| With the advent of Fifth-Generation(5G),to meet the various requirements,smartphones need to provide more diversified services.Therefore,it is urgent to enhance channel capacity of smartphones.To enhance channel capacity,Multiple-input multiple-output(MIMO)technology is a promising option.MIMO technology can improve not only channel capacity but also spectrum efficiency.Accordingly,the Sub-6 GHz MIMO array antenna design for 5G mobile communication is getting a lot of attention.To achieve various functions,a lot of antennas have been integrated in a smartphone.Therefore,the available space for 5G MIMO array is very limited.Due to the limited space,severe coupling between MIMO elements is inevitable.Although loading the external decoupling structures can improve isolation effectively,the employment of the external decoupling structures will increase complexity of the MIMO system and occupy extra spaces.Accordingly,it is still a challenging task to realize desirable isolation without any external decoupling structure within a compact size.This dissertation focuses on improving the isolation and ECC of the MIMO array and reducing the size of the MIMO system without loading any external decoupling structure.In this dissertation,the main works can be summarized as follows: 1.An eight-antenna MIMO array based of the theory of characteristic mode(TCM)is proposed.Recently,the TCM is widely applied to design MIMO array antennas.In the traditional method,the TCM is employed to analyze the structure and find the optimal placement of multi-antenna so that the modes excited by the different antennas are completely different.However,this method is not effective for multi-mode structures.When many same modes are excited by two antennas,a new method is proposed to improve the Envelope Correlation Coefficient(ECC)of the two antennas.According to the theoretical analysis,the same modes excited by two antennas can be divided into two types: equiphase modes and antiphase modes,and the tradeoff of the equiphase modes and the antiphase modes is first proposed and analyzed as an efficient way to improve the ECC.An eight-antenna MIMO array antenna is proposed,and the new method is employed to improve ECC.Finally,the isolation of the proposed MIMO system is higher than 15 d B and the ECC is lower than 0.16.2.Two compact building blocks with two shared-aperture antennas are proposed.The first building block is composed of a coupled feed slot antenna and a loop antenna,and the two antennas are overlapped completely.Compared with conventional arrangement(loop antenna and slot antenna separately disposed,not overlapped),the length reduction of the proposed building block is more than 50%.Based on the first building block,the second building block with higher isolation is proposed,which is easier to fabricate.The second building block is composed of a coupled feed slot antenna and a coupled feed loop antenna.The isolation of the building block is better than 30 d B.Four proposed building blocks are utilized to constitute an eight-element MIMO array antenna.The designed eight-antenna MIMO array can yield desirable isolation of better than 18.5 d B,and the ECC is lower than 0.02.3.A high-isolation building block using stable current nulls existed in the standing wave region is proposed.Here,a two-antenna building block that is composed of a gap-coupled loop antenna and a loop antenna is meticulously designed to allow Ant 1 to excite a standing-wave region that can be formed on the structure of loop antenna.Then,the feed point of the loop antenna is designed to fall at the stable current mull.Under this circumstance,very slight current following to the feeding port of the loop antenna,when the gap-coupled loop antenna works.Therefore,the isolation of the building block is higher than 26 d B over the band of interest.By employing the proposed building blocks,a four-antenna MIMO array,an eight-antenna MIMO array and a ten-antenna MIMO array can be implemented.The isolations of the four-antenna,eight-antenna and ten-antenna MIMO arrays are higher than 23 d B,17.9 d B and 14.4 d B,respectively.Furthermore,desirable ECCs of lower than 0.032 and 0.075 of the proposed four-antenna and eight-antenna MIMO arrays were also calculated,respectively.Therefore,the design method in this dissertation is suitable for multi-antenna systems applications and the proposed four-antenna,eight-antenna and ten-antenna MIMO arrays are very promising for future 5G smartphone applications.4.Two eight-antenna MIMO arrays with unbroken metallic frames are proposed.It is still a challenging target to realize desirable performance for a multi-antenna MIMO array with an unbroken metallic frame.To obtain high isolation,the first MIMO array employs three types of antennas.Although the external decoupling structure is not utilized,the isolation of the eight-antenna MIMO array is better than 17 d B.Under the consideration of fabrication,the second MIMO array with an unbroken metallic frame is proposed.To realize high isolation,two types of antennas are utilized to constitute the eight-antenna MIMO array.The second MIMO array also realizes desirable measured isolation of higher than 20 d B without any external decoupling structure and the ECC is lower than 0.05.The proposed two MIMO arrays with good performances possess great potential for 5G smartphone with an unbroken metallic frame. |
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