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Ma C.,National University of Singapore | Yeo T.S.,National University of Singapore | Tan C.S.,DSO National Laboratory | Liu Z.,Zhongyuan University of Technology
IEEE Transactions on Geoscience and Remote Sensing | Year: 2011

Conventional inverse synthetic aperture radar image is a 2-D range-Doppler projection of a target and does not provide 3-D information. Its formation also requires complex motion compensation when the target is uncooperative and maneuvering. On the other hand, multiple-input and multiple-output (MIMO) radar, in addition to having a wide virtual aperture and high cross-range resolution, could also obtain a target's 3-D image in one snapshot and thus have avoided the complex motion compensation needed. In this paper, we propose a 3-D imaging algorithm using three MIMO configurations. The signal model is derived based on a modified zero correlation zone code. A strong scatterer selection criterion is also proposed for the construction of the target profile. © 2011 IEEE.

Ma C.,National University of Singapore | Yeo T.S.,National University of Singapore | Tan C.S.,DSO National Laboratory | Tan H.S.,National University of Singapore
IEEE Transactions on Image Processing | Year: 2010

Large 2-D sparse array provides high angular resolution microwave images but artifacts are also induced by the high sidelobes of the beam pattern, thus, limiting its dynamic range. CLEAN technique has been used in the literature to extract strong scatterers for use in subsequent signal cancelation (artifacts removal). However, the performance of DFT parameters estimation based CLEAN algorithm for the estimation of the signal amplitudes is known to be poor, and this affects the signal cancelation. In this paper, DFT is used only to provide the initial estimates, and the maximum likelihood parameters estimation method with steepest descent implementation is then used to improve the precision of the calculated scatterers positions and amplitudes. Time domain information is also used to reduce the sidelobe levels. As a result, clear, artifact-free images could be obtained. The effects of multiple reflections and rotation speed estimation error are also discussed. The proposed method has been verified using numerical simulations and it has been shown to be effective. © 2006 IEEE.

Yao J.,Institute for Infocomm Research | Lee C.,DSO National Laboratory | Yeo S.P.,National University of Singapore
IEEE Transactions on Microwave Theory and Techniques | Year: 2011

The branch-line coupler may be redesigned for crossover application. The bandwidth of such a coupler can be extended by suitably incorporating additional sections into the composite design. Laboratory tests on microstrip prototypes have shown the return loss and isolation of the three- and four-section couplers to be better than 20 dB over bandwidths of 22% and 33%, respectively. The insertion losses and group delays vary by less than ±dB and ±1, respectively, for both prototypes. © 2010 IEEE.

Geng L.,National University of Singapore | Zhang Y.F.,National University of Singapore | Wang J.J.,National University of Singapore | Fuh J.Y.H.,National University of Singapore | Teo S.H.,DSO National Laboratory
Control and Intelligent Systems | Year: 2014

This paper addresses the mission planning issues for guiding a group of unmanned aerial vehicles to carry out a series of tasks, namely classification, attack, and verification, against multiple targets. The flying space is constrained with the presence of flight prohibit zones (FPZs) and enemy radar sites. The solution space for task assignment and sequencing is modelled with a graph representation. With a path formation based on Dubins vehicle paths, a genetic algorithm has been developed for finding the optimal solution from the graph to achieve the following goals: (1) completion of the three tasks on each target, (2) avoidance of FPZs, (3) low level of exposure to enemy radar detection, and (4) short overall flying path length. A case study is presented to demonstrate the effectiveness of the proposed method.

