Studies On Key Technologies And Characteristic Mode Analysis Methods For 5G Base Station Antennas

With the rapid development of the fifth generation(5G)mobile communication system,massive Multiple-Input and Multiple-Output(Massive-MIMO)antenna arrays have been a core technology in 5G base station antennas for the requirements of point-to-multipoint(P2MP)application,low latency and high channel capacity.Due to extremely limited radiation aperture of base station towers,the 5G antenna arrays are expected to be intergrated into original 2G/3G/LTE antenna arrays to realize dual-band or multi-band operation properties.However,as the 5G antenna arrays are composed by massive of antenna elements,dual-band or multi-band base station antenna designs which contain 5G antenna arrays still face many challenges.According to the requirements of research projects,this work focuses on the dual-band base station antenna designs based on frequency selective surface(FSS)and the implementation of characteristic mode(CM)analysis in practical base station antenna arrays.The thesis mainly includes two parts.The first part is the dual-band base station antenna design based on FSS.It covers the design of a novel dual-band shared-aperture structure,a dual-band MIMO antenna array and an innovative kind of FSS with high transmission rate.The second part is the researches of the CM analysis methods for practical base station antenna with complex structures.The major work and achievements of this thesis are summarized as follows:1.A novel dual-band shared-aperture antenna topology based on FSS is proposed.A small-scale dual-band antenna array propotype is fabricated and measured to show the novel features of the proposed topology.In this topology,5G band antenna array(high band,HB)is placed upon the FSS while 2G/LTE band antenna element(low band,LB)is set betwenn the FSS and the ground.Owing to the reflection property of the FSS for HB electromagnetic(EM)waves,this dual-band configuration isolates the HB EM waves from the LB antenna.Therefore,a remarkable cross-band mutual-coupling reduction between the LB and HB antennas is achieved.The proposed topology also alleviates the limitation on placement of the HB antenna elements,which is essential for integrating a 2-D HB antenna array with the LB linear antenna array.Moreover,due to the capacitve loading effect of the FSS,the low-profile and broadband characterisitcs are achieved for the LB antenna,which leads to a compact dual-band antenna array design.2.Three additional decouple methods for large scale dual-band MIMO antenna array are proposed.Due to the HB and LB antennas are stacked in the proposed FSS based dual-band topology,the large-scale coaxial cables of the HB antenn array are tighly coupled with the LB antenna.Meanwhile,in-band mutual coupling in the HB antenna array needs to be further suppressed as well.In order to reduce the in-band and cross-band mutual coupling in large scale MIMO antenna array,three novel decoupling methods are proposed in this work simultaneously.Unequal width rectangular ring resonator FSS,distribute baffle structure,and helical torsion feeding cables are proposed to resolve the 2-D rectangular lattice of the HB antenna array,the in-band mutual coupling among the HB antnen array and the cross-band mutual coupling between the LB antenn and the coaxial cables of the HB antennas,respectively.Owing to these three additional decoupling methods,the FSS based dual-band topology is successfully implemented into dual-band large scale MIMO antenna array.A 5G MIMO antenna array with 32 ports(3.3-5.0 GHz)are integrated with a 2G/LTE antenna element with 2 ports(0.69-0.96 GHz)in the size of 250 mm× 344 mm× 73 mm.3.A novel grid frequency selecetive surface(GFSS)with high transmission rate is proposed in this work.The advantage of the proposed GFSS design method is that the low-stop and high-pass properties are simultaneously achieved on a single layer substrate.Owing to the perfect reflection property of the GFSS for 3G/LTE band(mid band,MB)antenna,the cross-band mutual coupling between 5G band(high band,HB)antennas and MB antenna is mitigated.Meanwhile,the GFSS possesses high transmission rate at the near-field rigion of the HB antenna,which maintains the radiation performance of the HB antennas and avoids a second-round optimizations for the HB antenna array.The relationships between the structures of the GFSS and the equivalent circuit model(ECM)are clear.The resonant behavior at low-stop and high-pass bands could be tuned independently according to the circuit analysis.Meanwhile,the scattered field analysis is also introduced for the first time,which help us understand the reflection/transmission principle of the GFSS from an intuitive view.By comparing the scattering properties of the GFSS and the traditional FSS,the transmission mechanisms for different FSSs are clearly illustrated.The influence of the traditional FSS to practical antenna array is also demonstrated by the scattered field analysis.Based on the GFSS,a dual-band base station antenna array operating at 1.7-2.4GHz and 3.3-3.8 GHz is fabricated for the first time.Both the simulation and the measurement results illustrate that the GFSS achieves remarkable decoupling effect between 3G/LTE and 5G band antennas.Meanwhile,it has no influence on radiation performance of the 5G band antenna.Furthermore,the proposed method is versatile to be applied in different scenarios.A GFSS with out-of-band radiation suppression property is designed.The simulation results shows the influence of the designed GFSS on realized gain of antenna array is less than 1 d B in the operation band,while the suppression of the out-of-band radiation is more than 11 d B.4.The implementation of CM theory in practical base station antenna with complex structures is studied.In 2001,substructure characteristic mode(SCM)and target coupling characteristic mode(TCCM)were firstly proposed by the B.D.Raines in his Ph D.dissertation.In this work,these two characteristic mode theories are introduced in the analysis of base station antennas.The working principles of the low-profile antenna and the decoupling mechanism of distribute baffle structure are demonstrated from the view of modal analysis.Speciffically,the FSS loaded antenna design method is firstly analyzed through SCM theory.The influence of the FSS on the radiation modes of the feeding strcutre is calculated.Meanwhile,the enhancement effect on radiation mode of the FSS due to the feeding strcture is also demonstrated.Therefore,the low-profile working mechinasm is illustrated by the SCM analysis.Then,TCCM is applied in in-band mutual coupling reduction design.The mapping relationships between radiation modes and coupling modes are demonstrated by TCCM analysis,which help us find the way to suppresse the unwanted mode.Then,the decoupling effect is revealed by examining the suppression effect of the unwanted radiation mode by the distribute baffle structure through the SCM analysis.