Abstract
In order to resolute the micro-Doppler frequency ambiguity caused by radar pulse repetition frequency not high enough (i.e., pulse dimension does not satisfy the requirement of Nyquist sampling theorem), this paper presents a micro-Doppler frequency ambiguity resolution method based on complex-valued U-net. The echo sequence is interpolated by zeros in the pulse dimension to increase the equivalent pulse repetition frequency, so that the echo sequence after zero interpolation contains the real micro-Doppler frequency; at the same time, some new frequency components are generated. The variation law of the echo sequence frequency after zero interpolation is analyzed. Then, the echo sequence in time domain after zero interpolation is transformed to the time-frequency domain by short-time Fourier transform (STFT). Finally, the time-frequency results can be segmented by the model, which is trained by complex-valued U-net to eliminate the redundant frequencies generated by zero interpolation; thus, the reconstruction of real micro-Doppler frequency is realized. Theoretical analysis and simulation results show that the proposed method can solve the problem of micro-Doppler frequency ambiguity. Compared with fully convolution network (FCN) and fully convolution residual network (FCRN), the proposed method has better performance and robustness.
Highlights
In modern warfare, helicopter has the advantages of vertical take-off and landing and strong transportation capabilities
By comparing the loss curves of three networks, we can find that the loss of fully convolution network (FCN) and fully convolution residual network (FCRN) is much greater than the loss of U-net
In order to resolute the problem of micro-Doppler frequency ambiguity, that is, the highest frequency in echo, which is higher than the sampling frequency, zero interpolation is proposed to increase the sampling frequency, and the complex-valued U-net is proposed to remove the redundant frequency components generated by zero interpolation after the real frequency is restored by zero interpolation. is method achieves the purpose of reconstructing the real micro-Doppler frequency
Summary
Helicopter has the advantages of vertical take-off and landing and strong transportation capabilities. Helicopter has become one of the main targets of air defense operations in various countries When this kind of target is hovering or flying at a slow speed, it is difficult to meet the requirements of detection and recognition by using traditional means, so it is important to use micromotion characteristics for feature extraction and recognition [3,4,5]. Range ambiguity or doppler ambiguity exists in different working modes. In view of this kind of problems, a lot of researches have been carried out on ambiguity resolution algorithms. Mathematical Problems in Engineering the relationship between traditional Doppler frequency and phase is generally linear, while the micro-Doppler frequency and phase satisfy the sinusoidal modulation relationship [18,19,20]
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