### Browsing by Subject "Signal to noise ratio"

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Item Open Access Anisotropy sensitivity of an acoustic lens with slit aperture(IEEE, 1993) Atalar, Abdullah; Ishikawa, I.; Ogura, Y.; Tomita, K.Show more A conventional spherical acoustic lens is modified by restricting its aperture in the form of a slit to provide directional sensitivity. The spacing between the two parallel absorbing sheets forming the slit is adjustable to obtain varying slit widths. The resulting lens can be used in conjunction with V(Z) method to obtain leaky wave velocities of the sample under investigation as a function of direction. The theoretical V(Z) analysis of the lens involves a two-dimensional integral rather than one-dimensional integral of the conventional lens. Single crystal anisotropic materials are chosen as test samples. Reflection coefficients for anisotropic single crystals of given surface cut and orientation are calculated. Numerically evaluated V(Z) curves are used to deduce the surface wave velocity of the object for the given orientation. This is compared with the surface wave velocity directly calculated from the elastic parameters of the object. Results show the compromise between signal-to-noise ratio and angular resolution as the slit width is varied. V(Z) measurement results of a slitted lens are presented to be compared with calculated curves. The new lens is used to measure the acoustic velocity on the (001) surface of GaAs along varying directions with differing slit widths.Show more Item Open Access Capacity bounds for an ultra-wideband channel model(IEEE, 2004-10) Arıkan, ErdalShow more There is an ongoing effort by the IEEE 802.15.3a subcommittee to reach a UWB personal area network standard. We estimate the achievable rates for such networks using a channel model specified by the same group. The analysis of this channel model is of interest in light of recent information-theoretic work on multipath fading channels which show that in order to take full advantage of such channels' capacity the transmitted signals have to be "peaky" in a certain sense. The immense bandwidth of the UWB channel also suggests at first that peaky signals should be used. However, unlike the many other wireless systems where the transmitter energy is limited, in the UWB channel only the power spectral density of the transmitted signal is constrained. As a result, the signal power can grow in proportion to the utilized bandwidth and peaky signals are not needed. © 2004 IEEE.Show more Item Open Access Çokyollu kanal parametre kestirimi için yeni bir dizilim sinyal işleme tekniği(IEEE, 2007-06) Güldoǧan, Mehmet Burak; Arıkan, OrhanShow more Bu bildiride, çarpraz belirsizlik işlevinin kullanıldığı yeni bir dizilim sinyal işleme tekniği önerilmektedir. Geliştirilen teknik bir algılayıcı dizilimine gelen sinyallerden herbirinin geliş yönünü (GY), zaman gecikmesini Doppler kaymasını ve genliğini dürümlü bir sekilde kestirir. Önerilen Çarpraz Belirsizlik İşlevi - Yön Bulma (ÇBI-YB) tekniği ile Çoklu Sinyal Sınıflandırması (MUSIC) algoritmasının performansları sentetik sinyaller kullanılarak kök Ortalama Karesel Hata (kOKH) cinsinden değişik işaret Gürültü Oranı (İGO) değerleri için karşılaştırılmıştır. Önerilen tekniğin başarımı kayıt edilmiş çokyollu yüksek-enlem iyonosfer verileri üzerinde irdelenmiştir. Elde edilen sonuçlar, düşük İGO değerlerinde dahi çokyollu sinyal kaynaklarını ayırmada önerilen ÇBİ-YB tekniğinin ciddi başarım artışı sağladığını göstermektedir.Show more Item Open Access Çokyollu ortamda çapraz belirsizlik işlevi-yön bulma tekniğinin başarım analizi(IEEE, 2008-04) Güldoǧan, Mehmet Burak; Arıkan, OrhanShow more Bu bildiride, Çapraz Belirsizlik İşlevi-Yön Bulma (CAFDF) tekniğinin çokyollu ortamlardaki sinyallerin zaman gecikmesi, Doppler kayması, geliş yönü(GY) ve genlik kestirimindeki başarımı ile yiiksek çözünürlüklü algoritmalar olan Uzay-Almaşan Genelleşmiş Beklenti-Enbüyüitme (SAGE) ve Çoklu Sinyal Sınıflandırılması(MUSIC)'in sentetik sinyaller iizerindeki başarımları kıyaslanmıştır. Algoritmalann performansları, kök Ortalama Karesel Hata (kOKH) cinsinden degişik işaret Gürültü Oranı (iGO) değerlerinde Monte Carlo denemelerine dayalı olarak sunulmuştur. Sentetik kanallarda istatiksel kıyaslama amaçlı Cramer-Rao alt sınırları eklenmiştir. Simülasyon sonuçları göstermektedir ki, orta ve düşük iGO değerlerinde CAF-DF diğer iki algoritmaya göre üstünlük sağlamaktadır.Show more Item Open Access Comparison of the CAF-DF and sage algorithms in multipath channel parameter estimation(IEEE, 2008-07) Güldoğan, M. Burak; Arıkan, OrhanShow more In this paper, performance of the recently proposed Cross Ambiguity Function - Direction Finding (CAF-DF) technique is compared with the Space Alternating Generalized Expectation Maximization (SAGE) technique. The CAF-DF, iteratively estimates direction of arrival (DOA), time-delay, Doppler shift and amplitude corresponding to each impinging signal onto an antenna array by utilizing the cross ambiguity function. On synthetic signals, based on Monte Carlo trials, performances of the algoritms are tested in terms of root Mean Squared Error (rMSE) at different Signal-to-Noise Ratios (SNR). Cramer-Rao lower bound is included for statistical comparisons. Simulation results indicate the superior performance of the