Cagatay Candan

Cagatay Candan
Middle East Technical University | METU · Department of Electrical and Electronics Engineering

Professor

About

67
Publications
3,647
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1,941
Citations

Publications

Publications (67)
Article
Maximum likelihood autoregressive (AR) model parameter estimation problem with independent snapshots observed under white Gaussian measurement noise is studied. In addition to the AR model parameters, the measurement noise variance is also included among the unknowns of the problem to develop a general solution covering several special cases such a...
Article
This study examines the effect of peak-to-average power ratio (PAPR) constraint on the transmit beamformer design problem with the goal of establishing a trade-off between the power efficiency (maximizing the average transmitted power) and other metrics such as the power level fluctuation in mainlobe, peak-sidelobe level (PSL), etc. Typically, unim...
Article
Conventional Mills Cross architecture suffers from poor direction-of-arrival (DOA) angle estimation accuracy in the dimension that the transmitter is aligned. To improve the estimation accuracy, a space-time coded, multiple-input multipleoutput (MIMO) direction finding method with complementary codes is presented. The performance of the suggested M...
Article
A two-stage method for the parameter estimation of Gaussian autoregressive models is proposed. The proposed first stage is an improved version of the conventional forward-backward prediction method and can be interpreted as its weighted version with the weights derived from the arithmetic mean of the log-likelihood functions for different condition...
Article
Functions of complex variables arise frequently in the formulation of signal processing problems. The basic calculus rules on differentiation and integration for functions of complex variables resemble, but are not identical to, the rules of their real variable counterparts. On the contrary, the standard calculus rules on differentiation, integrati...
Article
The resource management of a phase array system capable of multiple target tracking and surveillance is critical for the realization of its full potential. This paper aims to improve the performance of an existing method, time-balance (TB) scheduling, by establishing an analogy with a well-known stochastic control problem, the machine replacement p...
Article
The classical spectrum analysis methods utilize window functions to reduce the masking effect of a strong spectral component over weaker components. The main cost of side-lobe reduction is the reduction of signal-to-noise ratio (SNR) level of the output spectrum. We present a single snapshot method which optimizes the selection of most suitable win...
Article
Full-text available
The conventional sidelobe blanking system, known as the Maisel sidelobe blanker, uses two receiving channels with different gains to detect the presence of a jammer. The Maisel system is an ad-hoc detector without any optimality properties. Yet, it has been successfully utilised in numerous operational systems. Here, the authors study the optimum N...
Article
An efficient and low complexity frequency estimation method based on the discrete Fourier transform (DFT) samples is described. The suggested method can operate with an arbitrary window function in the absence or presence of zero-padding. The frequency estimation performance of the suggested method is shown to follow the Cramer–Rao bound closely wi...
Article
The problem of moving target indicator (MTI) filter design for radar systems with non-uniform (staggered) pulse repetition intervals is examined. The goal is to realise and then utilise a trade-off in the design of MTI filter between the conflicting requirements of high suppression of undesired signal (clutter echo) and minimal suppression of desir...
Conference Paper
The detection and estimation problems with large dimensional vectors frequently appear in the phased array radar systems equipped with, possibly, several hundreds of receiving elements. For such systems, a preprocessing stage reducing the large dimensional input to a manageable dimension is required. The present work shows that the subspace spanned...
Conference Paper
This study aims to experimentally investigate the feasibility of discriminating human motions with the help of micro Doppler features by using radar. In the first phase of the work, the synthetic data is generated through the human walking simulator by V. Chen and different time-frequency transformations are applied on the data and the results of t...
Conference Paper
A method is presented for the selection of analysis window length, or the number of input samples, for linear signal modeling without compromising the model assumptions. It is assumed that the signal of interest lies in a known linear space and noisy samples of the signal is provided. The goal is to use as many signal samples as possible to mitigat...
Conference Paper
