Research On Beamforming Based On Multi-Frequency Signals In Hf Radar

The operating frequency band of high-frequency radar is 3-30MHz, which will achieve detection over the horizon through the use of ionospheric effects on the reflection of electromagnetic wave or diffraction effects of long wave on the Earth's surface. However, compared with the microwave radar, the angle resolution and range resolution of high-frequency radar is lower. In order to improve the system resolution, we can transmit broadband signal. However, the high frequency band is so crowded that it is difficult to obtain a continuous clean band. In this paper, multi-frequency transmission signals are used to improve the spatial angle resolution of high frequency radar through the multiple carrier frequencies spatial processing, and adaptive algorithm is applied to multiple carrier frequencies beamforming. Meanwhile, this paper solves the problem leads from the main lobe beam interference.This paper makes a study of the multi-frequency signal beamforming method of high-frequency radar. Firstly, this paper outlines the basics of array signal processing, and describes the conventional beamforming method. After that, analyze the basic principle of multi-frequency signals, establish the multi-frequency signals model and study the principle of virtual aperture extension of the array, then research the multi-frequency signal frequency selection and the improvement of the system resolution capacity through MATLAB simulation.Based on the conventional beamforming, this paper focuses on several common adaptive beamforming algorithms. Minimum variance distortionless response beamformer (MVDR) can increase the output SINR compared with the conventional beamforming theoretically, but the performance of the actual MVDR is degradation. Diagonal loading method of beamforming (DLM) and norm constraint Capon beamforming (NCCB) improve the method of MVDR, whereas they are difficult to determine the values of the loading quantity and constraint. Robust Capon beamforming (RCB) and dual constraints robust Capon beamforming (DCRCB) also belong to the diagonal loading methods, while they select the loading quantity acoding to the uncertain set of steering vector, so that the stability of RCB and DCRCB method are better. However, if the uncertain set of steering vector error is very large, the output SINR decreases. Therefore, an improved adaptive beamforming based on the diagonal loading method is presented which can solve the problem well.As is to say, adaptive beamforming method can suppress interference effectively. However, if there is interference in the main beam, this method will both makes the interference suppression and leads to the distortion of the main beam. For this problem, we can use block matrix preconditioning method, which is valid in the single frequency adaptive beamforming case. This paper applies the method to multi-frequency signal adaptive beamforming, and illustrates the effectiveness of this method in suppressing the main lobe interference for multi-frequency signal using simulation.