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Space-time waveform-encoding (STWE) synthetic aperture radar (SAR) has great potential to improve the performance of the future spaceborne SAR system. With the aim of solving the problem of overlapped echoes, one effective approach involves the use of digital beamforming (DBF) in elevation combined with the null-steering techniques. However, the traditional real-time echo separation scheme with single-null-steering techniques occupies huge digital resources. In this letter, a novel real-time echo separation processing scheme in intermediate frequency domain is proposed to overcome the disadvantage of the conventional scheme. Based on the 2-D simulation results, the proposed scheme has the same accuracy of separating the overlapped echoes as the traditional scheme. The detailed analysis of the system complexity indicates that the proposed scheme can greatly reduce the resource occupation. Moreover, the effectiveness of the proposed scheme is further verified and demonstrated by the experimental results on the X-band 16-channel DBF-SAR system. [ABSTRACT FROM AUTHOR]

The appearance of fatigue is not conducive to driving activities because this state can affect driving performance and even cause life-threatening consequences. To reduce various traffic accidents caused by fatigue, researchers began to explore effective fatigue monitoring systems to detect this unfavorable state early. This paper systematically surveys the research on driving fatigue monitoring from three aspects: data acquisition, feature extraction, and fatigue assessment. Furthermore, this paper analyzes the differences between active and passive fatigue, fatigue and sleepiness, as well as fatigue and transportation scenarios. Finally, some open issues on driving fatigue monitoring are proposed, which will be the key research directions for future developments. [ABSTRACT FROM AUTHOR]

Multiple-input and multiple-output synthetic aperture radar (MIMO SAR) is a very potential technique for high-resolution and wide-swath (HRWS) imaging due to the fact that it can provide more degrees of freedom. In this article, an innovative MIMO-HRWS SAR imaging system is introduced with segmented phase coding (SPC) waveforms. However, it is well-known that the echo separation issue is the most technical challenge for MIMO SAR. It has shown that the separation of orthogonal echoes from cross correlation interference can be achieved by scan-on-receive (SCORE). However, this method often requires large computing resources on board, especially for null-steering process. For this, we propose a two-stage spaceborne–ground echo separation scheme. This scheme is mainly divided into two steps: the first step is a real-time SCORE beamforming process on board to reduce the downlink data volume, and the second step is a bandpass–null-steering process to suppress the interfering echoes on the ground. Thus, this scheme allows for the low computing resources on board and sufficient suppression of the interference simultaneously. Following this scheme, the MIMO-HRWS SAR system enables its number of equivalent phase centers nearly double that of the azimuth multichannel SAR system, leading to higher resolution and wider swath imaging. Finally, detailed simulation experiments for the MIMO-HRWS SAR imaging system are performed to verify the practicability and feasibility of the proposed scheme. [ABSTRACT FROM AUTHOR]

Digital beamforming (DBF) with scan-on-receive (SCORE) technique in elevation is a powerful technique that enables a spaceborne synthetic aperture radar (SAR) to achieve high-resolution wide swath (HRWS) imaging. In the spaceborne DBF-SAR system, sampling signals from each channel are weighted by weights generated by a digital signal processing system in real-time. However, the contradiction between the shortage of spaceborne hardware resources and resource demand of the multichannel real-time signal processing increases the difficulty of system design. In order to solve this problem, a novel weight generator and an improved intermediate frequency (IF) DBF real-time processing architecture are proposed in this article. By taking advantage of the special properties of the SCORE algorithm, the proposed weight generator calculates weights using a linear polynomial algorithm. The simulation result shows that a low-order approximation can achieve high performance. The proposed generator can correct multichannel amplitude and phase error at a low cost on hardware resources. The effectiveness of the proposed method is verified by experiments with a raw data processing instance of an X-band 16 channels DBF-SAR. [ABSTRACT FROM AUTHOR]

The displaced phase center antennas in azimuth and digital beamforming (DBF) in elevation are two state-of-the-art techniques for achieving high-resolution wide swath (HRWS) imaging in the multichannel synthetic aperture radar (SAR) systems. However, due to the atmospheric turbulence, airborne HRWS-SARs inevitably suffer trajectory disturbances, which will consequently defocus the SAR image. Although various motion compensation (MoCo) methods have been proposed, they are mostly designed for the traditional single-channel SAR and, therefore, ignore the channel-dependent posture error. The posture motion error will introduce residual motion and time-variant channel errors, which not only defocuses the image but also causes azimuth ambiguity and degrades the SNR improvement of the final images of the azimuth multichannel and DBF-SAR, respectively. To solve these problems, a novel MoCo scheme for 2-D multichannel SAR systems is proposed. The posture error is described and calculated in matrix form through the use of quaternions. Then, the posture error is transformed into the translational motion error for each receiving channel and is subsequently compensated precisely. To address the residual aperture-variant motion error, a modified aperture-dependent MoCo is integrated into the proposed scheme. Simulations and airborne experiments, including processing the data of a C-band SAR system with four azimuth channels and an X-band DBF-SAR system with 16 elevation channels, have been implemented to validate the effectiveness of the proposed MoCo scheme. [ABSTRACT FROM AUTHOR]