News Article | October 23, 2015
Site: news.mit.edu

The Dirac cone, named after British physicist Paul Dirac, started as a concept in particle and high-energy physics and has recently became important in research in condensed matter physics and material science. It has since been found to describe aspects of graphene, a two dimensional form of carbon, suggesting the possibility of applications across various fields. Now physicists at MIT have found another unusual phenomenon produced by the Dirac cone: It can spawn a phenomenon described as a “ring of exceptional points.” This connects two fields of research in physics and may have applications in building powerful lasers, precise optical sensors, and other devices. The results are published this week in the journal Nature by MIT postdoc Bo Zhen, Yale University postdoc Chia Wei Hsu, MIT physics professors Marin Soljačić and John Joannopoulos, and five others. This work represents “the first experimental demonstration of a ring of exceptional points,” Zhen says, and is the first study that relates research in exceptional points with the physical concepts of parity-time symmetry and Dirac cones. Individual exceptional points are a peculiar phenomenon unique to an unusual class of physical systems that can lead to counterintuitive phenomena. For example, around these points, opaque materials may seem more transparent, and light may be transmitted only in one direction. However, the practical usefulness of these properties is limited by absorption loss introduced in the materials. The MIT team used a nanoengineered material called a photonic crystal to produce the exceptional ring. This new ring of exceptional points is different from those studied by other groups, making it potentially more practical, the researchers say. “Instead of absorption loss, we adopt a different loss mechanism — radiation loss — which does not affect the device performance,” Zhen says. “In fact, radiation loss is useful and is necessary in devices like lasers.” This phenomenon could enable creation of new kinds of optical systems with novel features, the MIT team says. “One important possible application of this work is in creating a more powerful laser system than existing technologies allow,” Soljačić says. To build a more powerful laser requires a bigger lasing area, but that introduces more unwanted “modes” for light, which compete for power, limiting the final output. “Photonic crystal surface-emitting lasers are a very promising candidate for the next generation of high-quality, high-power compact laser systems,” Soljačić says, “and we estimate we can improve the output power limit of such lasers by a factor of at least 10.” “Our system could also be used for high-precision detectors for biological or chemical materials, because of its extreme sensitivity,” Hsu says. This improved sensitivity is due to another exotic property of the exceptional points: Their response to perturbations is not linear to the perturbation strength. Normally, Hsu says, it becomes very difficult to detect a substance when its concentration is low. When the concentration of the target substance is reduced by a million times, the overall signal also decreases by a million times, which can make it too small to detect. “But at an exceptional point, it’s not linear anymore,” Hsu says, “and the signal goes down by only 1,000 times, providing a much bigger response that can now be detected.” Demetrios Christodoulides, a professor of optics and photonics at the University of Central Florida who was not involved in this work, says, “This represents the first observation of an exceptional ring in a 2-D crystal associated with a two-dimensional band. The MIT work opens up a number of opportunities … in particular, around exceptional points where systems are known on many occasions to behave in a peculiar fashion.” The research team also included Yuichi Igarashi of NEC Corp. in Japan and MIT research scientist Ling Lu, postdoc Ido Kaminer, Harvard University graduate student Adi Pick, and Song-Liang Chua at DSO National Laboratory in Singapore. The work was supported, in part, by the Army Research Office through MIT’s Institute for Soldier Nanotechnologies, the National Science Foundation, and the Department of Energy.

Siauw W.L.,University of Poitiers | Siauw W.L.,DSO National Laboratory | Bonnet J.P.,University of Poitiers
International Journal of Heat and Fluid Flow | Year: 2016

We present the transient phenomena occurring during the impulsive control of flow separation over a NACA0015 airfoil at an incidence angle of 11° and a chord Reynolds number of 1. million. Actuation is performed via pneumatic vortex generators, impulsively activated in order to analyze the transient phenomena corresponding to the attachment process and, conversely, to transient re-separation occurring when the actuators are switched off. Measurements are performed using a linear array of unsteady pressure transducers and a single traversing crosswire. The pressure transducers are positioned in the separated region of the airfoil, which extends ∼. 0.3c upstream of the trailing edge at the above flow condition. To control the flow, the angled fluidic vortex generators are positioned in a single spanwise array located 0.3c downstream of the leading edge of the airfoil. We establish a statistical relationship between pressure and velocity signals during both the uncontrolled steady state and the transient processes of attachment and separation. The unsteady behavior of the attachment process is also qualitatively analyzed via a 0.3. million Reynold number visualizations. The emission of a "starting vortex" is evidenced. This corresponds to a transient increase of drag. © 2017.

