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Item Open Access Accurate positioning in ultra-wideband systems(Institute of Electrical and Electronics Engineers, 2011) Soganci, H.; Gezici, SinanShow more Accurate positioning systems can be realized via ultra-wideband signals due to their high time resolution. In this article, position estimation is studied for UWB systems. After a brief introduction to UWB signals and their positioning applications, two-step positioning systems are investigated from a UWB perspective. It is observed that time-based positioning is well suited for UWB systems. Then time-based UWB ranging is studied in detail, and the main challenges, theoretical limits, and range estimation algorithms are presented. Performance of some practical time-based ranging algorithms is investigated and compared against the maximum likelihood estimator and the theoretical limits. The trade-off between complexity and accuracy is observed.Show more Item Open Access Anchor placement in TOA based wireless localization networks via convex relaxation(IEEE, 2021-09-06) Öztürk, Cuneyd; Gezici, SinanShow more A wireless source localization network consisting of synchronized target and anchor nodes is considered. An anchor placement problem is formulated to minimize the Cramér-Rao lower bound (CRLB) on estimation of target node positions by anchor nodes. First, it is shown that the anchor placement problem can be approximated as a minimization problem of the ratio of two supermodular functions. Due to the lack of a polynomial time algorithm for such problems, an anchor selection problem is proposed to solve the anchor placement problem. Via relaxation of integer constraints, the anchor selection problem is approximated by a convex optimization problem, which is used to propose two algorithms for anchor selection. Furthermore, extensions to quasi-synchronous wireless localization networks are discussed. To examine the performance of the proposed algorithms, various simulation results are presented.Show more Item Open Access Autonomous navigation of robotic units in mobile sensor network(2012) Nazlibilek, S.Show more This work is motivated by the problem of detecting buried anti-tank and anti-personnel mines in roads or some border regions. The problem is tried to be solved by use of small mobile robotic sensors and their some abilities such as measurement of local fields, navigation around a region, communications with each other, and constituting team within a mission area. The aim of this work is to investigate the navigation problem for the team behavior of mobile sensors within a potential field available in a small-scale environment such as an indoor area or an outdoor region. The mobile sensor network here is a collection of robotic units with sensing capability of earth magnetic field anomalies. A new kind of positioning system is needed for their collective behavior. In this work, a new method of navigation is proposed as a local positioning system. It utilizes ultrasound and radio frequency information to determine the coordinates of the points inside the operational area. The method proposed here is compared with the ultra wideband ranging ping-pong method that is used widely in recent applications. A time division multiple access method is used for the communications among the mobile sensors. The results on the positioning methods together with several simulations and experimental works are given. It is shown that the positioning method utilizing ultrasound-radio frequency method can give fairly good results. © 2012 Elsevier Ltd. All rights reserved.Show more Item Open Access Bistable behavior of a two-mode Bose-Einstein condensate in an optical cavity(IOP Institute of Physics Publishing, 2013-01-11) Safaei, S.; Mustecaplioglu, Ö. E.; Tanatar, BilalShow more We consider a two-component Bose-Einstein condensate in a one-dimensional optical cavity. Specifically, the condensate atoms are taken to be in two degenerate modes due to their internal hyperfine spin degrees of freedom and they are coupled to the cavity field and an external transverse laser field in a Raman scheme. A parallel laser also excites the cavity mode. When the pump laser is far detuned from its resonance atomic transition frequency, an effective nonlinear optical model of the cavity-condensate system is developed under the discrete mode approximation (DMA), while matter-field coupling has been considered beyond the rotating wave approximation. By analytical and numerical solutions of the nonlinear dynamical equations, we examine the mean cavity field and population difference (magnetization) of the condensate modes. The stationary solutions of both the mean cavity field and normalized magnetization demonstrate bistable behavior under certain conditions for the laser pump intensity and matter-field coupling strength.Show more Item Open Access Comparative analysis of different approaches to target classification and localization with sonar(IEEE, 2001-08) Ayrulu, Birsel; Barshan, BillurShow more The comparison of different classification and fusion techniques was done for target classification