This letter presents an improved back-projection (BP) technique for far-field radar cross section (RCS) feature extraction from near-field measurement. The presented technique can synthesize a plane-wave response from a 2-D near-field planar scanner and obtain a 3-D scattering coefficient image. Due to the 3-D resolving power, it can spatially extract different parts of a complex object, or target’s scattering coefficient from the whole image which may include environment noise such as ground and support. From the 3-D scattering coefficient image, far-field RCS can be obtained as a function of direction and frequency. Simulations of RCS feature extraction for three squares and a 3-D complex-shaped electric-large car model are presented to validate the improved BP method.

Orthogonal frequency division multiplexing (OFDM) signals have been widely studied as a potential waveform used in integrated sensing and communication (ISAC) systems. High-computational effort, however, is required to estimate the azimuth of the target and suppress the interference from the nontarget directions. Moreover, along the nontarget directions, the transmitted confidential information can be easily intercepted by the eavesdroppers. In this article, directional modulation (DM) technology combined with OFDM waveforms, namely, OFDM-DM, is proposed for ISAC systems. From the sensing perspective, the interference from the nontarget direction can be suppressed, and 3-D radar images can be calculated without consuming extra computational resources. From a communication perspective, the OFDM-DM signals provide a secured physical-layer wireless transmission link and thus the confidential information can be securely delivered to the target. The efficacy of the proposed OFDM-DM ISAC waveforms is validated via numerical results for both sensing and communication functionalities by comparison with the traditional OFDM ISAC waveforms.

A popular method for nondestructive detection of underground targets is the remote sensing techniques. One of the most successful modalities for remote sensing of underground targets is the ground-penetrating radar (GPR), and it has been extensively used in many fields, such as military,^1,2 civil engineering,^3,4 and geological survey.⁵ GPR emits electromagnetic wave toward the ground, due to the different relative permittivity of the underground medium, the electromagnetic wave will be reflected back. When GPR antenna moves across the buried target, the time of arrival for responses from the target constitutes a hyperbola in the B-SCAN image. Therefore, the buried target detection can be viewed as mapping from the object space to an image space. Then, according to the characteristic curve of the target, the geometric parameters such as buried depth, width and height can be calculated.

Target position estimation is one of the important research directions in array signal processing. In recent years, the research of target azimuth estimation based on graph signal processing (GSP) has sprung up, which provides new ideas for the Direction of Arrival (DoA) application. In this article, by extending GSP-based DOA to joint azimuth and distance estimation and constructing a fully connected graph signal model, a multi-target joint azimuth and distance estimation method based on GSP is proposed. Firstly, the fully connection graph model is established related to the phase information of a linear array. For the fully connection graph, the Fourier transform method is used to solve the estimated response function, and the one-dimensional estimation of azimuth and distance is completed, respectively. Finally, the azimuth and distance estimation information are combined, and the false points in the merging process are removed by using CLEAN algorithm to complete the two-dimensional estimation of targets. The simulation results show that the proposed method has a smaller mean square error than the Multiple Signal Classification (MUSIC) algorithm in azimuth estimation under the condition of a low signal-to-noise ratio and more accurate response values than the MUSIC algorithm in distance estimation under any signal-to-noise ratio in multi-target estimation.

With the development of radio technology, passive bistatic radar (PBR) will suffer from interferences not only from the base station that is used as the illuminator of opportunity (BS-IoO), but also from the base station with co-frequency or adjacent frequency (BS-CF/AF). It is difficult for clutter cancellation algorithm to suppress all the interferences, especially the interferences from BS-CF/AF. The residual interferences will seriously affect target detection and DOA estimation. To solve this problem, a novel target detection and DOA estimation method for PBR based on compressed sensing sparse reconstruction is proposed. Firstly, clutter cancellation algorithm is used to suppress the interferences from BS-IoO. Secondly, the residual interferences and target echo are separated in spatial domain based on the azimuth sparse reconstruction. Finally, target detection and DOA estimation method are given. The proposed method can achieve not only target detection and DOA estimation in the presence of residual interferences, but also better anti-mainlobe interferences and high-resolution DOA estimation performance. Numerical simulation and experimental results verify the effectiveness of the proposed algorithm.

In this letter, a 3-D synthetic aperture radar with a frequency diverse array (3-D-FDA-SAR) is proposed for 3-D imaging. By applying an FDA to the 3-D-SAR system, the proposed 3-D-FDA-SAR can achieve real 3-D imaging using a simple hardware structure and a simple imaging algorithm. Specifically, the bandwidth needed for the range resolution is dispersed in space by the FDA, and the spatial resources (large aperture) needed for the along-track resolution are replaced by time resources. Therefore, the transmission and processing of wideband signals can be avoided, so the imaging algorithm only comprises a phase compensation step based on the back projection (BP) imaging algorithm. A resolution analysis of the range, along-track and across-track is presented to illustrate the rationality of the bandwidth dispersion and resource replacement. The simulation and experimental results verify the effectiveness of the proposed strategy.