Investigations On The Key Technologies Of Millimeter Wave Digital Multibeam Array

With the development of wireless communication in recent years,the operation band of radar and communication system is approaching the millimeter-wave(mm W)band.Considering the large propagation loss of electromagnetic waves at mm W frequencies,antennas with a directive beam need to be adopted.However,the conventional phased arrays,requiring expensive phase shifter in each channel,can only generate a single steerable beam at an instantaneous time.This do not meet the requirement of covering the whole field of view with multiple concurrent beams.Benefitting from enabling multiple simultaneous beam and non-phase shifter hardware structure,digital beamforming(DBF)technology has received more and more attention worldwide.Various kinds of applications based on DBF framework has been proposed,such as the 5G wireless communication,77GHz automotive radar,E-band broadband wireless communication and so on.Thus,it is of great significance to research key technologies of mm W digital multibeam array.The dissertation focuses on two key sections of mm W digital multibeam array,namely active system design and antenna array design.The main contents are summarized as follows.Chapter one mainly focuses on designs of frequency modulated continuous wave radar system.Two designs of FMCW radar are proposed in this chapter for 77GHz automotive radar application and X-band long range radar application,respectively.In this chapter,the system architecture,antenna design,RF circuit module measurement are demonstrated.The 77GHz automotive radar front-end,including two transmitters and six receivers,has high integrity and only occupies very small footprint.It is very suitable to be mounted on cars.It can be found from measurement result that moving targets more than 170 meters can be detected by this radar front-end.Another X-band radar prototype is proposed here for long-range detection application.The system involves one transmitting channel and six receiving channels.Noted that this radar enables detecting moving targets longer than 3km,from the measurement results.Chapter two designs a DDS-based digital beamforming array transmitter(DBFAT)system.Different sets of weighting coefficients are synthesized by the DDS module,which generates various types of multibeam patterns simultaneously.The system architecture,realization,generation,and measurement process of multiple beams are demonstrated.The transmitting array element consists of two kinds of tapered slot antennas with opposite fin orientations.They are placed in the side-by-side configuration such that the undesirable orthogonal electrical field is suppressed,thus improving the cross-polarization performance at the majority of the scanning angles.As the wavelength at the mm W band is quite short,it is very difficult to integrate all the channels on only one side of the board with such a narrow spacing(?0.5?0).In order to solve this problem,half of the channels were placed on the top side,while the others were located on the bottom side,offering enough space for encompassing adjacent channels and isolation.As demonstrated,a 1×15 DBFAT system was fabricated and measured.It can be found from measured results that the proposed DBFAT enables beam scanning within±50^o,less than 3d B fluctuation.The measured broadside gain of this array is about 18.3 d Bi,while its half power beam-width(HPBW)is about 6^o.The design of this part has been published in IEEE Trans.on Antennas Propag.Chapter three presents a mm W hybrid radar system with T-type array configuration.The hybrid radar system consists of a vertically distributed phased transmitting array(PTA)and a horizontally aligned DBF receiving array,thus enabling beam scanning in the elevation plane and azimuth plane respectively.The system architecture,design and measurement results are reported in this part.In addition,a comprehensive performance comparison between the hybrid radar concept and other existing radar solutions is conducted,to understand this hybrid concept clearly.The proposed radar system,including 15 transmitting channels and 16 receiving channels,emits FMCW signal with 150MHz(24.15-24.3GHz).It can be observed from measured beam-scanning results that the PTA and DBF receiving array enable beam-scanning within±40^o,thus meeting the requirement of the system.Compared with traditional planar 2-D radar(?M×N),the demonstrated radar realizes 2-D beam scanning with a much smaller number of channels(?M+N).Comparing to the multiple input multiple output(MIMO)radar solutions,the reported radar realize higher effective radiation power as all the transmitting channels work at the same time.Only 1-D DBF is required in the proposed concept,another dimension of beamforming has been accomplished by mm W PTA.This makes the baseband implementation is much easier than that of TDM MIMO radar.The design of this part has been submitted to IEEE Trans.on Antennas Propag.Chapter four designs a kind of novel substrate integrated waveguide(SIW)slot array for both77GHz medium and long range DBF radar application.Radars with different detection range have different requirements on the concerned field of view(FOV)such that LRR demands a high gain and a narrow FOV,while MRR needs a relatively lower gain and a wider FOV.The proposed antenna array consists of six 1×16 SIW slot linear array,while its radiation pattern looks like a combination of MRR and LRR radar antenna.This special radiation pattern is called“flat-shoulder”shaped radiation pattern in this chapter.A mixed optimization method,including non-linear fitting and aggressive space mapping,is utilized to synthesize this kind of special pattern.The excitation power and phase of each linear array input port are variables to be optimized for achieving the desired flat-shoulder pattern.In this mixed optimization process,the mutual coupling between adjacent arrays is considered,thus no more further adjustment is needed in the full-wave models when the mixed optimization procedure finishes.The measurement result agrees well with the simulated ones,thus confirming the correctness of the proposed concept.The impedance bandwidth(|S11|<-10d B)is about 3.76%(75.6-78.5GHz),and the measured peak gain achieves 21.7d Bi.The design of this part has been published in IEEE Trans.on Antennas Propag.Chapter five designs a new slot-loaded antenna element for E-band broadband mm W communication application.The array element consists of a corner-fed patch with a tilted slot cut at its edge,which suppresses unwanted modes in order to generate the desired 45°linear polarization.Compared with traditional 45°tilted patch element,the demonstrated element does not need the bending feedline and realizes a higher isolation level between adjacent patches in the side-by-side arrangement.At the same time,the direction of polarization can be switched from±45^o by changing the direction of etched slot easily.It is a promising element for designing45°or±45^o polarized antenna array.As demonstrations,three kinds of antenna arrays based on slot loaded element are fabricated and measured,including 1×8 45°polarized array,2×8 45°polarized array,1×8±45°polarized array.Good agreement is achieved between measured and simulated results,thereby confirming the validity of design.The reported arrays in this chapter owns the advantage of low profile(0.254 mm?0.067?0),single layer,low fabrication cost,simple structure,wideband(?10%),and flexibility of polarization.The designs of this part have been published in IEEE UCMMT 2017 and IEEE Trans.on Antennas Propag.Chapter six presents a novel beamforming patch array with a cosecant 4^th power pattern is proposed for millimeter-wave synthetic aperture radar(SAR)applications,with its concept and operational principal reported.The entire array consists of 12 series-fed microstrip patch sub-arrays and corresponding power distributing and phase shifting network.A mixed optimization method,including non-linear fitting(NLF)and active pattern method,is adopted for the array synthesis.In order to increase design reliability and tolerance to fabrication error,the excitation power ratio among the linear sub-array is limited to be an integer power of 2(2^M,M=0-3).Thus,the power dividing circuit can be realized by cascading several stages of different symmetrical SIW equal power dividers,which is much simpler than asymmetrical feeding structures.For demonstration,a 6×12 patch array prototype was fabricated and measured,exhibiting good performance of cosecant 4^th power pattern from 23.8 to 25.0 GHz.Simulated and measured results have been displayed,thus confirming the validity of the proposed concept.The presented array owns the advantage of low-profile(?0.09?0),compact,reliable,and low-cost.The design of this part has been accepted by IEEE Trans.on Antennas Propag.In general,all the works mentioned above have been published or submitted to IEEE Trans.on Antennas Propag.,with a total number of five first-author papers.The author has also published two conference papers and applies for six Chinese invention patents.