Showing papers by "Jie Chen published in 2014"

Bipartite consensus of general linear multi-agent systems

Abstract: Collective behaviors of multi-agent systems over signed graphs find applications in a variety of scenarios including social networks, predator-prey dynamics, which however have not been adequately addressed as their counterparts with nonnegative graphs. This paper studies bipartite consensus problem of general linear multi-agent systems over signed digraphs. First, we show that for general linear agents, bipartite consensus over signed graphs and ordinary consensus over nonnegative graphs are equivalent. This indicates that prevailing consensus controllers for nonnegative graphs can be adopted to solve bipartite consensus problems. Based on this observation, an existing Riccati equation based cooperative tracking controller is extended to solve the bipartite consensus problem for general linear systems.

Improved Compact Polarimetric SAR Quad-Pol Reconstruction Algorithm for Oil Spill Detection

Abstract: An improved reconstruction algorithm is proposed for compact polarimetric (CP) synthetic aperture radar (SAR) on oil spill detection. Based on the differences in statistical behavior between open and oil-covered sea surfaces, the proposed algorithm can iteratively reconstruct quad-pol SAR images from CP SAR data. During the experiment, it outperformed two existing compact SAR reconstruction algorithms in terms of both statistical and information theoretical analysis.

Consensus of second-order heterogeneous multi-agent systems under a directed graph

Abstract: In this paper, we study the consensus problem for second-order heterogeneous multi-agent systems under a directed graph. Unlike the existing consensus algorithms for second-order multi-agent systems in which all agents are assumed to have common unit inertias or share common control gains, we allow the inertias and the control gains to be heterogeneous for each agent. Fully distributed consensus algorithms are proposed when there exist, respectively, absolute velocity damping and relative velocity damping. Moreover, an adaptive σ-modification scheme for the gain adaption is proposed, which renders smaller control gains and thus requires smaller amplitude on the control input without sacrificing the consensus convergence.

Mitigation of Azimuth Ambiguities in Spaceborne Stripmap SAR Images Using Selective Restoration

Abstract: A novel framework is proposed for mitigating azimuth ambiguities in spaceborne stripmap synthetic aperture radar (SAR) images. The azimuth ambiguities in SAR images are localized by using a local mean SAR image, SAR system parameters, and a defined metric derived from azimuth antenna pattern. The defined metric helps isolate targets lying at locations of ambiguities. The mechanism for restoration of ambiguity regions is selected on the basis of size of ambiguity regions. A compressive imaging technique is employed to restore isolated ambiguity regions (smaller regions of interconnected pixels), whereas clustered regions (relatively bigger regions of interconnected pixels) are filled by using exemplar-based inpainting. The simulation results on a real TerraSAR-X data set demonstrated that the proposed scheme can effectively remove azimuth ambiguities and enhance SAR image quality.

Consensus of linear multi-agent systems with fully distributed control gains under a general directed graph

Abstract: In this paper, we study the leaderless consensus problem for general linear multi-agent systems under a general directed graph. A distributed consensus algorithm with gain adaption is proposed. A novel integral-type Lyapunov function is constructed to study the consensus convergence. The control gains are varying and updated adaptively by distributed adaptive laws. The proposed algorithms require no global information and thus can be implemented in a fully distributed manner.

On stabilizability of MIMO systems over parallel noisy channels

Abstract: In this paper we study the mean square stabilizability of MIMO discrete-time linear time-invariant plants over parallel additive white noise channels. We also consider a basic linear encoder-decoder scheme based on the design of a diagonal scaling matrix. We analyse the case of non minimum phase plants in an output feedback control setting. We obtain necessary and sufficient conditions for mean square stabilization when each SISO branch of the channel is subject to an individual signal-to-noise ratio constraint. We characterize a region where the set of constraints are compatible with mean square stability.

Output feedback stabilisation of single-input single-output linear systems with I/O network-induced delays. An eigenvalue-based approach

Abstract: This work addresses the output feedback stabilisation problem for a class of linear single-input single-output systems subject to I/O network delays. More precisely, we are interested in the characterisation of the set of delay and gain parameters guaranteeing the stability of the closed-loop system. To perform such an analysis, we adopt an eigenvalue perturbation based approach. Various illustrative numerical examples complete the presentation.

