Showing papers by "Jie Chen published in 2013"

Explicit conditions for stabilization over noisy channels subject to SNR constraints

Abstract: In this paper, we analyze the stability of networked control systems when parallel additive white noise (AWN) channels are considered. We assume that the parallel AWN channels are individually constrained in its signal to noise ratio (SNR). We obtain explicit conditions on the minimum requirements on the channel SNRs necessary for stability for a set of particular plant structures. Our results show explicitly the role played by the unstable plant pole directions in the stability of NCSs. We also show that, when state feedback control is used, a simple characterization on the channel SNRs for stability can be given.

Consensus over directed graph: Output feedback and topological constraints

Abstract: In this paper, the discrete-time multi-agent systems consensus problem over a directed, fixed network communication graph is studied. A distributed dynamic output feedback control protocol is employed. Drawing upon concepts and techniques from robust control, notably those concerning gain-phase margin optimization and analytic interpolation, we obtained explicit sufficient conditions for general linear agents to achieve consensus. The results display an explicit dependence of the consensus condition on the agent's unstable poles, non-minimum phase zeros and their relative degree.

Multi-agent consensus under delayed feedback: Fundamental constraint on graph and fundamental bound on delay

Abstract: In this paper, we study the consensus problem for discrete-time multi-agent systems over an undirected, fixed network communication graph. We assume that the agents' input is subject to a constant albeit possibly unknown time delay, and employ a distributed dynamic state feedback control protocol. Drawing upon concepts and techniques from robust control, notably those concerning gain margin optimization and analytic interpolation, we derive explicit, closed-form conditions for general linear agents to achieve consensus. Our results display an explicit dependence of the consensus condition on the agent's unstable dynamics and the delay value, showing that delayed communication between agents will generally hinder consensus and impose restrictions on the network topology. We also show that a lower bound on the maximal delay allowable for consensus can be computed by a simple line search method.

Assessment of Intratumoral Micromorphology for Patients with Clear Cell Renal Cell Carcinoma Using Susceptibility-Weighted Imaging

Abstract: Background Multiple treatment options exist for the management of renal cell carcinomas. Preoperative evaluation of clear cell renal cell carcinoma (CRCC) grades is important for deciding upon the appropriate method of therapy. We hypothesize that susceptibility weighted imaging (SWI) is sensitive enough to detect intratumoral microvessles and microbleeding in renal cell carcinoma, which can be used to grade CRCC. Material and Methods Retrospective reviews of 37 patients with pathologically proven CRCCs were evaluated. All patients underwent SWI examinations. The characteristics of intratumoral susceptibility signal intensity (ITSS) includes the likelihood of the presence of ITSS, morphology of ITSS, dominant structure of ITSS and ratio of ITSS area to tumor area, which were all assessed on SWI. The results were compared using the nonparametric Mann-Whitney test. Results ITSS was seen in all patients except 4 patients with low-grade CRCCs. There was no significant difference between low and high-grade CRCCs when looking at the likelihood of the presence of ITSS. There was a significant difference in the mean score of dominant structures between low and high-grade CRCCs. Specifically, more dominant vascular structures and less hemorrhage were seen in low-grade tumors (2.15±1.05) compared to high-grade tumors (1.27±0.47) (P<0.005). The ratio of ITSS area to tumor area was also significantly higher for the high-grade group (1.55±0.52) than that for the low-grade group (0.88±0.43) on SWI (P<0.005). Conclusion SWI is useful for grading CRCCs.

A refined Omega-K algorithm for focusing highly squint airborne stripmap SAR data

Abstract: The squint synthetic aperture radar (SAR) is capable of improving the coverage performance of SAR system while suffering from large range cell migration (RCM) and heavy computation load. To alleviate RCM, this paper proposes an innovative sliding receive-window (SRW) technique, in which the receive-window starting time varies pulse by pulse as a function of range-walk. Moreover, a refined Omega-K algorithm is deduced for focusing highly squint airborne stripmap SAR using SRW technique. In refined Omega-K, a new stolt interpolation relationship is found for the RCM is modified by the SRW technique. Finally, the imaging results justify the validity and accuracy of the refined Omega-K algorithm.

The mean-square stabilization of networked feedback systems with channel delays

Abstract: This work studies a stabilization problem via output feedback for networked linear time-invariant (LTI) discrete-time systems with quantization effects and network-induced delays. Logarithm quantization laws are adopted in the systems and the quantization errors are modeled as multiplicative noises. The network-induced delays under consideration are fixed delays. The necessary and sufficient condition of mean-square stabilization is studied for a SISO LTI networked control system. And then an analytic expression for the trade-off between quantization noise and channel delays in the mean-square stabilization problem is presented. The largest quantization noise variance with which the mean-square stability of the networked feedback system is guaranteed is obtained in terms of the fixed channel delay. While, the largest delay step which is allowed in the systems is obtained for a given quantization noise variance. Moreover, these results are extended to the case for minimum phase MIMO LTI systems.

Small-gain stability conditions for linear systems with time-varying delays

Abstract: Sufficient stability conditions for linear systems subject to time-varying delays are developed in this paper, which are in the form of scaled small gain conditions and can be checked readily by solving corresponding linear matrix inequality problems. From a robust control perspective, our development seeks to cast the stability problem as one of robust stability analysis, and the resulting stability conditions are also reminiscent of robust stability bounds typically found in robust control theory. Examples show significant improvement of our results over the existing criteria.

