Submit Manuscript
Peer Review
Editor Work2021 Vol. 10, No. 5
Previous Issue
2021, 10(5): 665-679.
Abstract
2057KB
Vortex electromagnetic wave carries the Orbital Angular Momentum (OAM), and thus has a spiral wavefront structure, which contains the azimuthal information of the target in the echo signal when performing radar imaging compared with planar waves. Hence, this kind of electromagnetic wave shows great potential for various applications in the field of radar detection and imaging, and it is expected to become the development direction of new-system radars. This paper describes in detail the research progress in vortex radar imaging technology in recent years. First, the characteristics of the vortex electromagnetic wave and the principles of imaging using a uniform circular array are introduced. Then the paper reviews the developmental history and research status of vortex radar imaging technology, according to three research directions: vortex radar imaging model, vortex radar gaze imaging algorithm, and vortex radar motion imaging. Finally, the prospects for the development of vortex radar imaging have been presented, along with the key scientific issues and trends for the future developments of vortex radar imaging.
Vortex electromagnetic wave carries the Orbital Angular Momentum (OAM), and thus has a spiral wavefront structure, which contains the azimuthal information of the target in the echo signal when performing radar imaging compared with planar waves. Hence, this kind of electromagnetic wave shows great potential for various applications in the field of radar detection and imaging, and it is expected to become the development direction of new-system radars. This paper describes in detail the research progress in vortex radar imaging technology in recent years. First, the characteristics of the vortex electromagnetic wave and the principles of imaging using a uniform circular array are introduced. Then the paper reviews the developmental history and research status of vortex radar imaging technology, according to three research directions: vortex radar imaging model, vortex radar gaze imaging algorithm, and vortex radar motion imaging. Finally, the prospects for the development of vortex radar imaging have been presented, along with the key scientific issues and trends for the future developments of vortex radar imaging.
2021, 10(5): 680-690.
The vortex ElectroMagnetic (EM) wave, whose phase wavefront is modulated by the Orbital Angular Momentum (OAM), has received immense attention, especially in the field of forward-looking radar imaging. Based on the fundamental principle and imaging method of the vortex EM radar, the azimuth resolution was studied in this paper. First, the circumstance of considering the Bessel amplitude term was analyzed, indicating that the azimuth resolution was determined by the effective scope of OAM modes. Then, an effective method for calculating the scope of OAM modes was proposed, including the expressions of azimuth resolution, spatial resolution, and super-resolution were characterized. Finally, the fundamental resolution performance with different influencing factors was analyzed via simulations. The analysis showed that changing the wavelength aperture ratio and imaging elevation could increase the effective scope of OAM modes, which improved the azimuth resolution. Through data fitting, the approximate expressions of the effective scope of OAM modes and the resolution of the super-real aperture radar with respect to the wavelength aperture ratio and imaging elevation were obtained separately, providing a reference for the parameters design and optimization of vortex EM wave radar.
The vortex ElectroMagnetic (EM) wave, whose phase wavefront is modulated by the Orbital Angular Momentum (OAM), has received immense attention, especially in the field of forward-looking radar imaging. Based on the fundamental principle and imaging method of the vortex EM radar, the azimuth resolution was studied in this paper. First, the circumstance of considering the Bessel amplitude term was analyzed, indicating that the azimuth resolution was determined by the effective scope of OAM modes. Then, an effective method for calculating the scope of OAM modes was proposed, including the expressions of azimuth resolution, spatial resolution, and super-resolution were characterized. Finally, the fundamental resolution performance with different influencing factors was analyzed via simulations. The analysis showed that changing the wavelength aperture ratio and imaging elevation could increase the effective scope of OAM modes, which improved the azimuth resolution. Through data fitting, the approximate expressions of the effective scope of OAM modes and the resolution of the super-real aperture radar with respect to the wavelength aperture ratio and imaging elevation were obtained separately, providing a reference for the parameters design and optimization of vortex EM wave radar.
2021, 10(5): 691-698.