Fundamental bounds on delay margin: When is a delay system stabilizable?

Abstract: This paper concerns the stabilization of linear systems subject to unknown, possibly time-varying delays. Drawing upon analytic interpolation and rational approximation techniques, we develop fundamental bounds on the delay margin, with which the delay plant is guaranteed to be stabilizable by a controller. Our contribution is threefold. First, for a single-input single-output system with an arbitrary number of plant unstable poles and nonminimum phase zeros, we provide an explicit, computationally efficient bound on the delay margin, which requires computing only the largest real eigenvalue of a constant matrix. Second, for multi-input multi-output systems, we show that estimates on the variation ranges of multiple delays can be obtained by solving LMI problems, and further, by computing the radius of delay variations. Third, we show that with appropriate care, these bounds and estimates can be extended to systems subject to time-varying delays. When specialized to more specific cases, e.g., to plants with one unstable pole and one nonminimum phase zero, our results give rise to analytical expressions exhibiting explicit dependence of the bounds and estimates on the pole and zero, thus demonstrating how fundamentally unstable poles and nonminimum phase zeros may limit the range of delays over which a plant can be stabilized.

Fast image-formation algorithm for ultrahigh-resolution airborne squint spotlight synthetic aperture radar based on adaptive sliding receive-window technique

Abstract: Adaptive sliding receive-window (ASRW) technique was usually introduced in airborne squint synthetic aperture radar (SAR) systems. Airborne squint spotlight SAR varies its receive-window starting time pulse-by-pulse as a function of range-walk, namely, the linear term of range cell migration (RCM). As a result, a huge data volume of the highly squint spotlight SAR echo signal can be significantly reduced. Because the ASRW technique changes the echo-receive starting time and Doppler history, the conventional image algorithm cannot be employed to directly focus airborne squint spotlight ASRW-SAR data. Therefore, a fast image-formation algorithm, based on the principle of the wave number domain algorithm (WDA) and azimuth deramping processing, was proposed for accurately and efficiently focusing the squint spotlight ASRW-SAR data. Azimuth deramping preprocessing was implemented for eliminating azimuth spectrum aliasing. Moreover, bulk compression and modified Stolt mapping were utilized for high-precision focusing. Additionally, geometric correction was employed for compensating the image distortion resulting from the ASRW technique. The proposed algorithm was verified by evaluating the image performance of point targets in different squint angles. In addition, a detailed analysis of computation loads in the appendix indicates that the processing efficiency can be greatly improved, e.g., the processing efficiency could be improved by 17 times in the 70-deg squint angle by applying the proposed image algorithm to the squint spotlight ASRW-SAR data.

Precise estimation of flight path for airborne SAR motion compensation

Abstract: Airborne synthetic aperture radar (SAR) systems are very sensible to deviations of the aircraft to the reference flight path. The trajectory errors can be divided into along-azimuth errors and the line-of-sight (LOS) displacement, while the latter one is the main source of motion errors. The LOS displacement can be obtained from the navigation data in which an ideal flight path should be assumed first. However, the accuracy of the assumed nominal trajectory is limited especially in high resolution SAR systems. To improve the precision of the estimation of LOS displacement, we present a novel method to acquire a precise flight path in the imaging intervals. This method can estimate trajectory deviation precisely by making full use of the navigation data.

Image formation algorithm for highly-squint strip-map SAR onboard high-speed platform using continuous PRF variation

Abstract: In highly-squint strip-map Synthetic Aperture Radar (SAR) onboard high-speed platform, the large range-walk is eliminated by continuously varying its pulse repetition interval (PRF), which overcomes the limitation of echo data collection. However, the accompanying problems of azimuth non-uniform sampling (ANS) and changing of Doppler history are inevitable. Therefore, an imaging formation algorithm for this SAR data imagery is proposed firstly. In the proposed algorithm, the range cell migration correction is performed to eliminate the changing of Doppler history. And the baseband Lagrange interpolation is implemented to reconstruct the ANS data. Then, the bulk compression function and stolt interpolation relationship are deduced in the focusing processing. Finally, the imaging results justify the validity and accuracy of the proposed algorithm.

