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

‍ ‍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.