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1.	Kolomvakis, Nikolaos, 1985, et al. (författare) Reconstruction of clipped signals 2023 Patent (övrigt vetenskapligt/konstnärligt)abstract Using information contained in clipped samples from aualog-to-digital (ADC) conversion to improve receiver performance, by, for example, reducing the clipping distortion caused by ADCs due to its data resolution constraints. This provides an advantage over existing solutions, which perform suboptimally because the existing solution discard information in tire clipped samples.
2.	Winges, Johan, 1987, et al. (författare) Compressed Sensing Applied to Non-Ideal Microwave Measurements in Metal Enclosures 2019 Ingår i: 13th European Conference on Antennas and Propagation, EuCAP 2019. - 9788890701887 Konferensbidrag (refereegranskat)abstract Compressed sensing can make use of a priori information in the sense that its dictionary may be constructed for an expected object of particular size and shape, should the application deal with the detection and positioning of exactly such expected objects. Here, we test such a compressed-sensing approach on measurements that include objects that are foreign to the dictionary. It is found that the compressed-sensing approach shows reasonable performance that degrades gracefully as the measurement region is contaminated by dielectric objects that are foreign to the dictionary, where the degradation also becomes more severe as more dielectric material is present in the measurement region due to mutual interactions not accounted for by the dictionary.
3.	Ozcelikkale, Ayca, 1982, et al. (författare) Remote Estimation of Correlated Sources under Energy Harvesting Constraints 2018 Ingår i: IEEE Transactions on Wireless Communications. - : Institute of Electrical and Electronics Engineers (IEEE). - 1558-2248 .- 1536-1276. ; 17:8, s. 5300-5313 Tidskriftsartikel (refereegranskat)abstract Remote estimation with an energy harvesting sensor with a limited data and energy buffer is considered. The sensor node observes an unknown temporally correlated field and communicates its observations to a remote fusion center using the energy it harvested. The fusion center employs linear minimum mean-square error (LMMSE) estimation to reconstruct the unknown field. We provide performance guarantees for the estimation error under a block transmission scheme, where at each transmission block, data and energy buffers are completely emptied. Our bounds provide insights into how statistical prop- erties of the energy harvesting process and buffer sizes may affect the estimation error. In particular, these bounds suggest insensitivity of the performance to buffer sizes for signals with low degree of freedom and suggest performance improvements with increasing buffer sizes for signals with relatively higher degree of freedom. Depending only on the mean, variance and finite support of the energy arrival process, these results provide insights for the energy and data buffer sizes for deployment in future energy harvesting wireless sensing systems.
4.	Trinh-Hoang, Minh, et al. (författare) AN IMPROVED DOA ESTIMATOR BASED ON PARTIAL RELAXATION APPROACH 2018 Ingår i: ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings. - 1520-6149. ; 2018-April, s. 3246-3250 Konferensbidrag (refereegranskat)abstract In the partial relaxation approach, at each desired direction, the manifold structure of the remaining interfering signals impinging on the sensor array is relaxed, which results in closed form estimates for the interference parameters. By adopting this approach, in this paper, a new estimator based on the unconstrained covariance fitting problem is proposed. To obtain the null-spectra efficiently, an iterative rooting scheme based on the rational function approximation is applied. Simulation results show that the performance of the proposed estimator is superior to the classical and other partial relaxation methods, especially in the case of low number of snapshots, irrespectively of any specific structure of the sensor array while maintaining a reasonable computational cost.
5.	Trinh-Hoang, Minh, et al. (författare) Improved DOA estimators using partial relaxation approach 2018 Ingår i: 2017 IEEE 7th International Workshop on Computational Advances in Multi-Sensor Adaptive Processing, CAMSAP 2017. ; 2017-December, s. 1-5 Konferensbidrag (refereegranskat)abstract © 2017 IEEE. In this paper, the partial relaxation approach is introduced and applied to DOA estimation using spectral search. Unlike existing methods like Capon or MUSIC which can be considered as single source approximations of multi-source estimation criteria, the proposed approach accounts for the existence of multiple sources. At each direction, the manifold structure of interfering signals impinging on the sensor array is relaxed, which results in closed form estimates for the interference parameters. The conventional multidimensional optimization problem reduces, thanks to this relaxation, to a simple spectral search. Following this principle, proposed estimators based on the Deterministic Maximum Likelihood, Weighted Subspace Fitting and Covariance Fitting method are derived. Simulation results show that the performance of the proposed estimators is superior to conventional methods especially in the case of low SNR and low number of snapshots, irrespectively of the special structure of the sensor array.