and localization with sonar. Target localization performance of artificial neural networks (ANN) was found to be better than the target differentiation algorithm (TDA) and fusion techniques. The target classification performance of non-parametric approaches was better than that of parameterized density estimator (PDE) using homoscedastic and heteroscedastic NM for statistical pattern recognition techniques.Show more Item Open Access Cooperative positioning in wireless networks(John Wiley & Sons, 2016) Gholami, M. R.; Keskin, M. F.; Gezici, Sinan; Jansson, M.; Webster, J. G.Show more In this article, we study cooperative positioning in wireless networks in which target nodes at unknown locations locally collaborate with each other to find their locations. We review different models available for positioning and categorize the model‐based algorithms in two groups: centralized and distributed. We then investigate a lower bound on the variance of unbiased estimators, namely the Cramer–Rao lower bound, which is a common benchmark in the positioning literature. We finally discuss some open problems and research topics in the area of positioning that are worth exploring in future studies.Show more Item Open Access Coupled optical microcavities in one-dimensional photonic bandgap structures(Institute of Physics Publishing, 2001) Bayındır, Mehmet; Kural, C.; Özbay, EkmelShow more We present a detailed theoretical and experimental study of the evanescent coupled optical microcavity modes in one-dimensional photonic bandgap structures. The coupled-cavity samples are fabricated by depositing alternating hydrogenated amorphous silicon nitride and silicon oxide layers. Splitting of the eigenmodes and formation of a defect band due to interaction between the neighbouring localized cavity modes are experimentally observed. Corresponding field patterns and the transmission spectra are obtained by using transfer matrix method (TMM) simulations. A theoretical model based on the classical wave analogue of the tight-binding (TB) picture is developed and applied to these structures. Experimental results are in good agreement with the predictions of the TB approximation and the TMM simulations.Show more Item Open Access Eavesdropper and jammer selection in wireless source localization networks(IEEE, 2021-07-26) Öztürk, Cüneyd; Gezici, SinanShow more We consider a wireless source localization network in which a target node emits localization signals that are used by anchor nodes to estimate the target node position. In addition to target and anchor nodes, there can also exist eavesdropper nodes and jammer nodes which aim to estimate the position of the target node and to degrade the accuracy of localization, respectively. We first propose the problem of eavesdropper selection with the goal of optimally placing a given number of eavesdropper nodes to a subset of possible positions to estimate the target node position as accurately as possible. As the performance metric, the Cramér-Rao lower bound (CRLB) related to the estimation of the target node position by eavesdropper nodes is derived, and its convexity and monotonicity properties are investigated. By relaxing the integer constraints, the eavesdropper selection problem is approximated by a convex optimization problem and algorithms are proposed for eavesdropper selection. Then, the problem of jammer selection is proposed where the aim is to optimally place a given number of jammer nodes to a subset of possible positions for degrading the localization accuracy of the network as much as possible. A CRLB expression from the literature is used as the performance metric, and its concavity and monotonicity properties are derived. Also, a convex optimization problem is derived after relaxation. Finally, the joint eavesdropper and jammer selection problem is proposed with the goal of placing certain numbers of eavesdropper and jammer nodes to a subset of possible positions.Show more Item Open Access Eavesdropper selection strategies in wireless source localization networks(IEEE, 2020) Öztürk, Cüneyd; Gezici, SinanShow more We consider a wireless source localization network in which eavesdropper nodes aim to estimate the position of a target node. We formulate the problem of selecting a set of N E positions out of N possible positions for placing eavesdropper nodes in order to estimate the target node position as accurately as possible. The Cramér-Rao lower bound related to the estimation of the target node position by eavesdropper nodes is derived, and its monotonicity and convexity properties are investigated. Via relaxation of the integer constraints, the eavesdropper selection problem is approximated by a convex optimization problem, which is used to propose two algorithms for eavesdropper selection. Moreover, in the presence of parameter uncertainty, a robust version of the eavesdropper selection problem is investigated. Simulation results are presented to examine performance of the proposed algorithms.Show more Item Open Access Enhanced coupling to microsphere resonances with optical fibers(Optical Society of America, 