Two of the most important criteria for a sensor array beam-pattern are beamwidth and side-lobe level. A narrower beamwidth means a better angular resolution and well separation of close sources. The beamwidth of a sensor array is directly proportional to array aperture, the beamwidth gets narrower as array aperture increases. However, increase in a...
Article
Using recently developed statistical target fluctuation models, the accuracy of sequential lobing is analytically studied. The study shows that the sequential lobing method suffers from a significant performance loss, in comparison with the monopulse method, for the Rayleigh fluctuation model. For other fluctuation models, the performance loss grad...
Article
A method for the frequency estimation of complex exponential signals observed under additive white Gaussian noise is presented. Unlike competing methods based on relatively few Discrete Fourier Transform (DFT) samples, the presented technique can generate a frequency estimate by fusing the information from all DFT samples. The estimator is shown to...
Article
The Savitzky–Golay (SG) filter design problem is posed as the minimum norm solution of an underdetermined equation system. A unified SG filter design framework encompassing several important applications such as smoothing, differentiation, integration and fractional delay is developed. In addition to the generality and flexibility of the framework,...
Conference Paper
Full-text available
We present an optimal sidelobe blanker (SLB) detector for Swerling-1 and Swerling-0 targets and compare the performances of the suggested detector with the classical Maisel SLB structure. The optimal SLB detector depends on the signal to noise ratio (SNR) and jammer to noise ratio (JNR) values and may not be practical for implementation in many app...
Conference Paper
This study aims to experimentally investigate the feasibility of discriminating human motions with the help of micro-Doppler features by using radar. In the first phase of the work, the synthetic data is generated through the human walking simulator by V. Chen and different time-frequency transformations are applied on the data. In the following ph...
Article
The moment function for the ratio of correlated generalized gamma variables is expressed in terms of special functions. The expression presented generalizes the known moment expression for the integer-valued moments to the real-valued moments. Approximate formulas, in terms of elementary functions, are provided for low and high correlation regions...
Article
The bias and mean square error (MSE) analysis of the frequency estimator suggested in is given and an improved version of the estimator, with the removal of estimator bias, is suggested. The signal-to-noise ratio (SNR) threshold above which the bias removal is effective is also determined.
Article
A remarkably simple upper bound on the capacity loss due to imprecise channel state information (CSI) is presented for single-input single-output (SISO) general memoryless fading channels, (Capacity Loss) ≤ log(1+var(h - ĥ)SNR) where var(h - ĥ) represents the variance of channel estimation error, i.e. CSI inaccuracy. The bound extends earlier work...
Conference Paper
In this work, design of non-uniform MTI filter for staggered MTI Radar systems is presented in the sense of min-max filter design. Corresponding design is studied to obtain the nearest improvement factor to the optimum MTI filter improvement factor. The performance of the design is examined with simulations and a detailed comparison with the non-un...
Conference Paper
The design of moving target indicator (MTI) filters with nonuniform interpulse periods is studied through the least square and min-max filter design methodologies. A trade-off between the contradictory objectives of maximum possible clutter suppression (maximum stopband attenuation) and minimum desired signal attenuation (minimum passband ripple) i...
Article
A two-stage detector is proposed to accommodate high computational load requirements of modern radar systems. The first stage of the proposed system is a low-complexity detector that operates at an unusually high false alarm probability value around 1/10. This stage is to prescreen and eliminate some of the test cells with relatively few operations...
Article
The capacity of zero-outage scheme with imprecise channel state information at the transmitter side (CSIT) for frequency flat, single-input multiple-output (SIMO) channels is examined. It is shown that when CSIT is not precise, the receiver signal-to-noise-ratio fluctuates and the scheme suffers from communication outages. Exact analytical expressi...
Article
An implementation for the post-Doppler adaptive target detectors enabling an efficient change of the subspace dimension is described. The proposed implementation uses the order recursive structure of the conjugate directions method and does not present any additional computational burden on the processor. The implementation can be particularly usef...
Article
Conventional constant false alarm rate (CFAR) methods use a fixed number of cells to estimate the background variance. For homogeneous environments, it is desirable to increase the number of cells, at the cost of increased computation and memory requirements, in order to improve the estimation performance. For nonhomogeneous environments, it is des...