6.	WINGES, JOHAN, 1987, et al. (författare) Compressed Sensing for the Detection and Positioning of Dielectric Objects Inside Metal Enclosures by Means of Microwave Measurements 2018 Ingår i: IEEE Transactions on Microwave Theory and Techniques. - 0018-9480 .- 1557-9670. ; 66:1, s. 462-476 Tidskriftsartikel (refereegranskat)abstract Based on compressed sensing and microwave measurements, we present a procedure for the detection and positioning of dielectric objects inside a metal enclosure, where the number of objects is unknown but assumed to be limited. The formulation features a convex quadratic optimization problem with 1-norm regularization, which allows for rapid detection and positioning given a precomputed dictionary. The dictionary consists of the scattering parameters computed from a single scattering object placed at the grid points of a structured grid that covers the entire measurement region. We test our method experimentally in a microwave measurement system that features a measurement region with a diameter of 11.6 cm. The measurement region is encircled by six aperture antennas, where each aperture is the end-opening of a rectangular waveguide operated from 2.7 to 4.2 GHz. We use acrylic-glass cylinders of radius 5.2 mm as scatterers and find that the compressed sensing method can correctly detect at least up to five scatterers with an average positioning accuracy of 3 mm. In addition, we investigate the performance of the method with respect to scarcity of data, where we omit scattering parameters or frequency points.
7.	Cerullo, Livia, 1982, et al. (författare) Microwave measurement system for dispersive dielectric properties of densely packed pellets 2017 Ingår i: Measurement: Journal of the International Measurement Confederation. - : Elsevier BV. - 0263-2241. ; 106, s. 179-189 Tidskriftsartikel (refereegranskat)abstract We present a microwave measurement system that is intended for the estimation of the dispersive dielectric properties for densely packed pellets. In particular, we estimate the Debye parameters for the effective permittivity of a mixture of air and densely packed moist microcrystalline cellulose pellets for the frequency band 2.7–5.1 GHz. The Debye parameters are estimated by means of minimizing the misfit between the measured scattering parameters and the corresponding computed response given a model of the measurement system. This minimization is performed by means of a nonlinear iterative optimization procedure, which is supported by continuum sensitivities, and the optimum is found in 10–60 iterations for the cases presented in this article. We find that increasing moisture content yields an increase in the static permittivity and a decrease in the relaxation time.
8.	Kolomvakis, Nikolaos, 1985, et al. (författare) Downlink Performance of Regularized ZF in Massive MIMO Systems Subject to IQ Imbalance 2017 Ingår i: IEEE International Conference on Communications. - 1550-3607. Konferensbidrag (refereegranskat)abstract This paper studies the impact of in-phase and quadrature imbalance (IQI) on the downlink of a single-cell multiuser multiple-input multiple-output (MU-MIMO) system with large antenna arrays. Moreover, we consider a time-division duplex (TDD) system where we assume uplink/downlink channel reciprocity in the downlink precoding design and study the impact of uplink/downlink channel mismatch on the downlink rate caused by different IQI on the base station (BS) and users equipment (UEs). A tractable analytical expression to predict the achievable downlink rates is derived for the regularized zero-forcing (RZF) and based on this expression, we prove that, as the number of BS antennas grow large, the achievable downlink rate of each UE is limited either by the RX IQI at the UEs or by the simultaneous presence of the transmit/receive IQI at the BS and TX IQI at the UEs, when there is uplink/downlink channel mismatch.