CAF-DF technique over SAGE technique for low and medium SNR values. © 2008 IEEE.Show more Item Open Access Compressive sensing-based robust off-the-grid stretch processing(Institution of Engineering and Technology, 2017) Ilhan, I.; Gurbuz, A. C.; Arıkan, OrhanShow more Classical stretch processing (SP) obtains high range resolution by compressing large bandwidth signals with narrowband receivers using lower rate analogue-to-digital converters. SP achieves the resolution of the large bandwidth signal by focusing into a limited range window, and by deramping in the analogue domain. SP offers moderate data rate for signal processing for high bandwidth waveforms. Furthermore, if the scene in the examined window is sparse, compressive sensing (CS)-based techniques have the potential to further decrease the required number of measurements. However, CS-based reconstructions are highly affected by model mismatches such as targets that are off-the-grid. This study proposes a sparsity-based iterative parameter perturbation technique for SP that is robust to targets off-the-grid in range or Doppler. The error between reconstructed and actual scenes is measured using Earth mover's distance metric. Performance analyses of the proposed technique are compared with classical CS and SP techniques in terms of data rate, resolution and signal-to-noise ratio. It is shown through simulations that the proposed technique offers robust and high-resolution reconstructions for the same data rate compared with both classical SP- and CS-based techniques.Show more Item Open Access Deep-collapse operation of capacitive micromachined ultrasonic transducers(IEEE, 2011) Olcum, S.; Yamaner F. Y.; Bozkurt, A.; Atalar, AbdullahShow more Capacitive micromachined ultrasonic transducers (CMUTs) have been introduced as a promising technology for ultrasound imaging and therapeutic ultrasound applications which require high transmitted pressures for increased penetration, high signal-to-noise ratio, and fast heating. However, output power limitation of CMUTs compared with piezoelectrics has been a major drawback. In this work, we show that the output pressure of CMUTs can be significantly increased by deep-collapse operation, which utilizes an electrical pulse excitation much higher than the collapse voltage. We extend the analyses made for CMUTs working in the conventional (uncollapsed) region to the collapsed region and experimentally verify the findings. The static deflection profile of a collapsed membrane is calculated by an analytical approach within 0.6% error when compared with static, electromechanical finite element method (FEM) simulations. The electrical and mechanical restoring forces acting on a collapsed membrane are calculated. It is demonstrated that the stored mechanical energy and the electrical energy increase nonlinearly with increasing pulse amplitude if the membrane has a full-coverage top electrode. Utilizing higher restoring and electrical forces in the deep-collapsed region, we measure 3.5 MPa peak-to-peak pressure centered at 6.8 MHz with a 106% fractional bandwidth at the surface of the transducer with a collapse voltage of 35 V, when the pulse amplitude is 160 V. The experimental results are verified using transient FEM simulations.Show more Item Open Access Downlink beamforming under individual SINR and per antenna power constraints(IEEE, 2007-08) Yazarel, Y. K.; Aktaş, DefneShow more In this paper we consider the problem of finding the optimum beamforming vectors for the downlink of a multiuser system, where there are individual signal to interference plus noise ratio (SINR) targets for each user. Majority of the previous work on this problem assumed a total power constraint on the base stations. However, since each transmit antenna is limited by the amount of power it can transmit due to the limited linear region of the power amplifliers, a more realistic constraint is to place a limit on the per antenna power. In a recent work, Yu and Lan proposed an iterative algorithm for computing the optimum beamforming vectors minimizing the power margin over all antennas under individual SINR and per antenna power constraints. However, from a system designer point of view, it may be more desirable to minimize the total transmit power rather than minimizing the power margin, especially when the system is not symmetric. Reformulating the transmitter optimization problem to minimize the total transmit power subject to individual SINR constraints on the users and per antenna power constraints on the base stations, the algorithm proposed by Yu and Lan is modified. Performance of the modified algorithm is compared with existing methods for various cellular array scenarios. ©2007 IEEE.Show more Item Open Access Fast processing techniques for accurate ultrasonic range measurements(Institute of Physics Publishing, 2000) Barshan, B.Show more Four methods of range measurement for airborne ultrasonic systems - namely simple thresholding, curve-fitting, sliding-window, and correlation detection - are compared on the basis of bias error, standard deviation, total error, robustness to noise, and the difficulty/complexity of implementation. Whereas correlation detection is theoretically optimal, the other three methods can offer acceptable performance at much lower cost. Performances of all methods have been investigated