1997) Serpengüzel, A.; Arnold, S.; Griffel, G.; Lock, J. A.Show more Morphology-dependent resonances (MDR's) of polystyrene microspheres were excited by an optical fiber coupler. For optical elimination of the air-cladding interface at the optical fiber coupler surface, the microsphere was immersed in an index-matching oil. MDR's were observed, even though the relative refractive index between the microsphere and the oil was only 1.09. The observed MDR spectra are in good agreement with the generalized Lorenz-Mie theory and the localization principle. The scattering efficiency into each MDR is estimated as a function of the impact parameter by means of generalized Lorenz-Mie theory. (C) 1997 Optical Society of America.Show more Item Open Access Estimation theoretic analyses of location secrecy and ris-aided localization under hardware impairments(2022-06) Öztürk, CüneydShow more In this thesis, we present estimation theoretic analyses of location secrecy and reconfigurable intelligent surface (RIS) aided localization under hardware impairments. First, we consider a wireless source localization network in which a target node emits localization signals that are used by anchor nodes to estimate the target node position. In addition to target and anchor nodes, there can also exist eavesdropper nodes and jammer nodes which aim to estimate the position of the target node and to degrade the accuracy of localization, respectively. We propose the problem of eavesdropper selection with the goal of optimally placing a given number of eavesdropper nodes to a subset of possible positions in the network to estimate the target node position as accurately as possible. As the performance metric, the Cramér-Rao lower bound (CRLB) related to the estimation of the target node position by eavesdropper nodes is derived, and its convexity and monotonicity properties are investigated. By relaxing the integer constraints, the eavesdropper selection problem is approximated by a convex optimization problem and algorithms are proposed for eavesdropper selection. Moreover, in the presence of parameter uncertainty, a robust version of the eavesdropper selection problem is developed. Then, the problem of jammer selection is proposed where the aim is to optimally place a given number of jammer nodes to a subset of possible positions for degrading the localization accuracy of the network as much as possible. A CRLB expression from the literature is used as the performance metric, and its concavity and monotonicity properties are derived. Also, a convex optimization problem and its robust version are derived after relaxation. Moreover, the joint eavesdropper and jammer selection problem is proposed with the goal of placing certain numbers of eavesdropper and jammer nodes to a subset of possible positions. Simulation results are presented to illustrate performance of the proposed algorithms. Second, a wireless source localization network consisting of synchronized target and anchor nodes is considered. An anchor placement problem is formulated to minimize the CRLB on estimation of target node positions by anchor nodes. It is shown that the anchor placement problem can be approximated as a minimization problem of the ratio of two supermodular functions. Due to the lack of a polynomial time algorithm for such problems, an anchor selection problem is proposed to solve the anchor placement problem. Via relaxation of integer constraints, the anchor selection problem is approximated by a convex optimization problem, which is used to propose two algorithms for anchor selection. Furthermore, extensions to quasi-synchronous wireless localization networks are discussed. To examine the performance of the proposed algorithms, various simulation results are presented. Third, we investigate the problem of RIS-aided near-field localization of a user equipment (UE) served by a base station (BS) under phase-dependent amplitude variations at each RIS element. Through a misspecified Cramér -Rao bound (MCRB) analysis and a resulting lower bound (LB) on localization, we show that when the UE is unaware of amplitude variations (i.e., assumes unit-amplitude responses), severe performance penalties can arise, especially at high signal-to-noise ratios (SNRs). Leveraging Jacobi-Anger expansion to decouple range-azimuth-elevation dimensions, we develop a low-complexity approximated mismatched maximum likelihood (AMML) estimator, which is asymptotically tight to the LB. To mitigate performance loss due to model mismatch, we propose to jointly estimate the UE location and the RIS amplitude model parameters. The corresponding Cramér -Rao bound (CRB) is derived, as well as an iterative refinement algorithm, which employs the AMML method as a subroutine and alternatingly updates individual parameters of the RIS amplitude model. Simulation results indicate fast convergence and performance close to the CRB. The proposed method can successfully recover the performance loss of the AMML under a wide range of RIS parameters and effectively calibrate the RIS amplitude model online with the help of a user that has an a-priori unknown location. Fourth, we consider RIS-aided