9.	Kolomvakis, Nikolaos, 1985, et al. (författare) Massive MIMO Systems with IQ Imbalance: Channel Estimation and Sum Rate Limits 2017 Ingår i: IEEE Transactions on Communications. - 0090-6778 .- 1558-0857. ; 65:6, s. 2382 - 2396 Tidskriftsartikel (refereegranskat)abstract This paper studies the impact of in-phase and quadrature imbalance (IQI) on single-cell multiuser multiple-input multiple-output (MU-MIMO) systems with large antenna arrays. Moreover, we consider a time-division duplex (TDD) system where we assume uplink/downlink channel reciprocity in the downlink precoding design. First, we investigate the effect of transceiver IQI on the uplink channel estimation by deriving the linear minimum-mean-square-error (LMMSE) estimator for the IQ-impaired model and prove that only the receiver IQI at the base station (BS) limits the estimation accuracy. Then, we study the impact of uplink/downlink channel mismatch on the downlink rate caused by different IQI on the BS and user equipments (UEs). We prove that the achievable downlink rate of each UE is limited either by the receiver IQI at the UEs or jointly by the transmit and receive IQI at the BS, when there is mismatch between the uplink and downlink channels.
10.	Ozcelikkale, Ayca, 1982, et al. (författare) Transmission Strategies for Remote Estimation with an Energy Harvesting Sensor 2017 Ingår i: IEEE Transactions on Wireless Communications. - 1558-2248 .- 1536-1276. ; 16:7, s. 4390-4403 Tidskriftsartikel (refereegranskat)abstract We consider the remote estimation of a time-correlated signal using an energy harvesting (EH) sensor. The sensor observes the unknown signal and communicates its observations to a remote fusion center using an amplify-and-forward strategy. We consider the design of optimal power allocation strategies in order to minimize the mean-square error at the fusion center. Contrary to the traditional approaches, the degree of correlation between the signal values constitutes an important aspect of our formulation. We provide the optimal power allocation strategies for a number of illustrative scenarios. We show that the most majorized power allocation strategy, i.e. the power allocation as balanced as possible, is optimal for the cases of circularly wide-sense stationary (c.w.s.s.) signals with a static correlation coefficient, and sampled low-pass c.w.s.s. signals for a static channel. We show that the optimal strategy can be characterized as a water-filling type solution for sampled low-pass c.w.s.s. signals for a fading channel. Motivated by the high-complexity of the numerical solution of the optimization problem, we propose low-complexity policies for the general scenario. Numerical evaluations illustrate the close performance of these low-complexity policies to that of the optimal policies, and demonstrate the effect of the EH constraints and the degree of freedom of the signal.
11.	Panahi, Ashkan, 1986, et al. (författare) Performance Analysis of Sparsity-Based Parameter Estimation 2017 Ingår i: IEEE Transactions on Signal Processing. - : Institute of Electrical and Electronics Engineers (IEEE). - 1941-0476 .- 1053-587X. ; 65:24, s. 6478-6488 Tidskriftsartikel (refereegranskat)abstract Since the advent of the l(1) regularized least squares method (LASSO), a new line of research has emerged, which has been geared toward the application of the LASSO to parameter estimation problems. Recent years witnessed a considerable progress in this area. The notorious difficulty with discretization has been settled in the recent literature, and an entirely continuous estimation method is now available. However, an adequate analysis of this approach lacks in the current literature. This paper provides a novel analysis of the LASSO as an estimator of continuous parameters. This analysis is different from the previous ones in that our parameters of interest are associated with the support of the LASSO solution. In other words, our analysis characterizes the error in the parameterization of the support. We provide a novel framework for our analysis by studying nearly ideal sparse solutions. In this framework, we quantify the error in the high signal-to-noise ratio regime. As the result depends on the choice of the regularization parameter, our analysis also provides a new insight into the problem of selecting the regularization parameter. Without loss of generality, the results are expressed in the context of direction of arrival estimation problem.
12.	Xu, Xiaowei, 1991, et al. (författare) Simultaneous Information and Power Transfer under a Non-Linear RF Energy Harvesting Model 2017 Ingår i: IEEE International Conference on Communications Workshops (ICC) (2017). - 9781509015252 ; , s. 179-184 Konferensbidrag (refereegranskat)abstract In the design of simultaneous wireless information and power transfer (SWIPT) systems, it has been typically as- sumed that energy conversion efficiency is independent from the level of the input power at the energy receiver. On the other hand, in practice the energy conversion efficiency exhibits a non- linear behavior and highly depends on the input power. This leads to a discrepancy between the practical energy harvesting (EH) hardware available and the resource allocation designs made for the SWIPT systems. This work is concerned with this issue. In particular, we propose a practical quadratic model for the power conversion efficiency in EH circuitry. Comparisons with the constant efficiency models used in conventional SWIPT system design as well as another non-linear model proposed in the literature are made. With its convexity properties together with the good match it provides for the measurement data from practical EH circuitry, the proposed model is shown to be a promising alternative to the existing EH approaches. Using the proposed model, the problem of resource allocation for a multi- user Orthogonal Frequency-Division Multiple Access (OFDMA) system is investigated. The performance improvement due to the usage of the proposed non-linear model is illustrated.
