| The internet of things(IoT)has gained unprecedented development opportunities since it was elevated to the height of national strategy in 2010.As one of the most important IoT techniques,radio-frequency-identification(RFID)technology has been widely used in biology,transportation,logistics,military,and other fields due to its efficient reading/writing ability,excellent environmental adaptability,and low cost.Among them,RFID technology working in UHF and MW band becomes a new research trend with better read/write rate and compact antenna structure.IoT proposes new RFID performance requirement of multi-band,fullpolarization,miniaturization,and anti-interference.RFID has kinds of application direction with different operating bands.For example,the IoT intelligent factory needs both active and passive RFID technique for logistics,warehousing,automatic management,real-time positioning and other scenarios.The miniaturized multi-frequency antenna integrating multiple communication standards can well suit the diversified application requirements of RFID antenna functions.MIMO technology can enhance the RFID system’s stability thanks to its spatial multiplexing and spatial diversity.The CP antenna has a higher recognition rate for labels placed in any direction and can better suppress the multipath effect compared with linear polarized(LP)antennas.Therefore,the CP MIMO-RFID antenna,which can work in multiple frequency bands at the same time,has obtained extensive attention and research in the field of scientific research due to its valuable research value and development potential.The main research contents are listed as follows:Firstly,the application of MIMO technology in RFID system is analyzed,and it is found that MIMO technology can effectively improve the system SNR,reduce the system bit error rate and improve the system read and write range.When the number of RFID reader antennas increases from one to two or three,the performance of the RFID system’s BER and identification range is improved most obviously.Then,this section proved that,compared with LP MIMO antenna,the mean channel capacity of CP MIMO antenna with good AR performance(AR<3d B when deflection Angle θ>40?)increases by 2.5B/s/Hz,and the ECC of CP MIMO antenna is much lower.When the deflection Angle changes continuously,the CP MIMO antenna has more stable received power.The above research results provide a clear design direction for the multi-band CP MIMO antenna,that is,to design the dual-cell CP MIMO antenna with high isolation,low ECC and miniaturization.Secondly,the traditional CP MIMO antenna usually requires feed network and multi-antenna element structure to achieve excellent port isolation and AR performance,resulting in complex and large antenna structure.The existing RFID reader antenna needs to adopt a double antenna structure or a single antenna equipped with an annular device(coupler)to realize the transceiver duplex function,which increases the system complexity and design cost.To solve this problem,this paper proposes unequal-gap shared-aperture technology and designs single-frequency(2.45 GHz)dual-port dual-CP MIMO antenna for RFID system.Under the compact structure,the high isolation,high gain,dua-port and dual-CP radiation mode is realized.The antenna unit can realize the transceiver duplex function of RFID reader,and the dual antenna unit can be applied to MIMO system by polarization diversity and spatial diversity technique.The feasibility of applying the unequal-gap shared-aperture technology to CP MIMO-RFID antenna system is verified.Thirdly,considering that the current multi-band CP MIMO antenna is difficult to obtain both good isolation and structure miniaturization,this paper adopts the shared radiation element technology and the shared floor gap coupling technology.Two dual-band CP MIMO antennas with different application directions in UHF/MW band(0.915/2.45 GHz)and MW band(2.45/5.8 GHz)are proposed.The effect of mutual coupling between ports is reduced by loading step impedance microstrip filter and defected ground structure(DGS).Through theoretical analysis,simulation,and experimental verification,the two antenna schemes realize dual-band CP mode ensuring the high isolation,and their compact dual-band four-port structure can meet the requirements of RFID system for antenna size and performance.Finally,based on the current multi-band MIMO antenna scheme only limited to the dual-band CP mode,this paper proposed the phase delay feed network combining with shared-floor antenna aperture coupled feeding technology.Step impedance of microstrip filter with DGS technology improve isolation between antenna units.The design of tri-band CP MIMO antenna with independent working band port is realized.With excellent performance,its miniaturization and low-profile structure characteristics are even better than those of relevant dualband CP MIMO antenna schemes,which can meet the strict requirements of RFID system for antenna size.A valuable performance breakthrough of multi-port triband CP MIMO mode is realized.To sum up,this paper takes IoT RFID reader antenna as the application background,analyzes and discusses the performance advantages and design difficulties of CP MIMO antenna compared with LP MIMO antenna,and aims at the multi-band,multi-port CP MIMO mode.The analysis and design method of multi-band and multi-port CP MIMO antenna applied to RFID is proposed(one single-band,two dual-band and one three-band CP MIMO antenna scheme is proposed in turn),and two optimization design schemes for improving port isolation are proposed.Theoretical analysis,numerical simulation and experimental verification of the proposed multi-band multi-port CP MIMO antennas are carried out.The multi-band CP MIMO antenna technique provides a new theoretical basis and design ideas,which will help promote the application and innovation process of multi-band CP MIMO-RFID antenna technology. |
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