localization scenarios with RIS pixel failures, where individual RIS elements can become faulty due to hardware imperfections. We explore the impact of such failures on the localization performance. To that aim, an MCRB analysis is conducted and numerical results indicate that performance loss for estimating the UE position can be significant in the presence of pixel failures. To remedy this issue, we develop two different diagnosis strategies to determine which pixels are failing, and design robust methods to perform localization in the presence of faulty elements. One strategy is based on the l_1-regularization method, the second one employs a successive approach. Both methods significantly reduce the performance loss due to pixel failures. The successive one performs very close to the theoretical bounds at high SNRs even though it has a higher computational cost than the l_1-regularization based method. In the final part of the dissertation, the optimal encoding strategy of a scalar parameter is performed in the presence of jamming based on an estimation theoretic criterion. Namely, the aim is to obtain the optimal encoding function at the transmitter that minimizes the expectation of the conditional Cramér -Rao bound (ECRB) at the receiver when the jammer has access to the parameter and alters the received signal by sending an encoded version of the parameter. Via calculus of variations, the optimal encoding function at the transmitter is characterized explicitly, and an algorithm is proposed to calculate it. Numerical examples demonstrate benefits of the proposed optimal encoding approach.Show more Item Open Access Fabrication, characterization and simulation of plasmonic cavities(2010) Karabıyık, MustafaShow more Surface plasmon polaritons (SPPs) originate from the collective oscillations of conduction electrons coupled with photons propagating at metal-dielectric interfaces. A uniform metallic gratings change the dispersion (energy-momentum relation) of a flat metal surfaces due to the interaction of SPPs with the periodic structure. By breaking the symmetry of the periodic plasmonic structure, SPP cavities can be achieved and SPPs can be localized inside the cavity regions. The aim of this thesis is to understand the physics of phase shifted grating based plasmonic cavities. To this end, we fabricated uniform gratings and phase shifted gratings using electron beam lithography, and optically characterized these SPP structures with polarization dependent reflection spectroscopy. We verified experimental results with numerical simulations SPP propagation and localization on the grating structures. Dispersion curves of SPPs have been calculated by solving Maxwell’s wave equations using finite difference time domain method (FDTD) with appropriate boundary conditions in agreement with experimentally obtained data. We studied the dispersion curve as a function of grating profile modulation where we vary the ridge height and width of the ridges. We find that the plasmonic band gap width increases as the ridge height of the ridges in the grating increases. Optimum duty cycle of grating to observe plasmonic band gap is determined to be half of the grating period. Amount of the phase shift added to the periodicity of the uniform grating defines the energy of the cavity state, which is periodically related to the phase shift. A plasmonic cavity with a quality factor 80 has been achieved. The propagation mechanism of SPPs on coupled cavities is plasmon hopping from a given cavity to the next one.Show more Item Open Access Grating based plasmonic cavities(2009) Şenlik, Servet SeçkinShow more Surface plasmon polaritons are dipole carrying electromagnetic excitations occur- ing at metal-dielectric interfaces. Metallic periodic structures exhibit modi¯ed transmission and re°ection spectra owing to the interaction of propagating SPPs with the periodicity. These periodic surfaces are used to demonstrate localiza- tion of propagating SPPs. Thin metallic ¯lms surrounded by Bragg re°ectors, selective loading of biharmonic metallic surfaces and Moire patterns are used to demonstrate plasmonic cavity formation. The quality factor, Q, a characteristic value that indicates rate of energy loss relative to the stored energy in the cavity is a crucial parameter for classifying these cavities. It was proposed that the Q factor should strongly depend on the surface geometry. However, there was not a sytematic study on the Q factor of these cavity structures. In this work, we report on a comparative study of grating based plasmonic band gap cavities. Numerically, we calculate the quality factors of the cavities based on three types of grating surfaces; uniform, biharmonic and Moirµe surfaces. Experimentally, we demonstrate the existence of plasmonic cavities based on uniform gratings. E®ective index perturbation and cavity geometries are obtained by additional dielectric loading. Furthermore, we fabricate 2D plasmonic structures, observe plasmonic band gaps in the symetry axis and propose cavity geometries for this structure.Show more Item Open Access Hybrid TDOA/RSS based localization for visible