| The traditional broadband receiving/transmitting structure is restricted by the topology of array and the developments of electronic components.So it is difficult to further expand the receiving/transmitting bandwidth.Besides,the traditional multibeamforming technology is mainly achieved by subarray division which greatly reduces the efficiency of array elements.Therefore,the researches on the structures of broadband receiving/transmitting and the methods of digital transmit multi-beamforming with a same aperture have become the research hotspots.In this dissertation,the array structures of broadband receiver/transmitter is studied at first,the corresponding optimization methods for the topology of array and beamforming weight vector is proposed.Secondly,two novel multi-channel broadband receiving/ transmitting structures are proposed.Based on the novel structures,the procedure of signal processing and the estimation method of key parameters are studied.Moreover,the adaptive multi-beamforming algorithms and the low-side-lobe transmit multi-beamforming algorithms are studied,respectively.In the end,a new method for the transmit multi-beamforming is proposed by improving the transmitting mode of phased array.Therefore,this dissertation aims to further enhance receiving/transmitting bandwidth and improve the performance of transmit multi-beamforming.This dissertation carries out the researches from the system structure to the algorithm module.The main works of this dissertation and related research results are as follows:1)The signal model of the sparse circular array is analyzed.Besides,the objective function and the solution vector are constructed.Based on the new selection operator,the modified genetic algorithm is proposed,which improves the diversity of populations.With the modified genetic algorithm,the probability of falling into local optimal solution is reduced.Therefore,the modified genetic algorithm is suitable for fast global optimization.The pattern function of circular array can be expanded to the first-order Taylor series by setting a minimal increment of the solution.The pattern optimization problem is then converted to an iterative optimization problem.Each iteration can be treated as a second-order cone programming optimization problem.The iterative optimization algorithm usually obtains the local optimal solution,thus it is suitable for high-precision optimization in a limited area.In this section,a new pattern optimization algorithm for the sparse circular array is proposed which combines the modified genetic algorithm and the iterative optimization algorithm.The global optimal solution can be obtained by the joint optimization of array element position and array weight vector.Experiments show that the side-lobe level of the proposed algorithm is lower and more stable than the existing genetic algorithms for the sparse circular array.2)Based on the modified Nyquist folding structure,a multi-channel Nyquist folding receiving/transmitting structure with a sinusoidal frequency modulation(SFM)local oscillator is proposed.The procedures of signal processing and the constraint conditions of SFM modulation local oscillator are analyzed,afterwards.A new method for Nyquist zone(NZ)index estimation and the transmit beamforming method under the proposed structure are studied in this section.According to different types of modulation,a multichannel Nyquist folding receiving/transmitting structure based on a linear frequency modulation(LFM)local oscillator is proposed.The procedures of signal processing and the NZ index estimation method based on the de-chirp function(DF)are also studied.Additionally,the transmit beamforming method based on the NZ estimation is proposed.Finally,Experiments show that the two kinds of broadband receiving/transmitting structure is feasible,and NZ estimation is the key to the performance of the transmit beamforming.3)The mathematical model of adaptive multi-beamforming is constructed and the robustness of conventional adaptive multi-beamforming algorithms is analyzed.In order to deal with the limited snapshots and self-cancellation,this section firstly studies the dynamic diagonal loading algorithm and provides the loading values.Secondly,this section proposes a correlation vector subspace projection algorithm.The error of covariance matrix caused by the strong desired signal is eliminated.The covariance matrix is compensated for the weak desired signal.Therefore,the proposed adaptive beamforming algorithm has a better robustness for both strong and weak desired signals.Finally,a new adaptive multi-beamforming algorithm based on the covariance matrix reconstruction is proposed,which combines the dynamic diagonal loading algorithm and correlation vector subspace projection algorithm.The robustness of the proposed algorithm is improved under limited snapshots and self-cancellation.In addition,this section studies a steering vector correction method for the estimating error of the steering vectors.This method further enhances the robustness of adaptive multi-beamforming.4)The principles of optimal frequency focusing,the construction of optimal focusing matrix and focusing frequency are studied.Further,the mathematical model of the transmit multi-beamforming algorithm for the narrowband and wideband signals is proposed.For the narrowband signal,a transmit multi-beamforming algorithm is proposed based on the second-order cone programming optimization algorithm.For the wideband signal,a transmit multi-beamforming algorithm is proposed combining the optimal frequency focusing and the second-order cone programming optimization.The simulations show that the proposed method is better than the existing algorithms in main beam stabilization,frequency sensitivity and side-lobe level.5)Based on the design and transmit beam pattern of the frequency diversity array,this section proposes a modified circular phased array whose transmitting model is improved.The transmit pattern of the modified phased array is analyzed according to the mathematics and experiments.The coupling between distance and angle is analyzed.Besides,the coupling between distance and time is also analyzed.Considering the actual dynamic constraints for the transmitter,this section transforms the dynamic constraint problem of array weight into a convex optimization problem.A joint optimization algorithm for the dynamic of array weight and the time-invariant characteristic of beam is proposed.The weight with smaller dynamic range and the multi-beamforming with better performance is obtained.The simulations show that the modified phased array can effectively form multiple spot-beams in space,and the values of the joint optimization performs better than that of the existing methods. |
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