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Item Open Access Clutter detection algorithms for airborne pulse-Doppler radar(IEEE, 2010) Güngör, Ahmet; Gezici, SinanShow more Clutter detection is an important stage of target detection. Clutter may not always appear around zero Doppler frequency when realistic terrain models and moving platforms are considered. Two algorithms developed for clutter detection using range-Doppler matrix elements and their performance analysis are presented in this paper. The first algorithm has higher error rates but lower computational complexity whereas the second one has lower error rates but higher computational complexity. The algorithms detect clutter position by filtering range-Doppler matrix elements via non-linear filters. ©2010 IEEE.Show more Item Open Access Controlled surface structuring with nonlinear laser lithography(Bilkent University, 2018-01) Yavuz, ÖzgünShow more Self-organisation has always fascinated researchers from all branches of sciences and engineering. Despite its ubiquity, our present understanding of its core principles and in particular how to control self-organised phenomena is at its infancy. A particularly rich case of self-organisation arises from the interactions of intensely powerful laser beams with material surfaces. As this phenomenon leads to formation of sub-wavelength, thereby, nanoscale periodic structures through a simple, one-step process performed in ambient atmosphere, there has been tremendous interest in its use in applications, ranging from tribology to data storage. However, there remains much to be desired in terms of our ability to control, regulate, dynamically modify the resulting structures. A technique recently demonstrated in our group, Nonlinear Laser Lithography (NLL), has made possible the creation of extremely uniform, virtually perfectly periodic self-organised nanostructures, which are in the form of parallel nanoscale lines. These nanostructures can be used to cover or tile indefinitely large areas without any apparent loss in quality or uniformity. Armed with this advance, we are now in a position to look beyond getting simply periodic structures and to develop conceptual tools and practical techniques for advanced control of the self-organisation process and to create a vast array of self-organised structures. In this thesis, we first develop a rigorous theoretical model for NLL, which we then show to possess excellent predictive power and can efficiently guide the experiments. We first reveal an interesting, self-organised effect, namely that the nanostructures respond to a tilting of the laser beam's wavefront in a manner that is strongly analogous to the well-known Doppler effect. Further, building on the rigorous model developed in this thesis, we propose and experimentally demonstrate that noise or modulations in the laser beam or defects on the surface can each steer the self-organised process. We further show that by deliberately introducing noise or defects, we can achieve patterns that are impossible to achieve otherwise. As an ultimate demonstration of this capability, we report on the creation of all the Bravais lattices possible for a surface. While the main results to be reported concern the NLL technique, the conceptual tools developed in this thesis rely on general properties of selforganisation through an interplay of positive and negative nonlinear feedback mechanisms. This defines a broad class of self-organising systems. As such, it is likely that the techniques we introduce can be appropriately adapted to achieve similar control over self-organised patterns forming in entirely different physical systems.Show more Item Open Access Doppler effect on nanopatterning with nonlinear laser lithography(OSA, 2017) Yavuz, Özgün; Kara, Semih; Tokel, Onur; Pavlov, Ihor; İlday, Fatih ÖmerShow more Summary form only given. Just five years after invention of the laser, laser induced periodic structures (LIPSS) had been reported. However, the structure period is not very uniform in LIPSS. Recently, with nonlinear laser lithography (NLL), long range ordered periodic surface structures had been maintained by exploiting various feedback mechanisms and nonlinearities. Albeit, fine tuning of structure period remains challenging. Here, we present an analogy between Doppler effect and structure period of the NLL which adds a capability of changing the structure period.Show more Item Open Access Multiple-resampling receiver design for OFDM over Doppler-distorted underwater acoustic channels(2013) Tu, K.; Duman, T. M.; Stojanovic, M.; Proakis J. G.Show more In this paper, we focus on orthogonal frequency-division multiplexing (OFDM) receiver designs for underwater acoustic (UWA) channels with user-and/or path-specific Doppler scaling distortions. The scenario is motivated by the cooperative communications framework, where distributed transmitter/receiver pairs may experience significantly different Doppler distortions, as well as by the single-user scenarios, where distinct Doppler scaling factors may exist among different propagation paths. The conventional approach of front-end resampling that corrects for common Doppler scaling may not be appropriate in such scenarios, rendering a post-fast-Fourier-transform (FFT) signal that is contaminated by user-and/or path-specific intercarrier interference. To counteract this problem, we propose a family of front-end receiver structures that utilize multiple-resampling (MR) branches, each matched to the Doppler scaling factor of a particular user and/or path. Following resampling, FFT modules transform the Doppler-compensated signals into the frequency domain for further processing through linear or nonlinear detection