13.	Zhang, Xinlin, 1986, et al. (författare) Hybrid beamforming in uplink massive MIMO systems in the presence of blockers 2017 Ingår i: ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings. - 1520-6149. ; , s. 6503-6507 Konferensbidrag (refereegranskat)abstract Hybrid beamforming (HBF) is a potential solution to reduce the baseband hardware cost in massive multiple-input multiple-output (MIMO) systems that has drawn considerable attention recently. In this paper, we consider the uplink of a multiuser massive MIMO system in the presence of blockers. Such blockers, which arise from, for example, users served by other non-cooperative base stations (BSs), can limit the system performance if not handled properly. We propose a HBF scheme that can remove the impact of blockers, while preserving signals from intended users. Specifically, in our two-step receive beamforming scheme, the analog beamformer (ABF) is designed based on channel covariance matrices to minimize the powers of blockers, while the digital beamformer (DBF) deals with inter-user interference. We propose an iterative algorithm that efficiently gives a good sub-optimal solution to the NP-hard problem in the ABF design. Moreover, we consider a more complete BS architecture by incorporating automatic gain control (AGC) and analog-to-digital converter (ADC). As we show in the simulations, for a system with full-precision ADCs, our scheme approaches the sum rate of an ideal fully-digital system. Interestingly, for a system with low-resolution ADCs, our HBF scheme can even outperform a fully-digital system.
14.	Ozcelikkale, Ayca, 1982, et al. (författare) Performance Bounds for Remote Estimation with an Energy Harvesting Sensor 2016 Ingår i: IEEE International Symposium on Information Theory - Proceedings. - 2157-8095. - 9781509018062 ; 2016-August, s. 460-464 Konferensbidrag (refereegranskat)abstract Remote estimation with an energy harvesting sensor with a limited data buffer is considered. The sensor node observes an unknown correlated circularly wide-sense stationary (c.w.s.s.) Gaussian field and communicates its observations to a remote fusion center using the energy it harvested. The fusion center employs minimum mean-square error (MMSE) estimation to reconstruct the unknown field. The distortion minimization problem under the online scheme, where the sensor has only access to the statistical information for the future energy packets is considered. We provide performance bounds on the achievable distortion under a slotted block transmission scheme, where at each transmission time slot, the data and the energy buffer is completely emptied. Our bounds provide insight to the trade-off between the buffer sizes and the achievable distortion. These trade-offs illustrate the insensitivity of the performance to the buffer size for signals with low degree of freedom and suggest performance improvements with increasing buffer size for signals with relatively higher degree of freedom.
15.	Ozcelikkale, Ayca, 1982, et al. (författare) Simultaneous Information and Power Transfer with Transmitters with Hardware Impairments 2016 Ingår i: Proceedings of the International Symposium on Wireless Communication Systems. - 2154-0217 .- 2154-0225. - 9781509020614 ; 2016-October, s. 114-118 Konferensbidrag (refereegranskat)abstract We investigate the performance of a communication system with simultaneous wireless information and power transfer capabilities under non-ideal transmitter hardware. We adopt an experimentally validated additive noise model in which the level of the noise at an antenna is proportional to the signal power at that antenna. We consider the linear precoder design problem and focus on the problem of minimizing the mean-square error under energy harvesting constraints. This set-up, in general, constitutes a non-convex formulation. For the single antenna information user case, we provide a tight convex relaxation, i.e. a convex formulation from which an optimal solution for the original problem can be constructed. For the general case, we propose a block coordinate descent technique to solve the resulting non-convex problem. Our numerical results illustrate the effect of hardware impairments on the system.