as a function of target range, azimuth, and signal-to-noise ratio. Curve fitting, sliding window, and thresholding follow correlation detection in the order of decreasing complexity. Apart from correlation detection, minimum bias and total error is most consistently obtained with the curve-fitting method. On the other hand, the sliding-window method is always better than the thresholding and curve-fitting methods in terms of minimizing the standard deviation. The experimental results are in close agreement with the corresponding simulation results. Overall, the three simple and fast processing methods provide a variety of attractive compromises between measurement accuracy and system complexity. Although this paper concentrates on ultrasonic range measurement in air, the techniques described may also find application in underwater acoustics.Show more Item Open Access The fractional fourier transform and its applications to image representation and beamforming(ASME, 2003-09) Yetik, I. Ş; Kutay, M. A.; Özaktaş, Haldun. M.Show more The ath order fractional Fourier transform operator is the ath power of the ordinary Fourier transform operator. We provide a brief introduction to the fractional Fourier transform, discuss some of its more important properties, and concentrate on its applications to image representation and compression, and beamforming. We show that improved performance can be obtained by employing the fractional Fourier transform instead of the ordinary Fourier transform in these applications.Show more Item Open Access Huber function based reconstruction in accelerated phase-cycled bSSFP acquisitions for increased detection performance(IEEE, 2017) Ilıcak, Efe; Çukur, TolgaShow more Balanced steady-state free precession imaging suffers from irrecoverable signal losses, called banding artifacts. A common way to alleviate banding artifacts without sacrificing scan-efficiency is to use multiple-acquisition methods that combine phase-cycled images. However, soft thresholding applications used during the recovery can reduce the detection performance and image quality. In this study, a reconstruction strategy that applies Huber function to increase detection sensitivity on small coefficients is evaluated. This strategy is compared with conventional methods in terms of peak signal to noise ratio and structural similarity index.Show more Item Open Access Image reconstruction for Magnetic Particle Imaging using an Augmented Lagrangian Method(IEEE, 2017) Ilbey S.; Top C.B.; Çukur, Tolga; Sarıtaş, Emine Ülkü; Guven H.E.Show more Magnetic particle imaging (MPI) is a relatively new imaging modality that images the spatial distribution of superparamagnetic iron oxide nanoparticles administered to the body. In this study, we use a new method based on Alternating Direction Method of Multipliers (a subset of Augmented Lagrangian Methods, ADMM) with total variation and l1 norm minimization, to reconstruct MPI images. We demonstrate this method on data simulated for a field free line MPI system, and compare its performance against the conventional Algebraic Reconstruction Technique. The ADMM improves image quality as indicated by a higher structural similarity, for low signal-to-noise ratio datasets, and it significantly reduces computation time. © 2017 IEEE.Show more Item Open Access In situ synthesis of biomolecule encapsulated gold-cross-linked poly(ethylene glycol) nanocomposite as biosensing platform: A model study(Elsevier BV, 2010) Odaci, D.; Kahveci, M.U.; Sahkulubey, E.L.; Ozdemir, C.; Uyar, Tamer; Timur, S.; Yagci Y.Show more In situ synthesis of poly(ethylene glycol) (PEG) hydrogels containing gold nanoparticles(AuNPs) and glucose oxidase (GOx) enzyme by photo-induced electron transfer process was reported here and applied in electrochemical glucose biosensing as the model system. Newly designed bionanocomposite matrix by simple one-step fabrication offered a good contact between the active site of the enzyme and AuNPs inside the network that caused the promotion in the electron transfer properties that was evidenced by cyclic voltammetryas well as higher amperometric biosensing responses in comparing with response signals obtained from the matrix without AuNPs. As well as some parameters important in the optimization studies such as optimum pH, enzyme loading and AuNP amount, the analytical characteristics of the biosensor (AuNP/GOx) were examined by the monitoring of chronoamperometric response due to the oxygen consumption through the enzymatic reaction at − 0.7 V under optimized conditions at sodium acetate buffer (50 mM, pH 4.0) and the linear graph was obtained in the range of 0.1–1.0 mM glucose. The detection limit (LOD) of the biosensor was calculated as 0.06 mM by using the signal to noise ratio of 3. Moreover, the presence of AuNPs was visualized by TEM. Finally, the biosensor was applied for glucose analysis for some beverages and obtained data were compared with HPLC as the reference method to test the possible matrix effect due to the nature of the samples.Show more Item Open Access Microfabricated ultrasonic transducers: towards robust models and immersion devices(IEEE, 1996-11) Ladabaum, I.; Jin, X.; Soh, H. T.; Pierre, F.; Atalar, Abdullah; Khuri-Yakub, B. T.Show more The successful fabrication of ultrasonic immersion transducers is reported. Transducers are