.20–3.88 GHz to 2.83–3.65 GHz,and its efficiency can reach 45.5%,as well as a 2d B gain.This decoupling method has the potential to replace the traditional loading lossy lumps decoupling method.Secondly,a similar π-shaped self-decoupling structure is proposed to solve the problems of high profile,large size,and ambiguous decoupling frequency adjustment in the current shared radiator decoupling structure.By using the mode conversion of the self-radiating current of the π-shaped shared radiator from the spherical monopole radiation mode to the hemispherical dipole radiation mode,a low-coupling eightelement antenna system is suitable for the current ultra-thin 5G mobile phone is implemented,with the profile only 3.7mm.The proposed decoupling structure can adjust the decoupling frequency through several key parameters in the π-shaped feeding branch.The experimental results show that dual-unit can effectively cover the LTE Band 42 band,with isolation from-16.08 d B to-11.04 d B and efficiency from 80 to 94.1%.The 8×8 MIMO mobile phone antenna isolation is-20.1d B to-12.17 d B,efficiency is 79.72–93.7%,and the ECC is below 0.05,which is very suitable for ultrathin smartphones.Finally,given the low isolation effect of the decoupling methods currently proposed,a novel and efficient decoupling method,the phase-decoupler method,with isolation less than-28 d B,is proposed for the first time by using the frame structure.Using this phase decoupler in the MIMO antenna,the additional current path reverses the phase of the coupled current flowing through the floor to the adjacent unit,weakens the coupling current intensity,and has a significant regulatory effect on the coupling.Selecting a mobile phone frame-antenna and using the phase decoupler,the results show that without adding any lossy elements,only a very small area can be used to achieve efficient decoupling,and the maximum efficiency can reach 85.53%.an In8×8 MIMO antenna system,the coupling can be reduced below-24.6d B with a maximum efficiency of 82.17%.This decoupling method has important application value in the future decoupling of mobile phone frame-antenna with border. |
