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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/6
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Publication Open Access Early/fast VLF events produced by the quiescent heating of the lower ionosphere by thunderstorms(American Geophysical Union (AGU), 2017) Kabirzadeh, R.; Marshall, R. A.; Department of Electrical and Electronics Engineering; Department of Electrical and Electronics Engineering; İnan, Umran Savaş; Faculty Member; College of Engineering; 177880Large and easily distinguishable perturbations of the VLF transmitter signals due to interactions with thundercloud-driven ionospheric modifications have been observed and studied for about three decades. These events are called "early/fast VLF" or "early VLF" events due to their immediate detection (similar to 20 ms) after the causative lightning flash on the ground and the fast rise time of the perturbed signal. Despite many years of study, the physical mechanisms responsible for these perturbations are still under investigation. Modifications of the sustained heating level of the ionosphere due to a lightning flash has been previously proposed as the causative mechanism of early/fast VLF events. The perturbations predicted by this mechanism, however, have been much smaller than experimental observations of 0.2-1 dB or higher. In this study, by using an improved 3-D thundercloud electrostatic upward coupling model which uses a realistic geomagnetic field, we find that the sustained heating model can predict perturbations that are consistent with reported experimental observations. Modifications in the quiescent heating of the lower ionosphere by thundercloud fields by individual lightning flashes may thus account for some observations of early/fast VLF events. Large and easily distinguishable perturbations of the VLF transmitter signals due to interactions with thundercloud-driven ionospheric modifications have been observed and studied for about three decades. These events are called "early/fast VLF" or "early VLF" events due to their immediate detection (similar to 20 ms) after the causative lightning flash on the ground and the fast rise time of the perturbed signal. Despite many years of study, the physical mechanisms responsible for these perturbations are still under investigation. Modifications of the sustained heating level of the ionosphere due to a lightning flash has been previously proposed as the causative mechanism of early/fast VLF events. The perturbations predicted by this mechanism, however, have been much smaller than experimental observations of 0.2-1 dB or higher. In this study, by using an improved 3-D thundercloud electrostatic upward coupling model which uses a realistic geomagnetic field, we find that the sustained heating model can predict perturbations that are consistent with reported experimental observations. Modifications in the quiescent heating of the lower ionosphere by thundercloud fields by individual lightning flashes may thus account for some observations of early/fast VLF events.Publication Open Access Comparative spectroscopic investigation of Tm3+: tellurite glasses for 2-mu m lasing applications(Multidisciplinary Digital Publishing Institute (MDPI), 2018) Kurt, Adnan; Speghini, Adolfo; Bettinelli, Marco; Department of Electrical and Electronics Engineering; Department of Physics; Department of Electrical and Electronics Engineering; Department of Physics; Çankaya, Hüseyin; Görgülü, Adil Tolga; Sennaroğlu, Alphan; Researcher; Master Student; Faculty Member; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Engineering; College of Sciences; N/A; N/A; 23851We performed a comparative spectroscopic analysis on three novel Tm3+: tellurite-based glasses with the following compositions Tm2O3: TeO2-ZnO (TeZnTm), Tm2O3: TeO2-Nb2O5 (TeNbTm), and Tm3+: TeO2-K2O-Nb2O5 (TeNbKTm), primarily for 2-mu m laser applications. Tellurite glasses were prepared at different doping concentrations in order to investigate the effect of Tm3+ ion concentration as well as host composition on the stimulated emission cross sections and the luminescence quantum efficiencies. By performing Judd-Ofelt analysis, we determined the average radiative lifetimes of the H-3(4) level to be 2.55 +/- 0.07 ms, 2.76 +/- 0.03 ms and 2.57 +/- 0.20 ms for the TeZnTm, TeNbTm and TeNbKTm samples, respectively. We clearly observed the effect of the cross-relaxation, which becomes significant at higher Tm2O3 concentrations, leading to the quenching of 1460-nm emission and enhancement of 