A multiband fusion (MF) method can estimate a full-band echo (FBE) with a bandwidth larger than the sum of bandwidths of several sub-bands echoes. To improve estimation speed and estimation accuracy of the FBE, a novel MF method based on a small multiband-measurement matrix and a nonconvex log-sum regularization is proposed and experimentally verified. In the method, the multiband-measurement matrix which is in a sub-bands-echoes expression provides a beneficial tool for speeding up the estimation of the FBE due to its small size. The small size is obtained by constructing the matrix only using parameters of existing sub-bands and ignoring parameters of gaps between sub-bands. The log-sum regularization is developed in a multiband cost function to improve the estimation accuracy of the FBE, for its corresponding geometric property is similar to a geometric property of an l ₀ -norm regularization that can guarantee an unbiased estimate of FBE. Additionally, since the imaging scene is sparse and the lack of several pulses does not affect the sparsity, the proposed method is robust to missing pulses. Simulation-data and real-date experimental results proved that the bandwidth of a fused FBE is 1.67 times the sum bandwidth of two sub-bands, the estimation accuracy and the estimation speed corresponding to the proposed method are 5.56 times and 1.11 times those of traditional MF methods, respectively, and the proposed MF method is still feasible even when 10 % of pulses are randomly missing.

A resolution-improving method for multiband imaging based on an extrapolated RELAX algorithm (ERA) is proposed. The proposed method improves image resolution by reconstructing a wideband signal through nonadjacent narrow subbands. The key points of this method are the coherent compensation of the subbands and the accurate extraction of target point scatters’ parameters from each subband. For coherent compensation, a two-step phase compensation (TSPC) is used to precisely compensate for the phase incoherence terms between the subbands. In order to accurately extract the parameters of point scatters (PSs), the ERA is proposed, which establishes a cost function between the extrapolated subbands and the PSs’ parameters. Furthermore, to enhance the robustness of PSs’ parameters extraction, a local band purification (LBP) process is proposed to resist the non-Gaussian clutter in the subbands. To validate the performance of the proposed method, Monte Carlo experiments using simulated data are carried out, whose results show that the proposed method can successfully improve the image resolution through the subbands under low signal-to-clutter ratio (SCR). Moreover, experiments with real measured data are conducted, using two 0.75 GHz subbands to reconstruct a 5 GHz signal, whose 2D image result is approximated to that based on the real 5 GHz signal. Image parameters are also compared, whose results demonstrate that the proposed method has satisfactory accuracy and robustness on improving image resolution.

Vortex electromagnetic waves carrying orbital angular momentum (OAM) enable differentiated scaling of multiple targets within the beam coverage, granting them superior theoretical performance in multi-dimensional multi-target staring imaging. Owing to the approximate dual relationship between OAM modes and target azimuth angles, multi-mode vortex scanning can rapidly and accurately determine spatial azimuth information of targets, significantly enhancing radar imaging resolution and speed in the azimuthal dimension. Recently, multiple factors, including insufficient OAM mode diversity, complex noise environments and low mode purity, collectively hinder the practical implementation of existing vortex imaging algorithms for high-resolution OAM based radar systems. To address these challenges, this article proposes a high-resolution RELAX algorithm for vortex imaging technology based on precise analysis of echo signal components. The method treats echo signals as superpositions of multiple complex sinusoidal signals and achieves accurate extraction of target positional information through iterative refinement. A comparative analysis of spectral estimation methods, sparse recovery methods and the RELAX method is presented. Experimental results demonstrate that compared to conventional imaging algorithms, the RELAX algorithm exhibits superior target recovery capability, effectively suppresses spurious targets, mitigates noise interference, and significantly improves image resolution, which is particularly pronounced under limited OAM mode diversity. The experimental findings reveal that under with the OAM modes of [-7,7], the RELAX algorithm achieves a limiting resolution of 6.7cm×19°, with the normalized mean-square error of 0.4557 and the image correlation value of 0.6917. This approach substantially alleviates the inherent dependence on OAM mode quantity in vortex radar systems, providing a robust resolution for practical scenarios with constrained electromagnetic (EM) resources. The proposed method notably expands the application scope of vortex EM waves in radar imaging architectures, establishes a solid new technical pathway for vortex radar signal processing, and facilitates the practical deployment of vortex radar.

A photonic signal generation scheme based on recirculating frequency-shifting and channelized sub-band stitching is proposed to synthesize high-quality multi-octave broadband linear frequency-modulated (LFM) signals. The scheme employs a hierarchical two-stage approach to expand the bandwidth of a narrowband seed signal. First, recirculating frequency-shifting generates and stitches frequency-shifted signals to obtain a broadband sub-band waveform with first-stage bandwidth expansion. Then, multiple broadband sub-band signals in different channels are precisely aligned and stitched to achieve secondary bandwidth extension, effectively breaking the octave-spanning limitation. The partitioned-channel structure ensures superior spurious suppression performance. A proof-of-concept experiment is designed to demonstrate the generation of a 5–17 GHz cross-octave LFM signal with a 12 GHz bandwidth, a relative bandwidth of 1.09, and an in-band spurious suppression ratio (SSR) of 35 dB, covering the C, X and Ku bands. To verify system flexibility, signals with varying center frequencies and bandwidths are generated without any hardware modification. To assess the practicality of the generation in an actual imaging system, microwave imaging experiments are also conducted. To the best of our knowledge, the proposed ultra-wideband generation has the best SSR in the generation of cross-octave signals, which holds significant application value for high-quality, high-resolution radar imaging.