Modified reconstruction method of squinted multi-channel SAR sigal

Abstract: Squinted multi-channel SAR is a available mode to achieve the high-resolution and wide-swath, because it is more flexible than broadside SAR. However, the traditional processing algorithm of broadside multi-channel mode is not suitable for squinted multi-channel mode, because the high squint angle will lead the reconstruction to fail. In this paper, a modified reconstruction method adapted to squinted multichannel SAR signal is proposed. The large central Doppler frequency of the squinted Doppler spectrum is taking into account, and the reconstruction method is modified accordingly. Meanwhile, the range walk correction is performed before the reconstruction to remove the mismatch between the reconstruction filters and the squinted signal with large range cell migration. Furthermore, the processing algorithm based on the wave-number algorithm framework with range walk correction is proposed. Finally, computer simulation results are presented to demonstrate the validity of the proposed algorithm.

Optimal regulation of linear discrete-time systems with multiplicative noises

Abstract: This paper studies optimal regulation problem for networked linear discrete-time systems with fading channel. The uncertainties in fading channels are modeled as multiplicative noises. The regulation performance is measured by a quadratic function. The optimal state feedback is designed by the mean-square stabilization solution to a modified Algebraic Riccati equation (MARE). The necessary and sufficient condition to the existence of the mean-square stabilization is presented in terms of the inherent characterizations of the systems. It is a nature extension for the result in standard optimal discrete-time linear quadratic regulation (LQR) problem. We also show that this optimal state feedback design problem is an eigenvalue problem (EVP). And then a design algorithm is developed for this optimal control problem.

[Evaluation of signal noise ratio on analysis of clear cell renal cell carcinoma using DWI with multi-b values].

Abstract: Objective To explore the influence of signal noise ratio (SNR) on analysis of clear cell renal cell carcinoma (CCRCC) using DWI with multi-b values. Methods The images of 17 cases with CCRCC were analyzed, including 17 masses and 9 pure cysts. The signal intensity of the cysts and masses was measured separately on DWI for each b value. The minimal SNR, as the threshold, was recorded when the signal curve manifest as the single exponential line. The SNR of the CCRCC was calculated on DWI for each b value, and compared with the threshold by independent Two-sample t Test. Results The signal decreased on DWI with increased b factors for both pure cysts and CCRCC. The threshold is 1.29±0.17, and the signal intensity of the cysts on DWI with multi-b values shown as a single exponential line when b≤800 s/mm2. For the CCRCC, the SNR is similar to the threshold when b=1 000 s/mm2 (t=0.40, P=0.69), and is lower when b=1 200 s/mm2 (t=-2.38, P=0.03). Conclusion The SNR should be sufficient for quantitative analysis of DWI, and the maximal b value is 1000 s/mm2 for CCRCC. Key words: Magnetic resonance imaging; Diffusion tensor imaging; Clear cell renal cell carcinoma

Accurate compensation of stop-go approximation for high resolution spaceborne SAR using modified hyperbolic range equation

Abstract: For high resolution spaceborne SAR system, the errors caused by straight trajectory assumption and stop-go approximation cannot be ignored. In this paper, an algorithm is proposed to compensate the above errors. First, a novel modified hyperbolic range equation (MHRE) is introduced to describe the satellite's orbit which is more precise than the traditional hyperbolic range equation (THRE). Then, the accurate signal model without stop-go approximation is deduced based on the MHRE. What's more, the residual phase errors in two-dimensional frequency domain caused by the two error sources are obtained. At last, the accuracy and validity of the proposed compensation algorithm are verified by the computer simulation results.