1860-nm emission. Furthermore, with increasing Tm2O3 concentrations, we observed a decrease in the fluorescence lifetimes as a result of the onset of non-radiative decay. For the H-3(4) level, the highest obtained quantum efficiency was 32% for the samples with the lowest Tm2O3 ion concentration. For the 1860-nm emission band, the average emission cross section was determined to measure around 6.33 +/- 0.34 x 10(-21) cm(2), revealing the potential of thulium-doped tellurite gain media for 2-mu m laser applications in bulk and fiber configurations.Publication Open Access Compressed training based massive MIMO(Institute of Electrical and Electronics Engineers (IEEE), 2019) Yılmaz, Baki Berkay; Department of Electrical and Electronics Engineering; Department of Electrical and Electronics Engineering; Erdoğan, Alper Tunga; Faculty Member; College of Engineering; 41624Massive multiple-input-multiple-output (MIMO) scheme promises high spectral efficiency through the employment of large scale antenna arrays in base stations. In time division duplexed implementations, co-channel mobile terminals transmit training information such that base stations can estimate and exploit channel state information to spatially multiplex these users. In the conventional approach, the optimal choice for training length was shown to be equal to the number of users, K. In this paper, we propose a new semiblind framework, named as "MIMO Compressed Training," which utilizes information symbols in addition to training symbols for adaptive spatial multiplexing. We show that this framework enables us to reduce (compress) the training length down to a value close to log(2) (K), i.e., the logarithm of the number of users, without any sparsity assumptions on the channel matrix. We also derive a prescription for the required packet length for proper training. The framework is built upon some convex optimization settings that enable efficient and reliable algorithm implementations. The numerical experiments demonstrate the strong potential of the proposed approach in terms of increasing the number of users per cell and improving the link quality.Publication Open Access On the capacity of diffusion-based molecular communications with SiNW FET-based receiver(Institute of Electrical and Electronics Engineers (IEEE), 2016) Department of Electrical and Electronics Engineering; Department of Electrical and Electronics Engineering; Kuşcu, Murat; Akan, Özgür Barış; Faculty Member; College of EngineeringMolecular communication (MC) is a bio-inspired communication method based on the exchange of molecules for information transfer among nanoscale devices. Although MC has been extensively studied from various aspects, limitations imposed by the physical design of transceiving units have been largely neglected in the literature. Recently, we have proposed a nanobioelectronic MC receiver architecture based on the nanoscale field effect transistor-based biosensor (bioFET) technology, providing noninvasive and sensitive molecular detection at nanoscale while producing electrical signals at the output. In this paper, we derive analytical closed-form expressions for the capacity and capacity-achieving input distribution for a memoryless MC channel with a silicon nanowire (SiNW) FET-based MC receiver. The resulting expressions could be used to optimize the information flow in MC systems equipped with nanobioelectronic receivers.Publication Open Access HF beam parameters in ELF/VLF wave generation via modulated heating of the ionosphere(American Geophysical Union (AGU), 2012) Cohen, M. B.; Golkowski, M.; Lehtinen, N. G.; McCarrick, M. J.; Department of Electrical and Electronics Engineering; Department of Electrical and Electronics Engineering; İnan, Umran Savaş; Faculty Member; College of Engineering; 177880ELF/VLF (0.3–30 kHz) wave generation is achievable via modulated HF (3–30 MHz) heating of the lower ionosphere in the presence of natural currents such as the auroral electrojet. Using the 3.6 MW High Frequency Active Auroral Research Program (HAARP) facility near Gakona, AK, we investigate the effect of HF frequency and beam size on the generated ELF/VLF amplitudes, as a function of modulation frequency, and find that generation in the Earth-ionosphere waveguide generally decreases with increasing HF frequency between 2.75–9.50 MHz. HAARP is also capable of spreading the HF power over a wider area, and we find that a larger beam area yields larger generated amplitudes on the ground. Measurements are shown to generally agree with a theoretical