16.	Ozcelikkale, Ayca, 1982, et al. (författare) Transmission Strategies for Remote Estimation under Energy Harvesting Constraints 2016 Ingår i: European Signal Processing Conference. - 2219-5491. - 9780992862657 ; 2016-November, s. 572-576 Konferensbidrag (refereegranskat)abstract We consider the remote estimation of a time- correlated field using an energy harvesting (EH) sensor. The sensor observes the unknown field and communicates its observa- tions to a remote fusion center using an amplify-forward strategy. We consider the design of optimal transmission strategies in order to minimize the mean-square error (MSE) at the fusion center. Contrary to traditional approaches, the degree of correlation between the field values constitutes an important aspect of our formulation. We provide the optimal power allocation strategies for a number of illustrative scenarios, including the circularly wide-sense stationary (c.w.s.s.) signals with static correlation coefficient and the sampled low-pass c.w.s.s. signals. Based on these results, we propose low-complexity policies for the general case. Numerical evaluations illustrate the performance of the optimal and the low-complexity policies.
17.	Yang, Bin, 1982, et al. (författare) Array response interpolation and DOA estimation with array response decomposition 2016 Ingår i: Signal Processing. - : Elsevier BV. - 0165-1684. ; 125, s. 97-109 Tidskriftsartikel (refereegranskat)abstract High-resolution direction-of-arrival (DOA) estimation requires an accurate array response model, which is usually achieved by measuring the response for given directions of the sources and employing interpolation. This paper considers the array response interpolation and DOA estimation by coherently combing parametric modeling and interpolation techniques. The proposed method is to decompose the array response as a product of a mutual coupling matrix, an ideal array response vector (dependent on the geometry of antenna array) and a DOA-dependent correction vector. Our idea is now that the correction vector will be a smoother function of DOA as compared to direct interpolation, since parts of the phase variation in one embedded element are due to the mutual coupling and the ideal array response. In numerical experiments of a real antenna array, the method is found to improve the performance of array response interpolation and DOA estimation significantly when the array response is updated by interpolation of the correction vector. In addition, we show how the DOA estimation performance can be predicted using a simple model of the interpolation error statistics.
18.	Ozcelikkale, Ayca, 1982, et al. (författare) MIMO Linear Precoder Design with Non-Ideal Transmitters 2015 Ingår i: Proceedings of the International Symposium on Wireless Communication Systems. - 2154-0217 .- 2154-0225. ; 2016-April, s. 681-685 Konferensbidrag (refereegranskat)abstract We investigate the linear precoder design problem for multiple-input multiple-output (MIMO) channels under non-ideal transmitter hardware.We consider two different non-ideal hardware models: i) an additive noise model in which the level of the noise at an antenna is proportional to the signal power at that antenna, ii) an additive precoder error model. We focus on the problem of minimizing mean-square error at the receiver under transmit power constraints at the transmitter. For the first hardware impairment model, this scenario leads to a non-convex formulation for which we propose a block-coordinate descent technique. The proposed method has a convergence guarantee and provides rank-constrained solutions.For the second model, analytical expressions for the optimum designs are provided. We compare the performance of our hardware impairment aware designs with that of designs developed with ideal hardware assumptions. Our results suggest that significant gains can be obtained by the proposed designs for sufficiently high signal-to-noise ratio values.
19.	Ozcelikkale, Ayca, 1982, et al. (författare) Wireless Information and Power Transfer in MIMO Channels under Rician fading 2015 Ingår i: ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings. - 1520-6149. - 9781467369978 ; 2015-August, s. 3187-3191 Konferensbidrag (refereegranskat)abstract Simultaneous wireless information and power transfer (SWIPT) offers an attractive alternative to the traditional battery limited or grid dependent communication system design. In this paper, we investigate robust precoder designs for systems with SWIPT capabilities under a stochastic Rician fading framework. Under a multiple-input multiple-output (MIMO) channel model, we formulate the problem of minimizing the average mean-square error at the information receiver (IR) while keeping the average energy harvested at the energy receiver (ER) above given levels. We consider two different strategies that can be adopted by the IR: i) simple estimation filters based solely on the channel mean, ii) robust estimation filters aiming to minimize the average mean-square error. Both of these scenarios lead to non-convex formulations. For the first scenario, we propose a convex relaxation that is tight. For the second scenario, we propose an alternating minimization method that provides precoder designs even in the scenarios where the number of transmit antennas is larger than the length of data vector. Our numerical results show that the proposed designs provide significant performance gains especially when the scattering component of the channel is strong.