observed to operate from 1 MHz to 20 MHz in water, with the frequency range limited by electronics, not the transducers. Transmission results are included which show that a single pair of transducers is able to operate in water at 4, 6, and 8 MHz with a signal to noise ratio of at least 48 dB. The same transducer pair is shown to operate in air at 6 MHz. A model is introduced which highlights the significant parameters of transducer design. The model enables the design of optimized transducers.Show more Item Open Access Mode separation and direction of arrival estimation in HF links(Wiley-Blackwell Publishing, 2003) Arikan, F.; Yilmaz, N.; Arıkan, Orhan; Miled, M. K. B. H.Show more Estimation of arrival angles and incoming signals is a challenging problem for HF channels where the signals are correlated and the separation between the signals can be as low as a couple of degrees. In this paper, a new algorithm, Multipath Separation- Direction of Arrival (MS-DOA), is developed to estimate both the arrival angles in elevation and azimuth and the incoming signals at the output of the reference antenna with very high accuracy. The MS-DOA algorithm provides reliable angle and signal estimates even with small separation of arrival angles and for low SNRs. The minimum number of antennas that are required by the algorithm is only one more than the number of incoming signals. In a narrowed down region of interest and for a few incoming signals, the computational search time for MS-DOA is only a couple of minutes in a Standard PC.Show more Item Open Access On probability of success in linear and differential cryptanalysis(Springer New York LLC, 2008-01) Selçuk, A. A.Show more Despite their widespread usage in block cipher security, linear and differential cryptanalysis still lack a robust treatment of their success probability, and the success chances of these attacks have commonly been estimated in a rather ad hoc fashion. In this paper, we present an analytical calculation of the success probability of linear and differential cryptanalytic attacks. The results apply to an extended sense of the term "success" where the correct key is found not necessarily as the highest-ranking candidate but within a set of high-ranking candidates. Experimental results show that the analysis provides accurate results in most cases, especially in linear cryptanalysis. In cases where the results are less accurate, as in certain cases of differential cryptanalysis, the results are useful to provide approximate estimates of the success probability and the necessary plaintext requirement. The analysis also reveals that the attacked key length in differential cryptanalysis is one of the factors that affect the success probability directly besides the signal-to-noise ratio and the available plaintext amount. © 2007 International Association for Cryptologic Research.Show more Item Open Access Penetration depth of the scanning acoustic microscope(IEEE, 1985-03) Atalar, AbdullahShow more A definition for the penetration depth of the scanning reflection acoustic microscope is given. With this definition it is possible to calculate the penetration depth of a given lens geometry for a given material. The penetration depth depends on the elastic parameters of the object, the signal-to-noise ratio, and the operation frequency of the acoustic microscope. Calculations show that for high-impedance materials, the penetration depthis limited by the wavelength of the surface waves and hence by frequency. For low-impedance materials the opening angle of the lens can be properly selected to make the longitudinal or shear wave penetration dominant, effectively increasing the penetration well above the wavelength limit of the surface wave.Show more Item Open Access Performance comparison of four time-of-flight estimation methods for sonar signals(1998-08-06) Barshan, B.; Ayrulu, B.Show more Performances of four methods of time-of-flight estimation for sonar signals are compared in terms of their bias, standard deviation and complexity: thresholding, curve fitting, m-out-of-N sliding-window, and correlation detection. Whereas correlation detection represents the theoretical optimum, simpler and faster suboptimal methods can offer acceptable performance at much lower cost. The experimental results are in close agreement with the simulations.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 Positioning algorithms for cooperative networks in the presence of an unknown turn-around time(IEEE, 2011) Gholami, M.R.; Gezici, Sinan; Ström, E.G.; Rydström, M.Show more This paper addresses the problem of single node positioning in cooperative network using hybrid two-way time-of-arrival and time-difference-of-arrival where, the turn-around time at the target node is unknown. Considering the turn-around time as a nuisance parameter, the derived maximum likelihood estimator (MLE) brings a difficult global optimization problem due to local minima in the cost function of the MLE. To avoid drawbacks in solving the MLE, we obtain a linear two-step estimator using non-linear pre-processing which is algebraic and closed-form in each step. To compare different methods, Cramér-Rao lower bound (CRLB) is derived. Simulation results confirm that the proposed linear estimator attains the CRLB for sufficiently high signal-to-noise ratios. © 2011 IEEE.Show more