model, which is then applied to also predict the effect of HF beam parameters on magnetospheric injection with HAARP.Publication Open Access Location-aware adaptive physical layer design for vehicular visible light communication(Institute of Electrical and Electronics Engineers (IEEE), 2019) Department of Electrical and Electronics Engineering; Department of Electrical and Electronics Engineering; Gürbilek, Gökhan; Koca, Mertkan; Uyrus, Ali; Soner, Burak; Ergen, Sinem Çöleri; Başar, Ertuğrul; Faculty Member; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; N/A; N/A; 7211; 149116Vehicular visible light communication (V2LC) is expected to complement radio frequency (RF) technologies for higher reliability in vehicular connectivity. Since high mobility makes the line-of-sight V2LC channel very dynamic, an adaptive physical layer (PHY) design is required for realizing a rate-optimal and reliable V2LC system. Existing studies on adaptive PHY designs have mostly considered indoor scenarios with low mobility and require a feedback channel for both reporting the received signal-to-noise ratio (SNR) to the transmitter and channel equalization (CE), which increases system complexity and introduces overhead. This paper presents a novel low-complexity adaptive PHY design that provides rate-optimal and reliable V2LC without a feedback channel. The proposed design utilizes a priori measurements of the BER with respect to SNR, which are static for V2LC on the road. SNR is predicted in real-time based on the relative locations of the transmitting (TX) and receiving (RX) vehicles using a path loss model based on a priori measurements of the SNR-distance relationship and the polar beam pattern for a given TX/RX pair, in a given setting. The proposed design is validated via night-time experiments with On-Off-Keying (OOK), 4-Pulse-Position Modulation (4-PPM) and Direct Current-Biased Optical OFDM (DCO-OFDM). The proposed location-aware adaptive PHY design can be expanded for general reliable rate-optimal V2LC use by updating the path loss model with additional measurements for different settings.Publication Open Access Optimum low-complexity decoder for ppatial modulation(Institute of Electrical and Electronics Engineers (IEEE), 2019) Al-Nahhal, İbrahim; Dobre, Octavia A.; Ikki, Salama; Department of Electrical and Electronics Engineering; Department of Electrical and Electronics Engineering; Başar, Ertuğrul; Faculty Member; College of Engineering; 149116In this paper, a novel low-complexity detection algorithm for spatial modulation (SM), referred to as the minimum-distance of maximum-length (m-M) algorithm, is proposed and analyzed. The proposed m-M algorithm is a smart searching method that is applied for the SM tree-search decoders. The behavior of the m-M algorithm is studied for three different scenarios: 1) perfect channel state information at the receiver side (CSIR); 2) imperfect CSIR of a fixed channel estimation error variance; and 3) imperfect CSIR of a variable channel estimation error variance. Moreover, the complexity of the m-M algorithm is considered as a random variable, which is carefully analyzed for all scenarios, using probabilistic tools. Based on a combination of the sphere decoder (SD) and ordering concepts, the m-M algorithm guarantees to find the maximum-likelihood (ML) solution with a significant reduction in the decoding complexity compared with SM-ML and existing SM-SD algorithms; it can reduce the complexity up to 94% and 85% in the perfect CSIR and the worst scenario of imperfect CSIR, respectively, compared with the SM-ML decoder. The Monte Carlo simulation results are provided to support our findings as well as the derived analytical complexity reduction expressions.Publication Open Access Augmented reality 3d display using head-mounted projectors and transparent retro-reflective screen(Society of Photo-optical Instrumentation Engineers (SPIE), 2017) Department of Electrical and Electronics Engineering; Department of Electrical and Electronics Engineering; Soomro, Shoaib Rehman; Ürey, Hakan; Faculty Member; College of Engineering; N/A; 8579A 3D augmented reality display is proposed that can provide glass-free stereo parallax using a highly transparent projection screen. The proposed display is based on a transparent retro-reflective screen and a pair of laser pico projectors placed close to the viewer's head. The retro-reflective screen directs incident light towards its source with little scattering so that each of the viewer's eyes only perceives the content projected by the