Conference Paper
A beamspace approach is presented to detect the multiplicity of impinging sources on the sensor array for the direction of arrival estimation problem. The method applies to the coherent signals and can also be used to detect the presence of interfering multipath signal. In many applications, the signal to noise ratio is not sufficient to resolve mu...
Conference Paper
A sequential method for the classification of the autoregressive processes is presented. Different from the conventional detectors having fixed sample size, the suggested method uses Wald's sequential probability ratio test (SPRT) which is known with faster decisions and has a variable sample size. It is assumed that the coefficients of the filters...
Conference Paper
It was shown in the literature that MIMO radar can resolve much more targets in the angle than phased array radars. In order to resolve targets close to each other in range, waveforms, whose side-lobes of the autocorrelation function are low, are designed by using pulse compression techniques or mismatched filters are used at the receiver. In this...
Article
A novel two-stage frequency domain channel estimation method especially suitable for the estimation of long channels such as ultra wide band channels is proposed. The proposed method can efficiently use the sequences with closed form analytical expressions such as the Legendre sequences. (The suggested method does not require a computationally inte...
Article
A new class of linear-phase infinite-impulse-response digital wideband integrators based on the numerical integration rules is presented. The proposed class of integrators exactly matches the desired phase response of the continuous-time integrator (after group delay compensation) and can approximate the magnitude response as closely as desired by...
Article
The parameter estimation of a complex exponential waveform observed under white noise is typically tackled in two stages. In the first stage, a coarse frequency estimate is found by the application of an N-point DFT to the input of length N . In the second stage, a fine search around the peak determined in the first stage is conducted. The method p...
Article
The discrete Fourier transform (DFT) not only enables fast implementation of the discrete convolution operation, which is critical for the efficient processing of analog signals through digital means, but it also represents a rich and beautiful analytical structure that is interesting on its own. A typical senior-level digital signal processing (DS...
Conference Paper
Opportunistic scheduling schemes maximizes the aggregate ca- pacity by using multiuser diversity. Users are chosen to serve according to their signal-to-noise ratios (SNR) for a particu- lar time slot and the user with the highest SNR is selected. Typically, a homogeneous system, in which every user has the same mean SNR, allocates time slots to al...
Conference Paper
Presence of a sinusoidal interference signal can inhibit the de- tection of another sinusoidal signal because of its sidelobes. By the use of conventional window functions, sidelobes can be suppressed with the price of frequency resolution loss. Spa- tially variant apodization (SVA), which uses an infinite number of window functions, can reduce spe...
Conference Paper
The use of waveform diversity and angular diversity provides significant improvements in the performance of Multi Input Multi Output (MIMO) radar compared to the conventional radars and the phased array radar systems. One of these improvement areas is the success of moving target detection. In this paper, the moving target detection performance of...
Article
We examine the application of transmit precoding in multiuser multi-input-multi-output (MIMO) communication systems with maximum ratio combining (MRC) receivers. In many multiuser applications, the maximum-likelihood or minimum mean-square error (MMSE) receivers can be prohibitive to implement due to their high implementation complexity. We examine...
Article
We propose a feedback quantization scheme for downlink multiuser diversity systems. The scheme is designed for heterogeneous users, i.e., users operating under different levels of mean signal-to-noise ratios (SNRs). The proposed scheme equally distributes the channel access time to user clusters and, at the same time, maximizes the sum-rate capacit...
Conference Paper
We present a method for the design of mismatched filters minimizing the interference from unwanted targets (point target or clutter) under the constraint of matched filtering loss. The method method seeks to find the optimum filter minimizing the interference and having a desired cross-correlation with a given transmitter waveform. The method is ap...
Article
The removal of coexisting land and weather clutter signals is a common problem in many radar applications. Although the optimal method for land-weather clutter suppression is known, it is usually not amenable to implementation due to computation and storage limitations of the processor. We propose an alternative method that employs low rank approxi...