light systems(Elsevier, 2019) Kazıklı, Ertan; Gezici, SinanShow more In a visible light positioning (VLP) system, a receiver can estimate its location based on signals transmitted by light emitting diodes (LEDs). In this manuscript, we investigate a quasi-synchronous VLP system, in which the LED transmitters are synchronous among themselves but are not synchronized with the receiver. In quasi-synchronous VLP systems, position estimation can be performed by utilizing time difference of arrival (TDOA) information together with channel attenuation information, leading to a hybrid localization system. To specify accuracy limits for quasi-synchronous VLP systems, the Cramér–Rao lower bound (CRLB) on position estimation is derived in a generic three-dimensional scenario. Then, a direct positioning approach is adopted to obtain the maximum likelihood (ML) position estimator based directly on received signals from LED transmitters. In addition, a two-step position estimator is proposed, where TDOA and received signal strength (RSS) estimates are obtained in the first step and the position estimation is performed, based on the TDOA and RSS estimates, in the second step. The performance of the two-step positioning technique is shown to converge to that of direct positioning at high signal-to-noise ratios based on asymptotic properties of ML estimation. Finally, CRLBs and performance of the proposed positioning techniques are investigated through simulations.Show more Item Open Access An inquiry into the metrics for evaluation of localization algorithms in wireless ad hoc and sensor networks(2008) Aksu, HidayetShow more In ad-hoc and sensor networks, the location of a sensor node making an observation is a vital piece of information to allow accurate data analysis. GPS is an established technology to enable precise position information. Yet, resource constraints and size issues prohibit its use in small sensor nodes that are designed to be cost efficient. Instead, most positions are estimated by a number of algorithms. Such estimates, inevitably introduce errors in the information collected from the field, and it is very important to determine the error in cases where they lead to inaccurate data analysis. After all, many components of the application rely on the reported locations including decision making processes. It is, therefore, vital to understand the impact of errors from the applications’ point of view. To date, the focus on location estimation was on individual accuracy of each sensor’s position in isolation to the complete network. In this thesis, we point out the problems with such an approach that does not consider the complete network topology and the relative positions of nodes in comparison to each other. We then describe the existing metrics, which are used in the literature, and also propose some novel metrics that can be used in this area of research. Furthermore, we run simulations to understand the behavior of the existing and proposed metrics. After having discussed the simulation results, we suggest a metric selection methodology that can be used for wireless sensor network applications.Show more Item Open Access Investigation of localized coupled-cavity modes in two-dimensional photonic bandgap structures(IEEE, 2002) Özbay, Ekmel; Bayındır, Mehmet; Bulu, I.; Cubukcu, E.Show more We present a detailed study of the localized coupled-cavity modes in 2-D dielectric photonic crystals. The transmission, phase, and delay time characteristics of the various coupled-cavity structures are measured and calculated. We observed the eigenmode splitting, waveguiding through the coupled cavities, splitting of electromagnetic waves in waveguide ports, and switching effect in such structures. The corresponding field patterns and the transmission spectra are obtained from the finite-difference-time-domain (FDTD) simulations. We also develop a theory based on the classical wave analog of the tight-binding (TB) approximation in solid state physics. Experimental results are in good agreement with the FDTD simulations and predictions of the TB approximation.Show more Item Open Access Jammer placement algorithms for wireless localization systems(2016-07) Kurt, Mehmet NecipShow more The optimal jammer placement problem is proposed and analyzed for wireless localization systems. In particular, the optimal location of a jammer node is obtained by maximizing the minimum of the Cram´er-Rao lower bounds (CRLBs) for a number of target nodes under location related constraints for the jammer node. For scenarios with more than two target nodes, theoretical results are derived to specify conditions under which the jammer node is located as close to a certain target node as possible, or the optimal location of the jammer node is determined by two of the target nodes. Also, explicit expressions are provided for the optimal location of the jammer node in the presence of two target nodes. In addition, in the absence of distance constraints for the jammer node, it is proved, for scenarios with more than two target nodes, that the optimal jammer location lies on the convex hull formed by the