Improvements of Performance and Reliability for Metal–Oxide–Nitride–Oxide–Silicon Flash Memory With NO- or $\hbox{N}_{2}\hbox{O}$ -Grown Oxynitride as Tunnel Layer

Abstract: The characteristics of oxynitride thermally grown in either NO or N2O ambient as a tunnel layer are investigated based on an Al/Al2O3/GdON/SiOxNy/Si structure. The physical thickness of each dielectric layer was measured and confirmed by multiwavelength ellipsometry and transmission electron microscopy. Experimental results reveal that better memory performances can be achieved for the metal-oxide-nitride-oxide-silicon (MONOS) device with NO-grown oxynitride as the tunnel layer, e.g., larger memory window, higher program/erase speed, better endurance, and retention characteristics, compared with devices with N2O -grown oxynitride and conventional SiO2 as the tunnel layer. The involved mechanisms lie in NO-nitridation-induced smaller hole barrier height, formation of more strong Si-N bonds at/near the oxynitride/Si interface due to more nitrogen incorporation in the tunnel layer. Therefore, the application of NO-grown oxynitride as tunnel layer is promising in advanced MONOS nonvolatile memory devices.

Optimal control of linear discrete-time systems with quantization effects

Abstract: This paper studies optimal control designs for networked linear discrete-time systems with quantization effects and/or fading channel. The quantization errors and/or fading channels are modeled as multiplicative noises. The H2 optimal control in mean-square sense is formulated. The necessary and sufficient condition to the existence of the mean-square stabilizing solution to a modified algebraic Riccati equation (MARE) is presented. The optimal H2 control via state feedback for the systems is designed by using the solution to the MARE. It is a nature extension for the result in standard optimal discrete-time H2 state feedback design. It is shown that this optimal state feedback design problem is eigenvalue problem (EVP) and the optimal design algorithm is developed.

Quantitative analysis of Faraday rotation impacts on image formation of spaceborne VHF/UHF-SAR

Abstract: The performance of spaceborne synthetic aperture radar (SAR) at lower frequencies, such as VHF/UHF bands, is affected by ionosphere effects, especially Faraday rotation (FR). A quantitative analysis of Faraday rotation impacts on spaceborne VHF/UHF-SAR image formation is presented in this paper. The error models of FR resulting from calm Total Electron Content (TEC) and fluctuant TEC, for Linearly Polarized (LP) and Dual Circularly Polarized (DCP) mode, are established. The simulation results indicate that for LP mode Faraday rotation effect on azimuth signal can be neglected, while the range signal is seriously affected, bur for DCP mode, the error in both range and azimuth signals is negligible. Therefore, DCP mode is a better choice to decrease the effect on image focusing caused by FR effect.

High accuracy SAR echo generation approach using space-time-variant backscattering characteristics

Abstract: In order to generate high accuracy data for space-borne Synthetic Aperture Radar (SAR) image interpretation, a novel parallel SAR echo generation approach is proposed in this paper. Target's backscattering coefficient is variable at the condition of wide bandwidth and large aspect angle. Considering targets' space-variant and time-variant backscattering, electromagnetic scattering at arbitrary aspect angle and frequency point is calculated accurately by Finite Difference Time Domain (FDTD). However, FDTD is a time-domain method causing large computation load, thus a parallel processing scheme is designed to reduce simulation time. It was observed that proposed approach provided higher image quality compared to classical method, and parallel scheme was also verified effective in the experiment.

Spaceborne SIMO-SAR for three-dimensional ionospheric irregularity sounding

Abstract: Ionospheric irregularity sounding is of significant importance in compensating for ionospheric scintillation effects on satellite communication, navigation, and remote sensing systems at lower frequencies, i.e., L band, P band, etc. A novel spaceborne nadir-looking single input and multiple outputs synthetic aperture radar (SAR) scheme is proposed for three-dimensional (3D) sounding of ionospheric irregularities. The 3D radar echo signal model is developed by examining the ionospheric effects on electromagnetic radar waveforms. These echoes can be used to produce 3D global maps of the distribution of ionospheric irregularities by means of a novel 3D SAR image formation algorithm. The effectiveness of the proposed SAR system was verified by means of computer simulations. The simulation results show that the proposed system has the capability to generate a 3D high-precision image of ionospheric irregularities with an altitude resolution as high as 0.9 km, an along-track resolution as high as 0.4 km, and an across-track angular resolution of 0.25.