Conference Paper
We study the detectors based on the ambiguity function. These detectors have been shown to be useful for the detection of linear frequency modulated (LFM) signals in the literature. We show that the detectors based on the projection of the cross-ambiguity surface are optimal for the detection of non-fluctuating targets in the Bayesian sense. The gi...
Article
Fast and accurate algorithms for digital computation of linear canonical transforms (LCTs) are discussed. Direct numerical integration takes O(N 2) time, where N is the number of samples. Designing fast and accurate algorithms that take \(O(N\log N)\) time is of importance for practical utilization of LCTs. There are several approaches to designing...
Conference Paper
We present a novel receiver structure for the detection and parameter estimation of linear frequency modulated signals. The proposed structure is based on the relations between the fractional Fourier transform and the ambiguity function. It has been shown that the optimal ML receiver, which is the peak detector in the ambiguity plane, can be implem...
Conference Paper
The joint suppression of returns from land and weather clutter is required in many radar applications. Although the optimal method of land-weather clutter suppression is known, the solution is usually not practical to implement. In this paper, we propose a method that employs rank-1 and rank-2 approximations on the weather clutter correlation matri...
Article
Discrete equivalents of Hermite-Gaussian functions play a critical role in the definition of a discrete fractional Fourier transform. The discrete equivalents are typically calculated through the eigendecomposition of a commutator matrix. In this letter, we first characterize the space of DFT-commuting matrices and then construct matrices approxima...
Article
A novel filtering structure with linear complexity is proposed for Lagrange interpolation. The structure is similar to the Farrow structure in principle, but it is more efficient and has the additional feature of being order updatable on-the-fly. The main application for the proposed structure is the implementation of fractional delay filters to mi...
Article
A method for the construction of complementary sets of sequences using polyphase representation of orthogonal filterbanks is presented. It is shown that the case of two-channel filterbanks unifies individually derived length extension formulas for complementary sequences into a common framework and the general M-channel case produces novel formulas...
Conference Paper
We present a data embedding method for image communication applications. Our goal is to implement novel multimedia applications such as multi-language captions, interactive programming and title specific features over the existing image communication channel. To this arm, we present a data embedding method for JPEG images which has the desired degr...
Conference Paper
We present a blind data hiding method for JPEG compressed images, which minimizes the perceptual distortion due to data embedding. The proposed system presents a number of options to the encoder to cast the given hidden bits in the compressed content signal. The perceptual distortion cost of each option is calculated from the parameters available t...
Conference Paper
A novel multiple description coding scheme based on the Chinese remainder theorem is presented. The main advantages of the scheme are the uniform diffusion of information to multiple packets, the flexible control mechanism over the representation redundancy, and the possibility of having a detail preserving post-processing routine to reduce the rec...
Conference Paper
We present a new definition of data hiding capacity which complements the established theory in the field and produces practical estimates under many attacks. We discuss the relation between the proposed definition and the current theoretical work on data hiding capacity. The definition proposed is applied to still images to estimate the hiding cap...
Article
We propose and consolidate a definition of the discrete fractional Fourier transform that generalizes the discrete Fourier transform (DFT) in the same sense that the continuous fractional Fourier transform generalizes the continuous ordinary Fourier transform. This definition is based on a particular set of eigenvectors of the DFT matrix, which con...
Article
We propose and consolidate a definition of the discrete fractional Fourier transform which generalizes the discrete Fourier transform (DFT) in the same sense that the continuous fractional Fourier transform (FRT) generalizes the continuous ordinary Fourier Transform. This definition is based on a particular set of eigenvectors of the DFF which cons...
Article
We present a novel data hiding method for compressed images. The method is designed to minimize the quality loss associated with data embedding into a JPEG image. The described technique uses the objective criterion such as the mean square error and the human visual system based criterion such as the Just Noticable Distortion metric for distortion...

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