In the Earth observation mission of the synthetic aperture radar (SAR), wide swath can be used to complete global monitoring in a short time and high resolution can provide rich detailed information about the feature space and prominent structure and texture. However, the traditional single-channel classical SAR system cannot meet high-resolution and wide-swath (HRWS) imaging demand due to the constraint of minimum antenna area. Fortunately, this fundamental limitation can be overcome by using multiple receive subapertures in combination with advanced digital beamforming (DBF) technique. DBF in elevation can provide high gain and better system performance and has recently gained much attention in the field of SAR imaging. This article presents a 16-channel in elevation airborne X-band DBF-SAR system with 500-MHz bandwidth, characterized by high speed data acquisition and storage, as a test bed to provide the technical reserves and support for a future spaceborne DBF-SAR system in China. The hardware configuration of this system is designed according to a realistic flight mission. To verify the feasibility and operability of this advanced 16-channel DBF-SAR system, an outfield airborne flight experiment was successfully conducted in eastern Guangdong Province in November 2019. Meanwhile, considering the inevitable channel mismatch from airborne system, a precise strategy as well as the underlying signal processing is proposed to process the experiment data. In addition to the channel mismatch due to the topographic height, the Scan-On-Receive (SCORE) pattern loss (SPL) is also an inherent factor, which will deteriorate the output SNR in final SAR images. Therefore, this article also implements a quantitative assessment of SPL combined with the practical flight parameters and the real airborne data. Finally, the corresponding processing results are presented and analyzed in detail. The practical SNR improvement of 11.23 dB emphasize that DBF technology can significantly improve the quality of SAR images and will make an essential contribution to next generation of HRWS technology for environment monitoring. [ABSTRACT FROM AUTHOR]

Arrhythmia is a kind of cardiovascular disease that seriously threats human health. Intelligent analysis of electrocardiogram (ECG) is an effective method for the early prevention and precise treatment to arrhythmia. In clinical ECG waveforms, it is common to see the multi-label phenomenon that one patient would be labeled with multiple types of arrhythmia. However, the current research is mainly to use the multiclass methods to solve the multi-label problem, ignoring the correlations between diseases and causing information loss. Therefore, this article aims: 1) to propose a multi-label feature selection method based on ECG (MS-ECG) and design an evaluation criterion of ECG features based on kernelized fuzzy rough sets so as to choose the optimal feature subset and optimize ECG feature space and 2) to propose the multi-label classification algorithm of arrhythmia based on ECG (MC-ECG) by establishing a multiobjective optimization model. This algorithm based on sparsity constraint explores the correlations between arrhythmia diseases and analyzes the mapping relationship between ECG features and arrhythmia diseases, so that one ECG signal would be automatically and accurately given multiple labels. Through sufficient experiments to prove the feasibility of our methods, we obtain the selected feature subset composed of 23 ECG features by MS-ECG. For the six evaluation criterions of MC-ECG, average precision is 0.8462, hamming loss is 0.1041, ranking loss is 0.1313, one-error is 0.2023, coverage is 0.4015, and micro-F1 is 0.6088. The outcome presents optimal to the current algorithms. [ABSTRACT FROM AUTHOR]

The azimuth phase coding (APC) technique is known for its very low implementation complexity and its effectiveness for point and distributed ambiguities in conventional synthetic aperture radar (SAR) systems. In recent years, as an extension, the APC technique has been briefly discussed for multichannel SAR systems. However, the properties of the APC technique are no longer guaranteed in the multichannel SAR systems based on the digital beamforming (DBF) on-receive, and only a slight APC gain in the suppression of the range ambiguity can be obtained. In this article, we first provide a more thorough analysis for an APC-multichannel SAR system with respect to a uniform pulse-repetition frequency (PRF). Then, the APC/multichannel SAR system with nonuniform spatial sampling is briefly discussed, and an improved reconstruction approach based on a quadratically constrained optimization model is proposed to increase greatly the APC gain with respect to existing multichannel reconstruction algorithms. This proposed approach allows the minimization of the range ambiguity with a given azimuth-ambiguity constraint. In particular, for some specific PRFs, the proposed method permits a cancellation of the odd-order range ambiguity. Finally, simulation experiments are performed to verify the advantages and effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

In this letter, we examine a new measure for high resolution and wide swath (HRWS) synthetic aperture radar (SAR) imaging based on an elevated frequency diversity array (EFDA). By highly integrating digital beamforming (DBF) and frequency diversity array (FDA) techniques, EFDA–SAR achieves range ambiguity resolution in the spatial frequency domain and range ambiguity suppression outside the observed swath in the range space domain. Moreover, the EFDA–SAR system improves the signal-to-noise ratio (SNR) due to its elevated antenna array design. A model is developed for the time-varying filtering of this novel EFDA-SAR system design. Simulation results are provided to demonstrate the efficiency of the proposed design. Using EFDA–SAR, we can obtain an HRWS SAR image without range ambiguity from the observed swath or outside it. Moreover, by combining DBF on reception, the SNR of the EFDA-SAR image is significantly improved. [ABSTRACT FROM AUTHOR]

Digital beamforming (DBF) is an effective method for improving the signal-to-noise ratio (SNR) in synthetic aperture radar (SAR) images. However, further enhancing the SNR in DBF-SAR is limited by the antenna size and the expensive system cost. To address this problem, in this letter we implement a hybrid strip-map/spotlight mode for DBF. By utilizing the proposed processing method, DBF is adapted for hybrid strip-map/spotlight mode data. Furthermore, the additional synthetic aperture acquired from the hybrid strip-map/spotlight mode is employed to implement multilook processing for further enhancing the SNR of DBF-SAR images. The proposed processing method was verified by an airborne X-band 16-channel DBF-SAR system. The results confirmed that a remarkable additional SNR improvement in DBF-SAR images can be obtained using the proposed method without increasing system complexity. [ABSTRACT FROM AUTHOR]