associated projector. Each projector displays one of the two components (left or right channel) of stereo content. The retro-reflective nature of screen provides high brightness compared to the regular diffused screens. The partially patterned retro-reflective material on clear substrate introduces optical transparency and facilitates the viewer to see the real-world scene on the other side of screen. The working principle and design of the proposed see-through 3D display are presented. A tabletop prototype consisting of an in-house fabricated 60x40cm(2) see-through retro-reflective screen and a pair of 30 lumen pico-projectors with custom 3D printed housings is demonstrated. Geometric calibration between projectors and optimal viewing conditions (eye box size, eye-to-projector distance) are discussed. The display performance is evaluated by measuring the brightness and crosstalk for each eye. The screen provides high brightness (up to 300 cd/ m2 per eye) using 30 lumens mobile projectors while maintaining the 75% screen transparency. The crosstalk between left and right views is measured as < 10% at the optimum distance of 125-175 cm, which is within acceptable range.Publication Open Access Highly intense lightning over the oceans: estimated peak currents from global GLD360 observations(American Geophysical Union (AGU), 2013) Said, R. K.; Cohen, M. B; Department of Electrical and Electronics Engineering; Department of Electrical and Electronics Engineering; İnan, Umran Savaş; Faculty Member; College of Engineering; 177880We present the first global distribution of the average estimated peak currents in negative lightning ?ashes using 1 year of continuous data from the Vaisala global lightning data set GLD360. The data set, composed of 353 million ?ashes, was compared with the National Lightning Detection NetworkTM for peak current accuracy, location accuracy, and detection efficiency. The validation results demonstrated a mean (geometric mean) peak current magnitude error of 21% (6%), a median location accuracy of 2.5 km, and a relative ground ?ash detection efficiency of 57% averaged over all positive and negative reference ?ashes, and 67% for all reference ?ashes above 15 kA. The distribution of peak currents for negative ?ashes shifts to higher magnitudes over the ocean. Three case study 10ı10ı regions are analyzed, in which the peak current enhancement is extremely sharp at the coastline, suggesting that the higher peak currents for oceanic lightning cannot be solely attributable to network artifacts such as detection efficiency and peak current estimation error. In these regions, the geometric mean and 95th percentile of the peak current distribution for negative cloud to ocean ?ashes is 22%–88% and 65%–121% higher, respectively, compared to cloud to ground ?ashes in nearby land regions. Globally, the majority of all negative ?ashes with estimated peak current magnitude above 75 kA occur over the ocean.Publication Open Access Nonlinear nanomechanical mass spectrometry at the single-nanoparticle level(American Chemical Society (ACS), 2019) Yüksel, Mert; Orhan, Ezgi; Yanık, Cenk; Arı, Atakan B.; Hanay, M. Selim; Department of Electrical and Electronics Engineering; Department of Electrical and Electronics Engineering; Demir, Alper; Faculty Member; College of Engineering; 3756Nanoelectromechanical systems (NEMS) have emerged as a promising technology for performing the mass spectrometry of large biomolecules and nanoparticles. As nanoscale objects land on NEMS sensors one by one, they induce resolvable shifts in the resonance frequency of the sensor proportional to their weight. The operational regime of NEMS sensors is often limited by the onset of nonlinearity, beyond which the highly sensitive schemes based on frequency tracking by phase-locked loops cannot be readily used. Here, we develop a measurement architecture with which to operate at the nonlinear regime and measure frequency shifts induced by analytes in a rapid and sensitive manner. We used this architecture to individually characterize the mass of gold nanoparticles and verified the results by performing independent measurements of the same nanoparticles based on linear mass sensing. Once the feasibility of the technique is established, we have obtained the mass spectrum of a 20 nm gold nanoparticle sample by individually recording about 500 single-particle events using two modes working sequentially in the nonlinear regime. The technique obtained here can be used for thin nanomechanical structures that possess a limited dynamic range.