面向小型无人机的OFDM-MIMO雷达信号设计和处理方法

Signal Design and Processing Method of OFDM-MIMO Radar for Small UAVs

随着空域的逐渐开放,以小型无人机为代表的低慢小目标,其有效检测对维护安全非常重要。然而,低慢小目标具有飞行高度低、飞行速度慢及雷达散射截面积(Radar Cross Section,RCS)小的特点,加之电磁环境复杂多变,使其难以被传统低分辨雷达发现。为了提升雷达的低截获性能和成像分辨率,本文提出了一种基于步进正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)的多输入多输出(Multiple Input Multiple Output,MIMO)雷达信号设计和处理方法。该方法是将窄带OFDM信号与非线性步进方案相结合,通过非线性步进方案控制MIMO发射信号的载频,获得发射端正交波形和大射频带宽,使得雷达接收机的天线孔径获得明显扩展,从而提升雷达的低截获性能和角度分辨率,同时实现低采样速率下的高距离分辨率。在雷达接收端,利用发射波形之间的正交性分离各通道回波,然后基于改进离散傅里叶变换(Discrete FourierTransform,DFT)和解码的处理方法,校正了非线性步进方案产生的相位误差,生成目标的三维高分辨成像。本文在77 GHz载频下设计仿真,验证所提方法的有效性。仿真结果表明,所提方法相比现有方法,在低信噪比下具有较高的三维成像分辨率,而且没有明显增加信号处理的复杂度。然而,该方法减小了最大无模糊速度。此外,本文讨论了步长数对雷达性能的影响,仿真结果表明,随着步长数的增加,最大无模糊速度减小,角度分辨率提升,为实际应用场景下步进OFDM-MIMO雷达的信号设计提供理论指导。

With the gradual opening of airspace,the effective monitoring of low slow small(LSS)targets represented by small unmanned aerial vehicles(UAVs)is important to maintain security.Detecting these targets using conventional low-resolution radars is challenging due to their low altitude,slow speed,and small radar cross section(RCS),particularly in complex and changeable electromagnetic environments.To improve radar interception performance and image resolution,we propose a signal design and processing approach for multiple input multiple output(MIMO)radar that employs stepped orthogonal frequency division multiplexing(OFDM)to detect small UAVs.First,the proposed waveform is designed by combining OFDM signals of small bandwidth with a nonlinear stepping scheme,denoted as stepped OFDM,which enables the transmission of consecutive low-bandwidth OFDM symbols on different carrier frequencies,thereby covering a much larger radio frequency(RF)bandwidth in a measurement frame.This also enables the high range resolution at a lower sampling rate compared to an equivalent wideband OFDM.In addition,the multiplexing of transmit(TX)antennas for the MIMO can be performed using stepped OFDM signals,which enables multiple TX antennas to operate simultaneously by assigning orthogonal subcarrier sets to each antenna,increasing the degrees of freedoms(DOFs)on both transmitters and receivers,eventually leading to a virtual array at receivers that significantly increase the antenna aperture size.Consequently,the stepped OFDM MIMO radar improves the azimuth resolution and low intercept performance.Then,the echoes of each channel are separated by utilizing the orthogonality between the transmit signals in the receiver.Additionally,a radar signal processing method to the stepped OFDM MIMO radar is proposed to improve the image resolution,where the modified discrete Fourier transform(DFT)and decoding processing is used to correct the phase errors introduced by the nonlinear stepping scheme,leading to separate range-velocity images for each virtual channel.In addition,the azimuth estimation exploiting the extended virtual array of the stepped OFDM MIMO radar is achieved by performing Fourier beamforming.Consequently,three dimensional(3D)high-resolution images of targets can be generated.Finally,the effectiveness of the proposed method is validated via simulations performed at a carrier frequency of 77 GHz.Notably,compared to the existing methods,the proposed method can achieve higher resolution in range,velocity,and azimuth dimensions in a low signal-to-noise ratio(SNR)regime and without increasing the more computational complexity of signal processing.However,high resolution is achieved at the expense of a reduced maximum unambiguous velocity.In addition,the effect of the number of steps on radar performance is examined.Results indicate that when the measuring time and the total RF bandwidth are the same,the maximum unambiguous velocity is decreased by the number of steps,and the achievable azimuth resolution is improved by the number of steps.Thus,this study can provide theoretical guidance for the signal design of stepped OFDM MIMO radars in practical application scenarios.