Research And Application Of The Theory Of Characteristic Modes In High Isolation MIMO Antenna Design

In the past decade,with the rapid development of mobile phones and other mobile terminals,people’s demands are becoming more and more diversified.It is urgent to improve the channel capacity and spectrum efficiency of mobile communication.However,mobile terminals are becoming more and more integrated,which will result the mutual coupling between MIMO units getting worse.The coupling will greatly affect the performance of the antenna.Therefore,it is a challenging task to design MIMO antennas with high isolation degree in limited space.In recent years,the combination of Theory of Characteristic Mode(TCM)and antenna design has become a research hot spot.TCM has a clear physical concept,which can essentially explain the radiation mechanism of the antenna and can facilitate the guidance of the antenna,such as offering valuable information for antenna design like correct resonance frequencies and right feed points.In this paper,the main contents can be summarized as follows:1.The development and application of TCM are briefly introduced,the related parameters of MIMO antenna are introduced as well,laying a foundation for the following research.The generalized characteristic mode theory of Perfect Electric Conductor(PEC)is derived from the Electric Field Integral Equation(EFIE).What’s more,the orthogonality of characteristic fields over the radiation sphere at infinity and characteristic current is verified.Finally,the relevant characteristic parameters are deduced and the physical meanings of these parameters is introduced.2.TCM is applied to the design of two dual-band MIMO antennas with high isolation degree.A properly designed metal neutralization line(NL)is loaded,reducing the mutual coupling between MIMO units.TCM analysis on the initial antenna structure is performed and the significant modes of the antenna are identified.Then,according to the direction of the current flow of significant characteristic modes,coupling and uncoupling modes are identified.The shape of NL is determined qualitatively based on the characteristic currents of coupling modes,and then,HFSS is used to optimize the key parameters of NL and determine it quantitatively.Compared with the traditional optimization design method,the proposed TCM based design procedure can save lots of optimal design time.Experimental results show that 20 d B of isolation degree between the ports is improved and the measured and simulated results are almost the same.3.Two mobile phone antennas are designed basing on the TCM,and the antennas achieve high isolation degree without additional decoupling structure.The traditional way to design high isolation MIMO antennas is to employ TCM to analyze the antenna structure and make different MIMO ports to excite different modes,which can make the isolation degree higher.However,this method is often difficult to implement when multiple modes are excited at the same time.The same modes excited by the ports can be divided into in-phase mode and out-ofphase mode.When the excitation degree of the two modes reaches a certain requirement,the antenna can achieve a good isolation degree.A two-port MIMO antenna was designed to achieve 30 d B isolation degree based on the proposed method.Finally,basing the orthogonality of the characteristic current and the characteristic field at infinity,another antenna unit is added to the two-port MIMO antenna and the isolation degree among the three MIMO ports is high.