Browsing by Author "Kalfa, Mert"
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Item Open Access Adaptive measurement matrix design in compressed sensing based direction of arrival estimation(IEEE, 2021) Kılıç, Berkan; Güngör, Alper; Kalfa, Mert; Arıkan, OrhanDesign of measurement matrices is an important aspect of compressed sensing (CS) based direction of arrival (DoA) applications that enables reduction in the analog channels to be processed in sparse target environments. Here, a novel measurement matrix design methodology for CS based DoA estimation is proposed and its superior performance over alternative measurement matrix design methodologies is demonstrated. The proposed method uses prior probability distribution of the targets to improve performance. Compared to the state-of-the-art techniques, it is quantitatively demonstrated that the proposed measurement matrix design approach enables significant reduction in the number of analog channels to be processed and adapts to a priori information on the target scene.Item Open Access Adaptive measurement matrix design in direction of arrival estimation(IEEE, 2022-09-26) Kılıç, Berkan; Güngör, Alper; Kalfa, Mert; Arıkan, OrhanAdvances in compressed sensing (CS) theory have brought new perspectives to encoding and decoding of signals with sparse representations. The encoding strategies are determined by measurement matrices whose design is a critical aspect of the CS applications. In this study, we propose a novel measurement matrix design methodology for direction of arrival estimation that adapts to the prior probability distribution on the source scene, and we compare its performance over alternative approaches using both on-grid and gridless reconstruction methods. The proposed technique is derived in closed-form and shown to provide improved compression rates compared to the state-of-the-art. This technique is also robust to the uncertainty in the prior source information. In the presence of significant mutual coupling between antenna elements, the proposed technique is adapted to mitigate these mutual coupling effects.Item Open Access Analysis of cylindrically conformal antennas using closed-form Green's function representations(IEEE, 2015-04) Kalfa, Mert; Karan, S.; Ertürk, Vakur B.Probe-fed microstrip patch antennas and slotted sectoral waveguide array antennas embedded in cylindrically stratified media are analyzed with a hybrid Method of Moments/Green's function technique, where closed-form Green's function representations for electric and magnetic current sources are used as the kernel of the associated integral equations. Various patch and slot antennas are analyzed using the proposed method. Numerical results in the form of input impedance, S-parameters, and radiation patterns are presented and compared to the results obtained from CST Microwave Studio™ and HFSS™.Item Open Access Analysis of slotted sectoral waveguide antenna arrays embedded in cylindrically stratified media(Bilkent University, 2013) Kalfa, MertSlotted waveguide antenna arrays with dielectric covers are widely used in both military and civil applications due to their low-profile, high power handling capacity, and the ability to conform to the host platform. Conformity is especially required for air platforms where aerodynamics and radar cross section (RCS) of the vehicle are of utmost importance. For an air platform, one or more dielectric layers (monolithic or sandwich radomes) can be used to protect the antenna from the extreme flight conditions. Although accurate and efficient design and analysis of low-profile conformal slotted waveguide arrays is of great interest, available solution methods in the literature usually suffer in terms of efficiency and memory requirements. Among the available solution methods, integral equation (IE) based ones that utilize the Method of Moments (MoM) are widely used. However, the IE solvers suffer from long matrix fill times, especially when cylindrically stratified media are considered. In this study, a slotted sectoral waveguide antenna, coaxially covered by multiple dielectric layers is analyzed with a hybrid MoM/Green’s function technique in the space domain. Only the fundamental mode of propagation (TE11) is assumed to be excited inside the sectoral waveguide. The longitudinal slots are on the broadside wall of the sectoral waveguide and are very thin in the transverse direction; therefore, only the TE modes are assumed to propagate. The solution domain is divided into two by using the equivalence theorem and fictitious magnetic current sources on the waveguide slots. Note that for the purposes of this study, the waveguide wall thickness is assumed to be zero. However, it can be incorporated into the problem by adding a third region which would be a sectoral cavity. The