The ElectroMagnetic Vortex (EMV) wave is named after the rotation around the wave propagation axis. This electromagnetic characteristic is called the Orbital Angular Momentum (OAM). Considering its azimuth angular resolution, this paper introduces the EMV wave into traditional Synthetic Aperture Radar (SAR) imaging and proposes a novel Three-Dimensional (3D) imaging scheme called EMV-SAR. In EMV-SAR, the echo is extended into 3D after involving the OAM mode domain. Based on the waveform diversity technology, the multiOAM-mode echo is obtained and simultaneously transformed into azimuthal angular signals via Fourier Transform (FT) to form the 3D data of range-azimuth-angular. In this study, we propose a joint two-dimensional azimuthal compression algorithm to generate 3D target imaging based on Radon FT. The simulation results validate the performance of the proposed system and algorithms and demonstrate the superiority of EMV-SAR in 3D imaging.
The ElectroMagnetic Vortex (EMV) wave is named after the rotation around the wave propagation axis. This electromagnetic characteristic is called the Orbital Angular Momentum (OAM). Considering its azimuth angular resolution, this paper introduces the EMV wave into traditional Synthetic Aperture Radar (SAR) imaging and proposes a novel Three-Dimensional (3D) imaging scheme called EMV-SAR. In EMV-SAR, the echo is extended into 3D after involving the OAM mode domain. Based on the waveform diversity technology, the multiOAM-mode echo is obtained and simultaneously transformed into azimuthal angular signals via Fourier Transform (FT) to form the 3D data of range-azimuth-angular. In this study, we propose a joint two-dimensional azimuthal compression algorithm to generate 3D target imaging based on Radon FT. The simulation results validate the performance of the proposed system and algorithms and demonstrate the superiority of EMV-SAR in 3D imaging.
2021, 10(5): 699-717.
The Orbital Angular Momentum (OAM)-based vortex radar has drawn increasing attention because of its potential for high-resolution imaging. The vortex radar high resolution imaging with limited OAM modes is commonly solved by sparse recovery, in which the prior knowledge of the imaging model needs to be known precisely. However, the inevitable phase error in the system results in imaging model mismatch and deteriorates the imaging performance considerably. To address this problem, the vortex radar imaging model with phase error is established for the first time in this work. Meanwhile, a two-step self-calibration imaging method for vortex radar is proposed to directly estimate the phase error. In the first step, a sparsity-driven algorithm is developed to promote sparsity and improve imaging performance. In the second step, a self-calibration operation is performed to directly compensate for the phase error. By alternately reconstructing the targets and estimating the phase error, the proposed method can reconstruct the target with high imaging quality and effectively compensate for the phase error. Simulation results demonstrate the advantages of the proposed method in enhancing the imaging quality and improving the phase error estimation performance.
The Orbital Angular Momentum (OAM)-based vortex radar has drawn increasing attention because of its potential for high-resolution imaging. The vortex radar high resolution imaging with limited OAM modes is commonly solved by sparse recovery, in which the prior knowledge of the imaging model needs to be known precisely. However, the inevitable phase error in the system results in imaging model mismatch and deteriorates the imaging performance considerably. To address this problem, the vortex radar imaging model with phase error is established for the first time in this work. Meanwhile, a two-step self-calibration imaging method for vortex radar is proposed to directly estimate the phase error. In the first step, a sparsity-driven algorithm is developed to promote sparsity and improve imaging performance. In the second step, a self-calibration operation is performed to directly compensate for the phase error. By alternately reconstructing the targets and estimating the phase error, the proposed method can reconstruct the target with high imaging quality and effectively compensate for the phase error. Simulation results demonstrate the advantages of the proposed method in enhancing the imaging quality and improving the phase error estimation performance.
2021, 10(5): 718-724.
In the Inverse Synthetic Aperture Radar (ISAR) imaging system, when the terahertz radar transmits the wide bandwidth signal and vortex electromagnetic wave, Three-Dimensional (3D) high-resolution imaging can be achieved through information decoupling based on the differential radiation field formed by the vortex electromagnetic wave and the synthetic aperture formed by the relative movement of the radar and the target. Accordingly, a 3D imaging model based on the terahertz vortex electromagnetic wave ISAR is established. A new image reconstruction method is proposed based on the Sparse Bayesian Learning (SBL) and subregion amplitude threshold setting methods. The proposed method can significantly simplify the imaging procedure and reduce the computational load. The simulation results indicate that the proposed SBL method can achieve a higher resolution than the conventional fast Fourier transform-based method, and its reconstruction performance increases with an increase in the signal-to-noise ratio.