schemes. As part of the overall receiver structure, a gradient-descent approach is also proposed to refine the channel estimates obtained by standard sparse channel estimators. The effectiveness and robustness of the proposed receivers are demonstrated via simulations, as well as emulations based on real data collected during the 2010 Mobile Acoustic Communications Experiment (MACE10, Martha's Vineyard, MA) and the 2008 Kauai Acomms MURI (KAM08, Kauai, HI) experiment.Show more Item Open Access Performance improvement of track initiation algorithms with the incorporation of doppler velocity measurement(IEEE, 2006) Kural, F.; Arıkan, F.; Arıkan, OrhanShow more In this study, to obtain the analytical expressions of false track initiation probability, elevation and Doppler velocity measurements extracted by a phased array radar are incorporated for the first time into the commonly used track initiation algorithms. With this study, the measurement set is expanded from a merely range and azimuth to include elevation and Doppler velocity. The analytical expressions of false track initiation probability depend on the parameters of the signal processing unit of the phased array radar, such as false alarm probability, true detection probability, signal-to-noise ratio and detector threshold. Furthermore, such expressions remove the necessity of very time-consuming simulations. The results indicate that using position and Doppler velocity measurements provide a reduction of false track initiation probability by a factor of 9 to 34 depending on the value of velocity and acceleration thresholds while supplying the design criterion of the true track initiation probability, ≥ 0.7.Show more Item Open Access Pulse doppler radar target recognition using a two-stage SVM procedure(IEEE, 2010-07-07) Eryildirim, A.; Onaran, I.Show more It is possible to detect and classify moving and stationary targets using ground surveillance pulse-Doppler radars (PDRs). A two-stage support vector machine (SVM) based target classification scheme is described here. The first stage tries to estimate the most descriptive temporal segment of the radar echo signal and the target signal is classified using the selected temporal segment in the second stage. Mel-frequency cepstral coefficients of radar echo signals are used as feature vectors in both stages. The proposed system is compared with the covariance and Gaussian mixture model (GMM) based classifiers. The effects of the window duration and number of feature parameters over classification performance are also investigated. Experimental results are presented.Show more Item Open Access Spatial and temporal symmetry breaking in nonlinear laser lithography(Bilkent University, 2023-01) Bin Aamir, AbdullahShow more Symmetry breaking is ubiquitous in nonlinear systems. This is also the case for Nonlinear Laser Lithography (NLL), in which an ultrafast laser beam incident on a material surface causes the infinite fold rotational symmetry of the material surface to be broken. In the case of linear polarization, line like structures are obtained that possess 2-fold rotational symmetry. We discuss two types of NLL, one due to the formation of oxide structures (Oxidation NLL) and the other due to material ablation (Ablation NLL). The existence of both types of structures is known for many years, however, although the regularity of oxidative structures has been significantly improved by our group earlier, the same was not true for ablative structures. Here, using the technique for Oxidation NLL and the parameters for ablative structures, we were able to achieve highly regular ablative structures which we call Ablation NLL. We demonstrate the coexistence of these two NLL structures on the same surface and how a plane can be tiled using them. Furthermore, we explore the phase space of NLL and determine the regions of the phase space occupied by the two NLL structures. We also demonstrate the versatility of NLL by obtaining Oxidation and Ablation NLL structures on several metals as well as on Silicon. We also discuss temporal symmetry breaking in NLL. If the laser beam is not incident normal to the surface and is tilted towards or away from the scanning direction, it can cause the period of the NLL structures to decrease or increase respectively. One can thus discern if a video of the beam creating a pattern while scanning over the surface along a line is run forward or backward. This dependence on the scanning direction leads to temporal symmetry breaking and is reminiscent of the Doppler effect. These symmetry breakings can be important for future research in this field along with possible commercial applications, some of which we have discussed here.Show more Item Open Access Successive cancelation approach for doppler frequency estimation in pulse doppler radar systems(IEEE, 2010) Soğancı, Hamza; Gezici, SinanShow more In this paper, a successive cancelation approach is proposed to estimate Doppler frequencies of targets in pulse Doppler radar systems. This technique utilizes the Doppler domain waveform structure of the received signal coming from a point target after matched filtering and pulse Doppler processing steps. The proposed technique is an iterative algorithm. In each iteration, a target that minimizes a cost function is found, and the signal coming from that target is subtracted from the total received signal. These steps are repeated until there are no more targets. The global minimum value of the cost function in each iteration is found via particle swarm optimization (PSO). Performance of this technique is compared with the optimal maximum likelihood solution for various signal-to-noise ratio (SNR) values based on Monte Carlo simulations.Show more