One height error compensation method in high resolution spaceborne SAR through self-focusing

Abstract: To high resolution spaceborne SAR, one problem is inevitable that as to targets with elevation downs in one scenario, their imaging quality will decline after processing with imaging algorithm. SAR images are confined to two-dimensional domain and it's hard to reflect targets' height property. Instead, target A with some height is projected to target B with no height while A and B are in the different range door but with the same slant range from the satellite. Doppler parameters of B are used to image A due to the two-dimensional limitation and then azimuth defocusing and range location shift will occur. Target height can be regarded as one error source. In this paper, imaging quality of targets with height error is improved through self-focusing method among three-step focusing algorithm as to high resolution sliding spotlight SAR. Experiment results show that it's effective.

Airborne geographically referenced stripmap SAR data processing

Abstract: This paper analyzes an innovative geographically referenced (GR) stripmap mode for the airborne synthetic aperture radar (SAR). In the GR stripmap SAR, the antenna beam illuminates orthogonally with the ground strip, which is not parallel to the SAR trajectory. Benefiting from the GR stripmap mode, the effective observation swath can be enlarged comparing to what can be realized by the traditional stripmap SAR. A modified nonlinear chirp scaling (MNLCS) method is suggested for the GR stripmap SAR imaging. It can solve the problem caused by scatterers' non-uniform Doppler history that disables most traditional imaging algorithms for the GR stripmap SAR. The MNLCS algorithm consists of three main steps. First, a bulk range migration compensation procedure eliminates the linear range migration (range walk). Second, an azimuth perturbation filter unifies the Doppler frequency modulation rate for each range cell. Lastly, a modified chirp scaling processing finishes the data focusing. Performance of the MNLCS algorithm was analyzed, followed by a series of computer simulation results, which validated the MNLCS for the GR stripmap SAR imaging.

Analysis of polarimetric features from CTLR compact polarimetric SAR data for discriminating oil slick damping status

Abstract: Polarimetric features retrieved from CTLR (circularly transmit and linearly receive) Synthetic Aperture Radar (SAR) data was analysed in details. A new parameter, namely, Damping Status Sensitivity Index (DSSI) was proposed for quantitatively evaluating the PolSAR characteristics' capability of discriminating different damping patterns of oil slicks and clean seawater. The L-band Uninhabited Aerial Vehicle SAR (UAVSAR) data was utilized in the experiments. It was shown that polarimetric characteristics retrieved from CTLR compact polarimetric SAR data were nearly as same as those derived from fully polarimetric SAR data and can be applied for discriminating different damping status of oil spill and look-likes.

Data-based onboard estimation of antenna phase center spacing in space-borne azimuth multi-channel SAR system

Abstract: In space-borne azimuth multi-channel SAR, the antenna phase center spacing should be measured precisely to obtain the reconstruction filters. In this paper, a data-based onboard estimation method of antenna phase center spacing in spaceborne multi-channel SAR system is proposed. Firstly, the principle of data-based onboard estimation is presented, then the estimation method in details is described step-by-step. Finally, simulations are carried out, with two influence factors, SCR and focusing accuracy, to demonstrate the validity of the proposed estimation method.

Signal model based on Maxwell's equations

Abstract: High resolution imaging is an important trend for SAR. The wide bandwidth signal is a must for high resolution. The error of traditional model based on the narrow bandwidth system may be inevitable for high resolution imaging. In this paper, a general SAR echo model is derived for the wide bandwidth system, and we also get the error factor between the traditional and general model. By simulating the two models, the conclusion is derived that if the transmitted signal is LFM waveform, the target is single and the receiving antenna is unit weight, the error of the two models can be neglected. The general echo model in more complicated situations is in the ongoing research. This study will make for the further research of high resolution SAR imaging algorithm.