Quasi-Vortex Electromagnetic Wave Radar Forward Looking based on Echo Phase Weighting
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摘要: 携带轨道角动量(Orbital Angular Momentum, OAM)的涡旋电磁波(Vortex Electromagnetic wave, VEMW)由于不同OAM模式之间的正交性，在雷达前视成像方面表现出重要的应用潜力，引起国内外学者的广泛关注。在VEMW成像技术中，通常采用均匀圆环阵列(Uniform Circular Array, UCA)来生成和发射多模态VEMW照射成像目标，以获得回波。然而，UCA产生的VEMW主瓣发散的特点，导致在自由空间中产生的电磁波能量分布不集中，从而使得雷达回波能量相对较弱。此外，传统涡旋电磁波雷达在工作过程中需要不断切换OAM模式，这一过程增加了系统的复杂性。为了应对上述挑战，该文首先建立了准圆环阵列(Quasi-Circular Array, QCA)的电场模型，通过QCA得到了能量更加集中的准涡旋电磁波。同时，提出了一种对每个阵元的接收回波进行多OAM模式相位加权的方法，得到等效多模式接收回波。仿真结果表明，相比于基于UCA的涡旋电磁波雷达成像方法，该文所提方法方位分辨率提高了两倍以上，并且有效地提高了回波能量，在信噪比低至–15 dB的环境下，重建的目标仍然清晰可见。该文所提方法在雷达前视成像领域具有较大的实际应用潜力。Abstract:
Objective Forward-looking radar imaging plays a critical role in multiple applications. Numerous algorithms have been proposed to enhance azimuth resolution; however, improvement remains difficult due to the limitations imposed by antenna aperture. Existing high-resolution techniques, including synthetic aperture radar and Doppler beam sharpening, rely on Doppler bandwidth and inevitably create blind spots in the forward-looking region. Vortex electromagnetic waves carrying orbital angular momentum offer potential in forward-looking scenarios because of the orthogonality between different orbital angular momentum modes. In conventional vortex electromagnetic wave imaging, a Uniform Circular Array (UCA) is used to generate and transmit multi-mode vortex electromagnetic waves. Yet, the UCA-generated waves suffer from main lobe divergence, which disperses energy and weakens echo signals, while multi-mode transmission increases system complexity. To address these issues, this paper proposes a Quasi-Circular Array (QCA) that reduces system complexity, produces vortex electromagnetic waves with more concentrated main lobes, and preserves phase linearity. In addition, a post-processing method based on echo phase weighting is introduced. By applying phase modulation to the single-mode echo received by each antenna element, a complete equivalent multi-mode echo is synthesized. The proposed method enhances azimuth resolution and exhibits strong anti-noise performance. Methods To obtain clear images under low Signal-to-Noise Ratio (SNR) conditions, a phase modulation echo post-processing method combined with a QCA is proposed. The QCA first generates a single-mode vortex electromagnetic wave to illuminate the region of interest. Each element of the array then receives and stores the echo. Phase modulation is subsequently applied to the stored echo to generate signals of specific modes, thereby synthesizing an equivalent multi-mode echo with enhanced amplitude that preserves target information. This approach demonstrates strong potential for practical applications in forward-looking radar imaging under low SNR conditions. Results and Discussions When noise is added to the echo and imaging is performed ( Figure 11 ), the proposed method achieves superior results under noisy conditions. As noise intensity increases, a clear target can still be reconstructed at a SNR of –10 dB. Even when the SNR is reduced to –15 dB and the target is submerged in noise, the contour features of the reconstructed target remain distinguishable. These results demonstrate that the method has strong anti-noise performance. In addition, when imaging is performed within a smaller mode range, the azimuth resolution achieved by the proposed method improves by an average factor of 2.2 compared with the traditional method (Figure 9 ). The improvements in resolution and anti-noise performance can be attributed to two factors: (1) The vortex electromagnetic waves generated by the QCA experience reduced destructive interference due to the asymmetric spatial distribution of array elements, producing waves with more concentrated main lobes, lower side lobes, and higher radiation gain. (2) Applying phase modulation in echo processing reduces the pulse repetition frequency of the vortex electromagnetic wave at the transmitting end, thereby lowering system complexity.Conclusions This study proposes a method capable of effective imaging under low SNR conditions. The echo expression of the electric field generated by the QCA is derived, and the radiation gain and phase characteristics of the quasi-vortex electromagnetic wave are analyzed. In addition, an echo post-processing method based on phase modulation is introduced. Simulation results demonstrate that, compared with the traditional UCA method, the proposed approach generates vortex electromagnetic waves with more concentrated main lobes, lower side lobes, and higher gain, while improving azimuth resolution by a factor of 2.2. Even at a SNR of –15 dB, the reconstructed imaging results remain distinguishable. -
表 2 雷达系统参数
符号 值 载频 $ f_{e} $ 9.6 GHz 带宽 $ B_{ \pm 0} $ 50 MHz 阵元个数 $ M $ 13 电尺寸半径 $ \bar{a} $ 2 发射的OAM模态 $ t_{r} $ 1 
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表 3 不同模态下的方位向评估指标
方法 方位向分辨率 (°) 峰值旁瓣比 (dB) [–4,4] [–5,5] [–6,6] [–4,4] [–5,5] [–6,6] 所提方法 12.4 9.3 4.8 –21 –20 –30 基于UCA的方法 23.9 18.1 16 –15 –11 –11
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