20.	Panahi, Ashkan, 1986, et al. (författare) A numerical implementation of gridless compressed sensing 2015 Ingår i: ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings. - 1520-6149. - 9781467369978 ; 2015-August, s. 3342-3346 Konferensbidrag (refereegranskat)abstract Atomic norm denoising has been recently introduced as a generalization of the Least Absolute Shrinkage and Selection Operator (LASSO) to overcome the problem of off-grid parameters. The method has been found to possess many interesting theoretical properties. However, its implementation has been only discussed in a special case of spectral line estimation by uniform sampling. In this paper, we propose a general numerical method to solve the atomic norm denoising problem. The complexity of the proposed algorithm is proportional to the complexity of a single-parameter search in the parameter space and thus in many interesting cases, including frequency estimation it enjoys fast realization.
21.	Panahi, Ashkan, 1986, et al. (författare) Wideband waveform design for robust target detection 2015 Ingår i: ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings. - 1520-6149. - 9781467369978 ; 2015-August, s. 3926-3930 Konferensbidrag (refereegranskat)abstract © 2015 IEEE. Future radar systems are expected to use waveforms of a high bandwidth, with an advantage of an improved range resolution. Herein, a technique to design robust wideband waveforms is developed. The context is detection of a single object with partially unknown parameters. The technique achieves an optimal detection speed for a desired resolution, maintaining a high detection performance. Many radar systems also require fast adaptation to a variable environment. Hence, the technique is devoted to rapidly design waveforms. In terms of probabilities of detection and false alarm, numerical evaluation shows the efficiency of the method when compared with a chirp signal and a Gaussian pulse.
22.	Razavi, Aidin, 1982, et al. (författare) Maximum Aperture Power Transmission in Lossy Homogeneous Matters 2015 Ingår i: IEEE Antennas and Wireless Propagation Letters. - 1548-5757 .- 1536-1225. ; 14, s. 175-178 Tidskriftsartikel (refereegranskat)abstract We apply array signal processing techniques to the transmitting aperture field modes in order to determine the optimal field distribution that maximizes the power transmission through lossy media between a transmitting and an ideally receiving antenna aperture. The optimal aperture distribution is then used as a reference field for developing curves applicable to the design of many near-field systems, such as for the detection of foreign objects in lossy matters (e.g. food contamination detectors), wireless charging of batteries of human body implanted devices, and for in-body communication systems.
23.	Razavi, Aidin, 1982, et al. (författare) Optimal Aperture Distribution for Maximum Power Transfer in Planar Lossy Multilayered Matters 2015 Ingår i: 9th European Conference on Antennas and Propagation, EuCAP 2015, Lisbon, Portugal, 13-17 May 2015. - 9788890701856 Konferensbidrag (refereegranskat)abstract We combine array-signal-processing and spectral domain techniques to determine the optimal aperture current distribution that maximizes the power coupling between two antenna apertures which are located on opposing sides of a multilayered lossy structure in the near-field. The resulting optimal distribution can be used as a reference in the design of antennas for near-field applications, such as for the detection of foreign objects in lossy media, wireless charging of implant batteries, or near-field communications with an implant. A MoM approach is formulated to study the performance of the optimal aperture currents in a simulated detection application in the presence of a small PEC patch, and it is shown that the optimal aperture current can improve the system sensitivity and increase the detection probability of the object.
24.	Ström, Marie, 1984, et al. (författare) Wideband Waveform and Receiver Filter Bank Design for Clutter Suppression 2015 Ingår i: IEEE Journal on Selected Topics in Signal Processing. - : Institute of Electrical and Electronics Engineers (IEEE). - 1941-0484 .- 1932-4553. ; 9:8, s. 1366-1376 Tidskriftsartikel (refereegranskat)abstract Modern wideband radar systems with long integration time, equipped with arbitrarily waveform generators, raise a demand for advanced signaling transmission schemes. In this paper, we propose two algorithms to select transmit waveforms and receiver filters. The techniques are based on a clutter suppression criterion. For the first algorithm, we employ an optimized filter bank, and for the second algorithm, we use a matched filter bank. Clutter suppression is achieved by minimizing the correlation between receiver filters and interfering clutter echoes. The algorithm, for the optimized filter bank, is extended to adapt the transmission scheme and receiver filters to a time-evolving scenario. Adaptation parameters are based on estimates of a clutter map and detected target characteristics. To estimate the clutter map we propose a Kalman filter, whereas target parameters are calculated using a least-squares fit to data. The efficiency of the algorithms and the adaptation scheme are visualized through a numerical simulation.