magnetic sources on the slots are expanded by piecewise sinusoid basis functions, and the continuity of the tangential magnetic fields across the iii iv slots is enforced to construct the integral equation. The integral equation is then converted into a matrix equation using Galerkin’s procedure. To compute the elements of the mutual admittance matrix, two Green’s function representations for the two solution regions are used. For the sectoral waveguide interior, the dyadic Green’s function components for a sectoral waveguide corresponding to longitudinal magnetic currents are rigorously derived. For the cylindrically stratified dielectric region, closed-form Green’s function representations for magnetic currents are developed, which are valid for all source and observation points, including the source region, where two magnetic current modes fully or partially overlap with each other. The proposed analysis method can be easily extended to include: slotted substrate integrated waveguides, slotted cavity antennas, and similar aperture type antennas embedded in cylindrically stratified media. Numerical results in the form of equivalent slot currents and far-zone radiation patterns for a generic slotted sectoral waveguide are presented, and compared to the results obtained from the commercially available full-wave electromagnetic solversItem Open Access Analysis of slotted sectoral waveguide array antennas with multilayer radomes and nonzero metal thickness(IEEE, 2015-07) Kalfa, Mert; Ertürk, Vakur B.Slotted waveguide array antennas offer low cross-polarization and high power handling capacity, and they are low-profile which enables them to be used in conformal and structurally-integrated antenna solutions. Hence, they are excellent candidates for phased array antennas in radar applications, especially in air platforms. Aerodynamics and radar cross section (RCS) for an air platform are critical design considerations for air platforms; therefore, conformal and structurally-integrated solutions with integrated multilayer (sandwich) radomes are desired. Although the accurate and efficient design and analysis of low-profile conformal slotted waveguide arrays is of great interest, available solution methods in the literature usually suffer in terms of efficiency and memory requirements. Among the available solution methods, one of the widely used solvers are integral equation (IE) based ones that utilize the method of moments (MoM). However, IE solvers suffer from long matrix fill times, especially when cylindrically stratified media are considered. © 2015 IEEE.Item Open Access Analysis of slotted sectoral waveguides embedded in cylindrically stratified media using closed-form Green's function representations(IEEE, 2013) Kalfa, Mert; Erturk, Vakur B.Slotted waveguide antenna arrays with dielectric covers are widely used in both military and civil applications due to their low-profile, high power handling capacity, and the ability to conform to the host platform. Conformity is especially required for air platforms where aerodynamics and radar cross section (RCS) of the vehicle are of utmost importance. For an air platform, one or more dielectric layers (monolithic or sandwich radomes) can be used to protect the antenna from the extreme flight conditions. Although the accurate and efficient design and analysis of low-profile conformal slotted waveguide arrays is of great interest, available solution methods in the literature usually suffer in terms of efficiency and memory requirements. Among the available solution methods, one of the widely used solvers are integral equation (IE) based ones that utilize the method of moments (MoM). However, the IE solvers suffer from long matrix fill times, especially when cylindrically stratified media are considered. © 2013 IEEE.Item Open Access Derivation of Green's function representations for the analysis of sectoral waveguides embedded in cylindrically stratified media(IEEE, 2013) Kalfa, Mert; Ertürk, Vakur B.The design and analysis of dielectric-covered slotted waveguide arrays are of great interest in many military and civil applications, because of their low-profile, high efficiency and high power handling capabilities. Regarding the efficient and accurate analysis of such antennas, integral equation (IE) based methods that utilize the method of moments (MoM) or a hybrid combination of MoM with an appropriate Green's function (referred to as MoM/Green's function technique) are widely used. For the hybrid MoM/Green's function technique, appropriate dyadic Green's function representations for the waveguide interior and the exterior stratified media are required. However, in the case of a cylindrically conformal dielectric-covered slotted waveguide array, which may be desired due to aerodynamic and/or radar cross section (RCS) concerns, the abovementioned IE-based analysis that utilize a hybrid MoM/Green's function technique becomes a greater challenge due to the difficulties in the evaluation of the entries of MoM impedance/admittance matrices, especially for the terms related to cylindrically stratified media. © 2013 IEEE.Item Open Access Design and analysis of slotted sectoral waveguide array antennas embedded in cylindrically stratified media(IEEE, 2014-07) Kalfa, Mert; Ertürk Vakur B.Slotted waveguide antennas are being widely used in military and commercial applications for many decades. Low cross-polarization, high power capacity, ease of fabrication in microwave bands, and the ability to form arrays make them excellent candidates for phased array antennas in radar applications. However, due to slots being highly resonant (narrow-band, high Q) radiators, their design parameters are very sensitive; hence, accurate design and analysis methods are required for a successful antenna design. Moreover, slotted waveguide array antennas are low-profile structures, which makes them suitable candidates for conformal and structure-integrated applications. Conformal and structure-integrated system solutions are especially required for air platforms, where aerodynamics, radar cross-section (RCS) and efficient use of real estate are of utmost importance. Although the accurate and efficient design and analysis of low-profile conformal slotted waveguide arrays are of great interest, available solution methods in the literature usually suffer in terms of efficiency and memory requirements. Among the available solution methods, one of the widely used solvers are integral equation (IE) based ones that utilize the method of moments (MoM). However, IE solvers suffer from long matrix fill times, especially for matrix entries related to the cylindrically stratified media. © 2014 IEEE.Item Open Access Error analysis of MLFMA with closed-form expressions(IEEE, 2021-04-06) Kalfa, Mert; Ertürk, Vakur B.; Ergül, ÖzgürThe current state-of-the-art error control of the multilevel fast multipole algorithm (MLFMA) is valid for any given error threshold at any frequency, but it requires a multiple-precision arithmetic framework to be implemented. In this work, we use asymptotic approximations and curve-fitting techniques to derive accurate closed-form expressions for the error control of MLFMA that can be implemented in common fixed-precision computers. Moreover, using the proposed closed-form expressions in conjunction with the state-of-the-art scheme, we report novel design curves for MLFMA that can be used to determine achievable error limits, as well as the minimum box sizes that can be solved with a given desired error threshold for a wide range of machine precision levels.Item Open Access Error control in MLFMA with multiple-precision arithmetic(Institution of Engineering and Technology, 2018-04) Kalfa, Mert; Ergül, Ö.; Ertürk, Vakur B.We present a new error control method that provides the truncation numbers as well as the required digits of machine precision for the translation operator of the multilevel fast multipole algorithm (MLFMA). The proposed method is valid for all frequencies, whereas the previous studies on error control are valid only for high-frequency problems (i.e., electrically large translation distances). When combined with a multiple-precision implementation of MLFMA, the proposed method can be used to solve low-frequency problems that are problematic with a fixed-precision implementation. Numerical results in the form of optimal truncation numbers and machine precisions for a variety of box sizes and desired relative error thresholds are presented and compared with the methods or numerical surveys available in the literature.Item Open Access Multiple-precision MLFMA for efficient and accurate solutions of broadband electromagnetic problems(Bilkent University, 2020-08) Kalfa, MertThe multilevel fast multipole algorithm (MLFMA) is a popular full-wave electromagnetic solver that enables the solution of electrically large problems with an extremely large number of unknowns. As with all computational electromagnetics solvers, active research is ongoing to extend the limitations of MLFMA for larger problems with finer geometrical details. For electrically small structures MLFMA suffers from the low-frequency breakdown, while more efficient schemes are required for electrically larger problems. We propose and demonstrate an elegant solution to the aforementioned problems by introducing a multiple-precision arithmetic (MPA) framework to the inherent hierarchical tree structure of MLFMA, dubbed the multiple-precision multilevel fast multipole algorithm (MP-MLFMA). With the introduction of MPMLFMA we show that a distinct machine precision can be assigned to each level of the