Segmented BAQ algorithm for SAR raw data of strong target in weak background

Abstract: In order to improve the performance of block adaptive quantization (BAQ) when dynamic range of the Synthetic Aperture Radar (SAR) echo is far larger than the radar receiver's, this paper proposes a segmented BAQ algorithm for SAR raw data of strong target in weak background. Firstly, unlike traditional ADC processing, the 12-bit ADC system is used before BAQ. The quantization and saturation error of 12-bit ADC system is analyzed. Moreover, the mapping relationship between standard deviation of output signal (SDOS) of the ADC and the average signal magnitude (ASM) for 12-bit ADC is deduced. Secondly, the concept of the segmented compression is proposed and a segmented non-uniform scalar quantizer for SAR raw data of strong target in weak background is designed. Thirdly, the encoding scheme is provided. Experiment results based on simulated data show that the performance of the proposed algorithm is better than that of traditional BAQ.

Spaceborne HF/VHF-radar system for ionosphere sounding

Abstract: Ionosphere sounding is of significant importance for compensating ionospheric effects on satellite communication, navigation and remote sensing systems at lower frequency, i.e. L-band, P-band etc.. A novel spaceborne HF/VHF-radar scheme is proposed for ionosphere sounding. Based on Frequency Division (FD) technique, the mode for large-scale ionospheric structure sounding can generate a two-dimensional (2D) topside electron density image. Based on synthetic aperture technique, the mode for small-scale ionospheric irregularities imaging can generate a 3D irregularities image. The effectiveness of the proposed SAR system was verified by means of computer simulations. The simulation results show that the resolution of large-scale sounding mode is as high as 0.49km, and the resolution of small-scale sounding mode is as high as range 0.9km, azimuth 0.27km and across-track 0.25°.

Attitude steering strategy for agile small SAR satellite with sliding spotlight mode

Abstract: Agile small SAR has broad applications, due to its high mobility and wide coverage. In this paper, we put forward an attitude steering strategy for agile small SAR satellite with sliding spotlight mode. This attitude steering strategy contains three steps. And according to the results of simulation, this attitude steering strategy has a very good performance.

Effects of satellite attitude jitter on spaceborne multi-channel SAR image qualities

Abstract: Attitude jitter will cause zygomorphous paired-echoes after azimuth focusing for traditional spaceborne SAR system. However, for multi-channel spaceborne SAR system, attitude jitter of one sub-antenna will not only cause the paired-echoes but also affect other sub-antennas. This paper first built the model of antenna jitter for multi-channel SAR system. As a next step, the amplitude and position of paired-echoes secondary derivative paired-echoes were illuminated by mathematic derivation. Moreover, the effects of satellite attitude jitter on multi-channel SAR image qualities were analyzed as well. Finally, the simulation results justified the validity of theoretical analysis results.

A novel subband fusion method for SAR echo combined with compressed sensing

Abstract: In this paper, a novel sub-band fusion technique for SAR echo using compressed sensing was proposed. As we all known, high resolution and wide swath are the trend of SAR systems, while the range resolution of SAR is limited by the bandwidth of transmitted signal. Our proposed algorithm aimed to take the compressed sensing and sub-band fusion into the SAR systems, realize high resolution and reduce the amounts of SAR conveyed data. Internal calibration was used to extract the phase error from each channel. And Orthogonal Matching Pursuit algorithm was used for range signal recovery. After coherent processing, we can acquire ultra-wide bandwidth echo. And finally, refined range Doppler algorithm was employed for range cell migration correction and azimuth compression. We proved the proposed algorithm used the fusion experiment at the end of the paper.

A refined chirp scaling algorithm for high-resolution spaceborne SAR based on the fourth-order model

Abstract: With the resolution of SAR developing continually, it is found that in the process of image processing, using equivalent squint range model would bring large phase error. So using traditional CS algorithm for processing will encounter difficulties. In order to solve this problem because of unsuitable slant range model, this paper introduces the fourth-order slant range model to replace the original model, and conducts a study on CS algorithm based on the fourth-order slant range model. This paper introduces the theoretical formulation and simulation results of the new algorithm. This algorithm can perform well for both low-resolution space-borne SAR and high-resolution space-borne SAR.

A refined three-step focusing algorithm based on spatial variation characteristic of spaceborne sliding spotlight SAR

Abstract: This paper deals with the problem of azimuth imaging worsening when three-step focusing algorithm fulfills imaging requirements of sliding spotlight SAR with wide azimuth swath. This is caused by spaceborne sliding spotlight SAR spatial variation characteristic: single point target's relatively large range cell migration and azimuth point targets' different Doppler experiences at different azimuth locations. To solve this problem phase error introduced by the equivalent strabismus model is compensated and the reference Doppler rate in the first step of TSFA is modified. A comparison of the imaging results to one azimuth 15km swath scene between refined TSFA and original TSFA is exhibited through simulations. The results prove that refined TSFA is able to get consistent good azimuth imaging quality of wide swath scene.