| The new generation mobile communication system has delineated multi-standard frequency bands and puts forward higher requirements for data throughput.Multiple-input multiple-output(MIMO)technique enables the channel capacity of wireless communication systems to be increased by taking full advantage of the spatial dimension.As a vital component in the MIMO system,the amount of antennas and the isolation between antennas in the MIMO system are significant factors influencing the channel capacity.However,the demand for portability of terminal devices and the application of full-screen make the space for antenna design inside the devices strictly limited.Tightly arranging multiple MIMO antenna elements in a limited design space can result in strong electromagnetic coupling,deteriorating the isolation between the elements and further reducing the channel capacity of the MIMO system.Therefore,in the compact terminal space,exploring efficient decoupling techniques to enhance the amount of MIMO antennas and achieving high isolation of MIMO antennas in multi-band and wide-band have become the current research emphasis and challenge in the field of terminal antennas.In addition,terminal devices generally work near human tissues,and long-term close proximity to human tissues will cause excessive electromagnetic radiation hazards to human tissues.To avoid the harmful effects of electromagnetic radiation on humans,the industry has introduced the specific absorption rate(SAR)to define the terminal electromagnetic safety limits.How to realize the low SAR design of the termination antenna while maintaining the radiated performance of the antenna has received widespread attention from both industry and academia.Therefore,this dissertation conducts research on the decoupling method of MIMO antenna array and low SAR antenna design in combination with the developing requirement of mobile communicating techniques.The study in this dissertation consists of:1.Aiming at the requirements of terminal equipment to increase the number of highly isolated MIMO antennas,a new multi-antenna self-decoupling method is presented by combining the orthogonal mode technique and the pattern diversity technique.Based on the proposed decoupling method,a closely spaced 4-port antenna group consisting of two highisolation antenna pairs is designed.By introducing the shielding effect of the terminal metal ground,good pattern diversity between two antenna pairs and self-decoupling of multiple antenna elements can be achieved.The isolation of the proposed 4-port antenna group is greater than 15 d B in the working frequency band without adding any additional decoupling structure.In addition,the proposed 4-port antenna group is used to design a 16-element MIMO antenna.Both measurement and simulation results demonstrate that the 16-element MIMO array has excellent antenna radiating properties,and the channel capacity of the proposed MIMO antenna is remarkably promoted compared with that of the general 8×8MIMO antenna system.The proposed self-decoupling method can realize the compact arrangement of multiple antenna elements on the basis of ensuring high isolation and good pattern diversity performance,which has well application perspective in the antenna design of 5G mobile terminals.2.For the demand of multi-band and wideband decoupling of terminal MIMO antenna,this dissertation provides a broadband self-isolation MIMO antenna element based on the principle of multi-adjacent modes coupling current cancellation for 5G mobile terminals.The coupling currents of the adjacent half-wavelength loop modes at the non-exciting ports are anti-phase and cancel,which can weaken the electromagnetic couplings between neighbouring antenna elements.With the combining function of five adjacent halfwavelength loop modes,the proposed antenna element can obtain broadband and selfisolation properties.The wideband high-isolation design of MIMO array can be realized by tightly arranging the proposed antenna elements.The experimental results indicate that the-6 d B in bandwidth of 8-element MIMO array ranges from 3.3 to 3.8 GHz and 4.4 to 5.0GHz,and without adding decoupling structure,the antenna array has a port isolation of more than 15 d B in both working frequency band.The proposed broadband self-isolation design method based on multi-adjacent modes coupling current cancellation can provide a new idea for the design of terminal self-isolation MIMO antenna.3.Aiming at the issue of low SAR for terminal antenna design,this dissertation conducts research on SAR reduction technique for antenna elements based on field regulation.Firstly,from the perspective of electric field regulation,a SAR reduction method based on characteristic mode theory is proposed.In this dissertation,the relationship between the induced electric field in human tissue and the antenna radiated electric field is established basing on the characteristic mode theory,and the low-SAR antenna design procedure and the methodology are investigated.The experimental results indicated a 26.1% reduction in the 10-g spatial average SAR peak for the low-SAR loop antenna,and its-10 d B reflection coefficient bandwidth is 320 MHz,which is 60% higher than that of the reference antenna(200 MHz).In addition,the measured efficiencies of the low-SAR antenna is better than 73%in the entire working frequency band.Moreover,the terminal antenna SAR reduction method basing on magnetic field control is studied.At first,the relationship for the SAR value of the antenna with the magnetic field is investigated based on the properties of the time-varying electromagnetic field.Then,combined with Maxwell equations and medium boundary conditions,the interaction of terminal antenna surface current,radiated magnetic field,and induced magnetic field in human tissue is explored.Finally,two low-SAR terminal antennas based on magnetic field regulation are proposed.Experimental results demonstrate that the SAR peaks of both proposed antennas are attenuated by more than 30% in comparison with the reference antenna.4.To address the issue of multi-antenna simultaneous SAR design,this dissertation put forward a multi-antenna simultaneous SAR value reduction method based on multi-antenna main excitation mode current modulation.First,the relationship between the multi-antenna surface current and the multi-antenna characteristic mode current and its mode weight coefficient is established.Then,the relationship between the multi-antenna main excitation mode current distribution,the antenna surface current distribution and the multi-antenna simultaneous SAR is studied.Finally,by reasonably designing the placement of the multiantenna elements on the ground,the uniform multi-antenna characteristic modes are effectively excited,thereby realizing the uniform distribution of the multi-antenna surface current and the reduction of the multi-antenna simultaneous SAR peak.The proposed multiantenna simultaneous SAR reduction method realizes the low SAR terminal multi-antenna design by optimizing the layout of antenna elements,and has obvious SAR reduction effect. |
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