tree structure of MLFMA, which enables accurate and efficient solutions of problems with deep tree-structures over arbitrarily large frequency bandwidths. To determine the required machine precisions for a given tree structure, as well as the number of harmonics required for an accurate error control of the translation operator of MP-MLFMA, we introduce and validate a novel error control scheme with accurate design curves that is valid at all frequencies, for the first time in the literature. Combined with the proposed error control scheme, we present the capabilities of MP-MLFMA over a wide range of broadband and deep tree-structure scattering problems. We also illustrate the true potential efficiency of MP-MLFMA, with a simple MPA framework implementation on a single-precision processor. With the hardware-defined implementation, we show the super-linear speed-up potential of MP-MLFMA for low-precisions.Item Open Access Parçacık süzgeci kullanarak uyarlamalı sıkıştırılmış algılama tabanlı geliş yönü kestirimi(IEEE, 2020-12-18) Kılıç, Berkan; Güngör, Alper; Kalfa, Mert; Arıkan, OrhanGeliş yönü kestirimi (GYK) problemlerinde sinyaller arkaplan uzayında, nispeten az boyutlu bir manifoldda yatar. Bu nedenle, sıkıştırılmış algılama teknikleri güvenilir geliş yönü kestirimine olanak verir. Ayrıca, ardışık Monte Carlo yöntemlerini kullanmak, tek bir noktasal kestirim yerine GYK için bir olasılık dağılımı elde etmeye olanak tanır. Sonuç olarak, bu olasılık dağılımı ölçüm matrisi tasarımında kullanıldığında, yüksek kestirim başarımı ile birlikte anten dizisi sinyal işlemede boyut indirgeme sağlanır. Bu çalışmada, olasılık dağılımı elde etmek için parçacık süzgeci kullanan, uyarlamalı sıkıştırılmış algılama tabanlı bir ızgarasız GYK yöntemi önerilmiştir. Ölçüm matrisinin rasgele seçilmesinin ve tasarlanmasının başarımları karşılaştırılmıştır. Ölçüm matrisi tasarımının, antenler üzerindeki ölçüm gürültüsüne bağlı olarak kestirim başarım artırımı, bir dizi benzetim ile gösterilmiştir.Item Open Access Reliable extraction of semantic information and rate of innovation estimation for graph signals(Institute of Electrical and Electronics Engineers , 2022-12-19) Kalfa, Mert ; Yetim, Sadık Yağız ; Atalik, Arda ; Gök, Mehmetcan; Ge, Y.; Li, R.; Tong, W.; Duman, Tolga Mete; Arıkan, OrhanSemantic signal processing and communications are poised to play a central part in developing the next generation of sensor devices and networks. A crucial component of a semantic system is the extraction of semantic signals from the raw input signals, which has become increasingly tractable with the recent advances in machine learning (ML) and artificial intelligence (AI) techniques. The accurate extraction of semantic signals using the aforementioned ML and AI methods, and the detection of semantic innovation for scheduling transmission and/or storage events are critical tasks for reliable semantic signal processing and communications. In this work, we propose a reliable semantic information extraction framework based on our previous work on semantic signal representations in a hierarchical graph-based structure. The proposed framework includes a time integration method to increase fidelity of ML outputs in a class-aware manner, a graph-edit-distance based metric to detect innovation events at the graph-level and filter out sporadic errors, and a Hidden Markov Model (HMM) to produce smooth and reliable graph signals. The proposed methods within the framework are demonstrated individually and collectively through simulations and case studies based on real-world computer vision examples.Item Open Access Towards goal-oriented semantic signal processing: Applications and future challenges(Elsevier, 2021-06-15) Kalfa, Mert; Gök, Mehmetcan; Atalık, Arda; Tegin, Büşra; Arıkan, Orhan; Duman, Tolga MeteAdvances in machine learning technology have enabled real-time extraction of semantic information in signals which can revolutionize signal processing techniques and improve their performance significantly for the next generation of applications. With the objective of a concrete representation and efficient processing of the semantic information, we propose and demonstrate a formal graph-based semantic language and a goal filtering method that enables goal-oriented signal processing. The proposed semantic signal processing framework can easily be tailored for specific applications and goals in a diverse range of signal processing applications. To illustrate its wide range of applicability, we investigate several use cases and provide details on how the proposed goal-oriented semantic signal processing framework can be customized. We also investigate and propose techniques for communications where sensor data is semantically processed and semantic information is exchanged across a sensor network.