In the Inverse Synthetic Aperture Radar (ISAR) imaging system, when the terahertz radar transmits the wide bandwidth signal and vortex electromagnetic wave, Three-Dimensional (3D) high-resolution imaging can be achieved through information decoupling based on the differential radiation field formed by the vortex electromagnetic wave and the synthetic aperture formed by the relative movement of the radar and the target. Accordingly, a 3D imaging model based on the terahertz vortex electromagnetic wave ISAR is established. A new image reconstruction method is proposed based on the Sparse Bayesian Learning (SBL) and subregion amplitude threshold setting methods. The proposed method can significantly simplify the imaging procedure and reduce the computational load. The simulation results indicate that the proposed SBL method can achieve a higher resolution than the conventional fast Fourier transform-based method, and its reconstruction performance increases with an increase in the signal-to-noise ratio.
2021, 10(5): 725-739.
Traditional radars can detect moving targets using the Doppler effect. However, traditional radars have shadow areas in detecting the angular motion of the rotating targets. The discovery of the rotational Doppler effect based on vortex electromagnetic waves helps solve the problem of detecting the angular motion of the rotating targets under direct vision, which has attracted considerable attention from domestic and foreign scholars. In this study, we discussed the recent research progress on the rotational Doppler effect of vortex electromagnetic waves, particularly for related results in the microwave band, including the rotational Doppler effects on the target under on-axis and off-axis cases; decoupling linear Doppler, micro-Doppler and rotational Doppler effects under complex motion cases; and rotational Doppler effects on the applications of radar imaging and velocity measurement. We summarized and analyzed the existing problems demanding prompt solutions in this field, and proposed future research directions and relative applications.
Traditional radars can detect moving targets using the Doppler effect. However, traditional radars have shadow areas in detecting the angular motion of the rotating targets. The discovery of the rotational Doppler effect based on vortex electromagnetic waves helps solve the problem of detecting the angular motion of the rotating targets under direct vision, which has attracted considerable attention from domestic and foreign scholars. In this study, we discussed the recent research progress on the rotational Doppler effect of vortex electromagnetic waves, particularly for related results in the microwave band, including the rotational Doppler effects on the target under on-axis and off-axis cases; decoupling linear Doppler, micro-Doppler and rotational Doppler effects under complex motion cases; and rotational Doppler effects on the applications of radar imaging and velocity measurement. We summarized and analyzed the existing problems demanding prompt solutions in this field, and proposed future research directions and relative applications.
2021, 10(5): 740-748.
The vortex ElectroMagnetic (EM) wave has a unique spiral phase wavefront. The rotational Doppler effect, induced by the lateral micromotion of a target is expected to provide a new mode of radar target detection. Under the illumination of vortex EM waves, the micromotion of a cone-shaped target periodically modulates the instantaneous frequency of the radar echo; this can effectively reflect the geometric characteristics and micromotion parameters of a cone-shaped target. This paper focuses on the estimation of cone-shaped target parameters under forward-looking radar conditions. First, based on the principle of vortex EM wave target rotational Doppler detection, a mathematical equation describing the vortex EM wave of a cone-shaped target echo is derived, and an echo rotational Doppler model is established. Second, a method for cone-shaped target parameter estimation under forward-looking conditions is proposed. Using the two-dimensional rotational Doppler information of the scattering points at the top and bottom of a cone-shaped target, the micromotion and geometric parameters of the target can be effectively estimated. The simulation results verify the effectiveness and robustness of the method proposed in this paper.
The vortex ElectroMagnetic (EM) wave has a unique spiral phase wavefront. The rotational Doppler effect, induced by the lateral micromotion of a target is expected to provide a new mode of radar target detection. Under the illumination of vortex EM waves, the micromotion of a cone-shaped target periodically modulates the instantaneous frequency of the radar echo; this can effectively reflect the geometric characteristics and micromotion parameters of a cone-shaped target. This paper focuses on the estimation of cone-shaped target parameters under forward-looking radar conditions. First, based on the principle of vortex EM wave target rotational Doppler detection, a mathematical equation describing the vortex EM wave of a cone-shaped target echo is derived, and an echo rotational Doppler model is established. Second, a method for cone-shaped target parameter estimation under forward-looking conditions is proposed. Using the two-dimensional rotational Doppler information of the scattering points at the top and bottom of a cone-shaped target, the micromotion and geometric parameters of the target can be effectively estimated. The simulation results verify the effectiveness and robustness of the method proposed in this paper.
2021, 10(5): 749-759.
