Research On Virtual Antenna Beamforming Based On Interpolated Array Transformation Technique

With the unceasing progress of modern science and technology, the wirelesscommunication system develops fast. As one of the key techniques of wirelesscommunication system, smart antenna can make its main lobe point at the direction of thedesired signal and adaptively inhibit interferers in the unknown directions; therefore, smartantenna can suppress multi-user interference and raise system capacity. However, when thenumber of signal and interferers is more than the degree of freedoms of an antenna array, thesmart antenna can not work effectively. Recently, the wireless communication equipmentsincrease day by day, the electromagnetic interference(EMI) problem gets serious increasingly,electromagnetic compatibility(EMC) becomes a research focus gradually. The EMC inantenna system influences the communication system greatly, it is one of effective andfeasible methods to reduce EMI by decreasing antenna elements so as to reduce radiationenergy and coupling interference.Virtual antenna is an advanced technology developed based on smart antenna, mainlystudies how to increase the degree of freedoms of an antenna array, so as to inhibit numerousinterference, or to process signals with the same amount via less antenna elements. Presently,the research is mainly focused on the DOA estimation aspect. This thesis takes the virtualantenna based on interpolated transformation technique as the research object, by theoreticalanalysis and simulation experiments, deeply studies the performance of applying virtualantenna in adaptive beamforming, and obtains some research results in raising interferenceinhibition gain, reducing side lobe level(SLL) and decreasing the interpolated transformationangle sensitivity effect, etc. The main research contents of this thesis are as follows:1. Aiming at the problem of when the existing interpolated virtual antenna was used inadaptive beamforming, the interference inhibition gain was relatively lower, the SLL was alittle higher and the antenna array output SINR was relatively lower, in this thesis, firstly, thereason of the problem was analyzed theoretically, and found that it was caused by thedisturbance of eigenvalues and eigenvectors of the covariance matrix brought by thetransformation error generated in the interpolated transformation process, and then the factorthat influences the inhibition gain is derived. By introducing the projection transformationthought into virtual antenna, the interference space projection(ISP) modified strategy is presented in this thesis, according to the different base vectors of interference space, ISPstrategy has two specific implementation methods, using the eigenvectors corresponding tobig eigenvalues obtained from covariance decomposition as base vectors to constructprojection operator, denoted as ISP-1method; using virtual antenna interference steeringvectors as base vectors to construct projection operator, denoted as ISP-2method. Throughtheoretical analysis, it is proved that via ISP strategy, the signal space of virtual antennacovariance can be enhanced and the disturbance of small eigenvalues and correspondingeigenvectors in noise space can get suppressed, therefore, the large and small eigenvaluesratio is reinforced, the virtual antenna beamforming null depth can be enlarged and the SLLcan be decreased.2. Aiming at the problem of virtual antenna beamforming inhibiting independentinterference, two Projected Virtual Antenna Beam Forming(PVABF) algorithms are proposed,the feasibility of suppressing interferers whose number is more than degree of freedoms of thereal antenna array, through simulation experiments, the results verifies PVABF algorithm caneffectively enlarge the virtual antenna beamforming null depth, improve the side lobeperformance, raise the antenna array output performance; on the premise of performanceindexes satisfying the system requirements, it can not only increase the degree of freedoms ofan antenna array, but also can enlarge the array antenna aperture to some extent, break thelimitation that the existing interpolated antenna is not beneficial to enlarge antenna arrayaperture.3. Aiming at the problem of virtual antenna beamforming inhibiting coherentinterference, combined with spatial smoothing technique, on the basis of Interpolated SpatialSmoothing(ISS) algorithm, introduce ISP strategy, propose Modified Interpolated SpatialSmoothing(MISS) algorithm, this algorithm can raise the inhibition gain on coherentinterference, improve the side lobe performance, through theoretical analysis and simulationexperiments, the results verify the validity of MISS algorithm, the different sub-array divisionform influences the performance of MISS algorithm.4. Aiming at the problem that virtual antenna interpolated transformation technique cannot work effectively over a large transformation area, when there exists burst interferencelying out of transformation area, the burst interference can not get inhibited, in this thesis,using differential geometry knowledge, analyze the mechanism of angle sensitivity problem, propose a arc length based virtual antenna beamforming method, this method allowsinterpolation over a large transformation area and ensures the transformation error keep inacceptable range, theoretical analysis and simulation results prove that this method caneffectively inhibit all the interference(including burst interference) arriving at the antennaarray.