25.	Veen, Alle Jan Van Der, et al. (författare) Minimal continuous state-space parametrizations 2015 Ingår i: European Signal Processing Conference. - 2219-5491. Konferensbidrag (refereegranskat)abstract We present a minimal continuous parametrization of all multivariate rational contractive transfer functions.In contrast to traditional minimal parametrizations, this parametrization does not contain any structural indices, which makes it very suitable for identification algorithms that use nonlinear optimization to estimate the parameters.
26.	Yang, Bin, 1982, et al. (författare) Array Calibration using Array Response Interpolation and Parametric Modeling 2015 Ingår i: Proc. European Signal Processing Conference 2015 (EUSIPCO 2015). - 2076-1465. - 9780992862633 ; , s. 1346-1350 Konferensbidrag (refereegranskat)abstract High-performance array applications often require an accurate array response model. A common way to achieve this is by array calibration which involves measuring the response for a finite number of given source directions and employing interpolation. This paper considers the array calibration problem by combing interpolation techniques and parametric modeling. The idea is to model the array response as a product of a mutual coupling matrix, an ideal array response vector (derived from the geometry of antenna array) and an angle-dependent correction vector. Since the major effects are captured by the physical model and the mutual coupling matrix, the correction vector will be a smoother function of angle as compared to direct interpolation of the measured array response. In numerical experiments of a real antenna array, the method is found to improve the performance of the array calibration significantly.
27.	Chung, Pei-Jung, et al. (författare) DOA Estimation Methods and Algorithms 2014 Ingår i: Academic Press Library in Signal Processing: Volume 3 Array and Statistical Signal Processing. - 9780124115972 ; , s. 599-650 Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract Estimation of direction of arrival (DOA) from data collected by sensor arrays is of fundamental importance to a variety of applications such as radar, sonar, wireless communications, geophysics and biomedical engineering. Significant progress in the development of algorithms has been made over the last three decades. This article provides an overview of DOA estimation methods that are relevant in theory and practice. We will present estimators based on beamforming, subspace and parametric approaches and compare their performance in terms of estimation accuracy, resolution capability and computational complexity. Methods for processing broadband data and signal detection will be discussed as well. Finally, a brief discussion will be given to application specific algorithms.
28.	Costa, Mario, et al. (författare) Array Processing in the Face of Nonidealities 2014 Ingår i: Academic Press Library in Signal Processing: Volume 3 Array and Statistical Signal Processing. - 9780124115972 ; , s. 819-857 Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract Real-world sensor arrays are typically composed of elements with individual directional beampatterns and are subject to mutual coupling, cross-polarization effects as well as mounting platform reflections. Errors in the array elements’ positions are also common in sensor arrays built in practice. Such nonidealities need to be taken into account for optimal array signal processing and in finding related performance bounds. Moreover, problems related to beam-steering and cancellation of the signal-of-interest in beamforming applications may be prevented. Otherwise, an array processor may experience a significant performance degradation. In this chapter we provide techniques that allow the practitioner to acquire the steering vector model of real-world sensor arrays so that various nonidealities are taken into account. Consequently, array processing algorithms may avoid performance losses caused by array modeling errors. These techniques include model-based calibration and auto-calibration methods, array interpolation, as well as the wavefield modeling principle or manifold separation technique. Robust methods are also briefly considered since they are useful when the array nonidealities are not described by the employed steering vector model. Extensive array processing examples related to direction-finding and beamforming are included demonstrating that optimal or close-to optimal performance may be achieved despite the array nonidealities.