Quantum Orbital Angular Momentum (OAM) indicates that each Electro-Magnetic (EM) photon of an EM wave carries OAM. In the microwave band, such an EM wave photon is called a vortex microwave photon. Physical properties distinguish between EM waves with vortex and plane wave photons. When illuminating a traditional stealthy target composed of absorbing materials, a vortex microwave photon can achieve higher echo power, thereby improving the Radar Cross Section (RCS), the corresponding receiving signal power, and detection probability. Hence, the vortex microwave photon shows promise in antistealth technology. In this paper, a vortex microwave quantum radar based on the OAM quantum state is proposed. Its basic physical architecture and corresponding mathematical model are given, and the high echo power characteristics of the vortex microwave photon are analyzed using Quantum Electro-Dynamics (QED). The correctness of the theoretical calculation was experimentally verified with an approximate 9 dB improvement in echo power. Moreover, the simulations are performed to clarify the improvement in radar performance, including the receiving power and detection probability, illustrating the capability of the vortex microwave photon when applied to antistealth radar.
Quantum Orbital Angular Momentum (OAM) indicates that each Electro-Magnetic (EM) photon of an EM wave carries OAM. In the microwave band, such an EM wave photon is called a vortex microwave photon. Physical properties distinguish between EM waves with vortex and plane wave photons. When illuminating a traditional stealthy target composed of absorbing materials, a vortex microwave photon can achieve higher echo power, thereby improving the Radar Cross Section (RCS), the corresponding receiving signal power, and detection probability. Hence, the vortex microwave photon shows promise in antistealth technology. In this paper, a vortex microwave quantum radar based on the OAM quantum state is proposed. Its basic physical architecture and corresponding mathematical model are given, and the high echo power characteristics of the vortex microwave photon are analyzed using Quantum Electro-Dynamics (QED). The correctness of the theoretical calculation was experimentally verified with an approximate 9 dB improvement in echo power. Moreover, the simulations are performed to clarify the improvement in radar performance, including the receiving power and detection probability, illustrating the capability of the vortex microwave photon when applied to antistealth radar.
2021, 10(5): 760-772.
Target detection based on space modulation requires a large number of test modes with space-time independence. The Orbital Angular Momentum (OAM) beams are orthogonal to each other and have infinite modes. Due to the strong dispersive materials, multi-mode OAM beams with the same scattering angle can be generated in the frequency domain. In this manuscript, the propagation characteristics of multi-mode OAM beams are analyzed, which can be utilized to improve detection efficiency. The echoes from the target illuminated by the multi-mode OAM beams are then investigated in three different application scenarios. A convolution neural network is employed to extract the relationship between the echo data and the target image based on prior knowledge. The target and the imaging scenarios can be distinguished with a high probability. Finally, the proposed method’s anti-noise performance is analyzed. The experimental results show that in the ideal state, the accuracy of target scene judgment can reach 97.5%. The accuracy of the target location recognition is higher than 80% when the interval between two adjacent targets in a scene is larger than a threshold. The accuracy of the target location recognition in three scenes is greatly reduced when SNR is less than 20 dB, depending on the scene.
Target detection based on space modulation requires a large number of test modes with space-time independence. The Orbital Angular Momentum (OAM) beams are orthogonal to each other and have infinite modes. Due to the strong dispersive materials, multi-mode OAM beams with the same scattering angle can be generated in the frequency domain. In this manuscript, the propagation characteristics of multi-mode OAM beams are analyzed, which can be utilized to improve detection efficiency. The echoes from the target illuminated by the multi-mode OAM beams are then investigated in three different application scenarios. A convolution neural network is employed to extract the relationship between the echo data and the target image based on prior knowledge. The target and the imaging scenarios can be distinguished with a high probability. Finally, the proposed method’s anti-noise performance is analyzed. The experimental results show that in the ideal state, the accuracy of target scene judgment can reach 97.5%. The accuracy of the target location recognition is higher than 80% when the interval between two adjacent targets in a scene is larger than a threshold. The accuracy of the target location recognition in three scenes is greatly reduced when SNR is less than 20 dB, depending on the scene.
2021, 10(5): 773-784.