locations of the target nodes and is determined by two or three of the target nodes, which have equalized CRLBs. Numerical examples are presented to provide illustrations of the theoretical results in different scenarios. Furthermore, an iterative algorithm is proposed for numerically determining the optimal jammer location. At each iteration of the algorithm, the jammer node is moved one step along a straight line with the purpose of increasing the CRLB(s) of the target node(s) with the minimum CRLB in the system. It is shown that the algorithm converges almost surely to the optimal jammer location under certain conditions for an infinitesimally small step size in the absence of location constraints for the jammer node. Simulations illustrate the effectiveness of the proposed algorithm in finding the optimal jammer location and its superiority in terms of the computational complexity compared to the exhaustive search over all feasible locations.Show more Item Open Access Jamming of Wireless Localization Systems(Institute of Electrical and Electronics Engineers Inc., 2016) Gezici, Sinan; Gholami, M. R.; Bayram, S.; Jansson M.Show more In this paper, the optimal jamming of wireless localization systems is investigated. Two optimal power allocation schemes are proposed for jammer nodes in the presence of total and peak power constraints. In the first scheme, power is allocated to jammer nodes in order to maximize the average Cramér-Rao lower bound (CRLB) of target nodes, whereas in the second scheme, the power allocation is performed for the aim of maximizing the minimum CRLB of target nodes. Both the schemes are formulated as linear programs, and a closed-form solution is obtained for the first scheme. For the second scheme, under certain conditions, the property of full total power utilization is specified, and a closed-form solution is obtained when the total power is lower than a specific threshold. In addition, it is shown that non-zero power is allocated to at most NT jammer nodes according to the second scheme in the absence of peak power constraints, where NT is the number of target nodes. In the presence of parameter uncertainty, robust versions of the power allocation schemes are proposed. Simulation results are presented to investigate the performance of the proposed schemes and to illustrate the theoretical results. © 2016 IEEE.Show more Item Open Access Line segment based range scan matching without pose information for indoor environments(2008) Yakın, İskenderShow more A mobile robot exploring an unknown environment often needs to keep track of its pose through its sensors. Range scan matching is a way of computing the pose difference of a robot at two different locations on the navigation path by finding common features observed in range sensor readings recorded at these locations. In this thesis, we introduce a new algorithm which computes this pose difference by matching common line segments extracted from two laser range scans taken from two different but unknown poses. In this algorithm, matching is performed by exploiting invariant geometric relations among line segments. The use of line segments instead of range points also reduces the computational complexity of determining the pose difference between two distinct scans. Compared to other scan matching algorithms, our method presents a powerful means for global scan matching, map building, place recognition, loop closing and multirobot mapping, all in real-time.Show more Item Open Access Localization and Tracking of Implantable Biomedical Sensors(MDPI AG, 2017) Umay, I.; Fidan, B.; Barshan, B.Show more Implantable sensor systems are effective tools for biomedical diagnosis, visualization and treatment of various health conditions, attracting the interest of researchers, as well as healthcare practitioners. These systems efficiently and conveniently provide essential data of the body part being diagnosed, such as gastrointestinal (temperature, pH, pressure) parameter values, blood glucose and pressure levels and electrocardiogram data. Such data are first transmitted from the implantable sensor units to an external receiver node or network and then to a central monitoring and control (computer) unit for analysis, diagnosis and/or treatment. Implantable sensor units are typically in the form of mobile microrobotic capsules or implanted stationary (body-fixed) units. In particular, capsule-based systems have attracted significant research interest recently, with a variety of applications, including endoscopy, microsurgery, drug delivery and biopsy. In such implantable sensor systems, one of the most challenging problems is the accurate localization and tracking of the microrobotic sensor unit (e.g., robotic capsule) inside the human body. This article presents a literature review of the existing localization and tracking techniques for robotic implantable sensor systems with their merits and limitations and possible solutions of the proposed localization methods. The article also provides a brief discussion on the connection and cooperation of such techniques with wearable biomedical sensor systems.Show more