Wang L.,Nanyang Technological University | Wan C.,DSO National Laboratory | Li S.,Shanghai JiaoTong University | Bi G.,Nanyang Technological University
Signal Processing | Year: 2015

Tonals generated by machineries with rotating elements typically have a harmonic structure with unknown fundamental frequencies, amplitude, harmonic order and phase. Detecting this type of signals is of great importance to numerous engineering applications. In the frequency domain, tonals are represented by a few harmonic frequencies, which appear in blocks, related to one or more fundamental frequencies. This block-sparsity property of the frequency content suggests alternative ways to recover and detect tonals by using sparse signal processing techniques. Motivated by the success of the block orthonormal greedy algorithm (BOGA), new detection architectures, which require no prior information about the number of the fundamental frequencies, are proposed for robust tonal detection in low signal to noise ratio (SNR) environments. The distributions of the test statistics of detection architectures are firstly analyzed theoretically and comprehensively based on the theory of order statistics. Detection performances are also analyzed and compared theoretically and experimentally. Significant improvements on detection performance in low SNR environments are shown over the conventional detectors that do not consider the harmonic structure and the sparsity of the tonals. © 2014 Elsevier B.V.

Kok A.,DSO National Laboratory | Donny T.,DSO National Laboratory | Shuichi O.,Hiroshima University | Takanori I.,Omron Corporation
Proceedings - 2010 IEEE International Conference on Wireless Communications, Networking and Information Security, WCNIS 2010 | Year: 2010

We study the application of turbo equalization in orthogonal frequency division multiplexing (OFDM) system over doubly-selective channel. Maximum A Posteriori (MAP) equalization that exploits characteristics of the banded channel structure in OFDM is used in our turbo equalization. In settings similar to Japan's ISDB-T standards, we find that banded channel structure that accounts for only 1 neighboring sub carrier (on one side) is sufficient to ensure a good equalization performance. At moderate Doppler frequency, turbo equalization does not enhance the system performance significantly. Furthermore, the effects of 1) initial channel estimation with turbo equalization; 2) channel re-estimation with turbo equalization are investigated, with the latter conferring greater bit error-rate (BER) enhancement than the former. Channel estimation has a more prominent effect on performance than turbo equalization, as verified by numerical simulations. ©2010 IEEE.

Wang L.,Nanyang Technological University | Bi G.,Nanyang Technological University | Wan C.,DSO National Laboratory | Lv X.,Nanyang Technological University
Signal Processing | Year: 2011

The block orthogonal greedy algorithm (BOGA) has been proven to successfully recover block-sparse signals in noiseless environments and the associated stability problem dealing with noisy signals has also been studied in the literature. This paper demonstrates that the recovery conditions of the BOGA previously reported can be relaxed by using a different definition of the block-coherence. The presented results in this paper provide a generalization of those reported by Tseng for block-sparse signal and serve as a complement of the BOGA reported by Eldar for noisy signals. © 2011 Elsevier B.V. All rights reserved.

Wang L.,Nanyang Technological University | Bi G.,Nanyang Technological University | Wang C.,DSO National Laboratory
IEEE Region 10 Annual International Conference, Proceedings/TENCON | Year: 2012

This paper presents detectors for signal with harmonics to fully explore the inherent harmonic structure of the signal with unknown fundamental frequency, amplitude and phase in complex white Gaussian noise. For signals with harmonic characteristics, a transformed power spectrum is proposed to find the energy integration of all the harmonic components. We analyze the statistical property of the transformed power spectrum and develop a harmonic tonal detector by using the method of generalized likelihood hypothesis test. Another detector which considers equal contributions of data sequences is also developed and analyzed theoretically. Both detectors fully utilize the harmonic information of the signal. Simulation results show their improvements on the detection performance. © 2012 IEEE.

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