The electromagnetic vortex wave has demonstrated excellent research value with potential applications in the fields of wireless communication and radar detection and imaging due to its unusual electromagnetic field distribution and theoretically infinite orthogonal Orbital Angular Momentum (OAM) modes. This study analyzes the anti-interference performance of OAM modes in the electromagnetic vortex Radio Frequency (RF) transceiver link primarily from the perspective of the electromagnetic vortex field distributions in space and the OAM modes orthogonality. Planar antenna arrays are designed to generate the electromagnetic vortex beams with respective OAM modes of and in the C band, and the corresponding RF transceiver links are established. The OAM modes’ anti-interference properties under different interference situations are analyzed in the electromagnetic vortex RF transceiver link by using a horn antenna as the interference source. Meanwhile, the corresponding OAM mode spectrum and the OAM modes’ orthogonality are employed as the primary methods in our analysis. Finally, the designed antenna models are fabricated, and the electromagnetic vortex RF transceiver links are measured. The corresponding analyses and conclusions are presented in this study. The OAM modes’ anti-interference performance analysis in the vortex electromagnetic wave’s RF transceiver link can provide a reference for exploring and designing a vortex electromagnetic wave in wireless communication and radar detection and imaging research.
The electromagnetic vortex wave has demonstrated excellent research value with potential applications in the fields of wireless communication and radar detection and imaging due to its unusual electromagnetic field distribution and theoretically infinite orthogonal Orbital Angular Momentum (OAM) modes. This study analyzes the anti-interference performance of OAM modes in the electromagnetic vortex Radio Frequency (RF) transceiver link primarily from the perspective of the electromagnetic vortex field distributions in space and the OAM modes orthogonality. Planar antenna arrays are designed to generate the electromagnetic vortex beams with respective OAM modes of and in the C band, and the corresponding RF transceiver links are established. The OAM modes’ anti-interference properties under different interference situations are analyzed in the electromagnetic vortex RF transceiver link by using a horn antenna as the interference source. Meanwhile, the corresponding OAM mode spectrum and the OAM modes’ orthogonality are employed as the primary methods in our analysis. Finally, the designed antenna models are fabricated, and the electromagnetic vortex RF transceiver links are measured. The corresponding analyses and conclusions are presented in this study. The OAM modes’ anti-interference performance analysis in the vortex electromagnetic wave’s RF transceiver link can provide a reference for exploring and designing a vortex electromagnetic wave in wireless communication and radar detection and imaging research.
2021, 10(5): 785-793.
Terahertz vortex beams can be used to improve the communication capacity of radar communication systems and the resolution of imaging systems. This paper presents a deflective vortex beam generation method based on a reflective metasurface working in the terahertz band. Without the limitations of traditional methods, metasurfaces are a good candidate to generate beams carrying an orbital angular momentum in the terahertz band. First, we designed and simulated a unit cell of the metasurface. The unit cell of our design consists of two metallic (gold) layers and one dielectric layer. An almost 360° phase shift was acquired by adjusting the length of the eight stubs of the top layer. The unit cell of the metasurface was simulated by CST Microwave Studio, and the simulation results showed that the co-polarization reflection efficiencies of the unit cells were more than 90%. To avoid performance degradation due to blockage of the feed horn, we controlled accurately the directions of vortex beams based on the concept of reflectarray. To verify the performance of our design, we simulated and measured five reflective metasurfaces. The results of simulation and measurement showed that these metasurfaces could generate five deflective vortex beams in the terahertz band. The topological charges of these beams are ±1, ±2, and 3, which account for the highest energy proportion in different vortex beams.
Terahertz vortex beams can be used to improve the communication capacity of radar communication systems and the resolution of imaging systems. This paper presents a deflective vortex beam generation method based on a reflective metasurface working in the terahertz band. Without the limitations of traditional methods, metasurfaces are a good candidate to generate beams carrying an orbital angular momentum in the terahertz band. First, we designed and simulated a unit cell of the metasurface. The unit cell of our design consists of two metallic (gold) layers and one dielectric layer. An almost 360° phase shift was acquired by adjusting the length of the eight stubs of the top layer. The unit cell of the metasurface was simulated by CST Microwave Studio, and the simulation results showed that the co-polarization reflection efficiencies of the unit cells were more than 90%. To avoid performance degradation due to blockage of the feed horn, we controlled accurately the directions of vortex beams based on the concept of reflectarray. To verify the performance of our design, we simulated and measured five reflective metasurfaces. The results of simulation and measurement showed that these metasurfaces could generate five deflective vortex beams in the terahertz band. The topological charges of these beams are ±1, ±2, and 3, which account for the highest energy proportion in different vortex beams.
微信 | 公众平台 