29.	Nohlert, Johan, 1987, et al. (författare) Global monitoring of fluidized-bed processes by means of microwave cavity resonances 2014 Ingår i: Measurement: Journal of the International Measurement Confederation. - : Elsevier BV. - 0263-2241. ; 55, s. 520-535 Tidskriftsartikel (refereegranskat)abstract We present an electromagnetic measurement system for monitoring of the effective permittivity in closed metal vessels, which are commonly used in the process industry. The measurement system exploits the process vessel as a microwave cavity resonator and the relative change in its complex resonance frequencies is related to the complex effective permittivity inside the vessel. Also, thermal expansion of the process vessel is taken into account and we compensate for its influence on the resonance frequencies by means of a priori information derived from a set of temperature measurements. The sensitivities, that relate the process state to the measured resonance frequencies, are computed by means of a detailed finite element model. The usefulness of the proposed measurement system is successfully demonstrated for a pharmaceutical fluidized-bed process, where the water and solid contents inside the process vessel is of interest.
30.	Panahi, Ashkan, 1986, et al. (författare) Basis pursuit over continuum applied to range-Doppler estimation problem 2014 Ingår i: Proceedings of the IEEE Sensor Array and Multichannel Signal Processing Workshop. - 2151-870X. - 9781479914814 ; , s. 381-384 Konferensbidrag (refereegranskat)abstract Sparse estimation and compressive sensing techniques have been recently considered for radar estimation problems. It is frequently observed that these methods are robust to model uncertainties and substantially improve performance in scenarios with a low signal-to-noise. However, since current sparsity-based techniques are computationally costly and require a suitable discretization (grid), which strongly restricts resolution, they practically receive less attention. In this work, we present an application of a new sparsity-based technique to the specific problem of range-Doppler estimation. The method, generalizing basis pursuit, is less computationally complex and its performance is independent of the grid selection. We demonstrate that the proposed technique can improve estimation performance in difficult cases, as compared to the SAGE technique.
31.	Panahi, Ashkan, 1986, et al. (författare) Gridless compressive sensing 2014 Ingår i: ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings. - 1520-6149. - 9781479928927 ; , s. 3385-3389 Konferensbidrag (refereegranskat)abstract The effect of off-grid atoms has become the prominent problem in application of the Compressed Sensing (CS) techniques to the cases where there is an underlying continuous parametrization. In this work, we develop a generalizing CS framework which shows that sampling to a finite grid is not necessary toward compressive estimation. We propose an alternative procedure over infinite dictionaries, which we show to be theoretically consistent in many cases of interest and then propose a robust implementation. We illustrate the general properties of our technique in some difficult practical instances of frequency estimation.
32.	Razavi, Aidin, 1982, et al. (författare) Optimal aperture distribution for near-field detection of foreign objects in lossy media 2014 Ingår i: 2014 IEEE-APS Topical Conference on Antennas and Propagation in Wireless Communications, APWC 2014. - 9781467356923 ; , s. 659-662 Konferensbidrag (refereegranskat)abstract We determine the optimal source aperture field distribution to maximize the power transfer through a lossy medium from the transmitter aperture to a receiver aperture located in the near-field region. The optimal distribution is determined by applying array signal processing techniques to aperture field modes. By designing the system to be sensitive to the presence of foreign objects in a homogenous matter, the synthesized optimal aperture distribution increases the detection probability of foreign objects in various near-field applications, such as those that can detect potentially hazardous contaminations in food.
33.	Ström, Marie, 1984, et al. (författare) Wideband waveform design for clutter suppression 2014 Ingår i: Proceedings of the IEEE Sensor Array and Multichannel Signal Processing Workshop. - 2151-870X. - 9781479914814 ; :Art. no. 6882399, s. 297-300 Konferensbidrag (refereegranskat)abstract Modern signal generators offer the capability to synthesize arbitrary wideband waveforms for radar and sonar applications. This makes it possible to optimize signals for specific purposes or scenarios. Herein, we discuss how to design a wideband waveform for clutter suppression. We formulate an optimization in terms of mainlobe and sidelobe properties of the ambiguity function, then we provide an approximate solution based on convex relaxation. Numerical evaluation shows the advantage of selecting a smart signal compared to a conventional waveform design. The advantages are shown as a lower probability of false alarm and a higher probability of correct target detection.
34.	Viberg, Mats, 1961 (författare) Introduction to Array Processing 2014 Ingår i: Academic Press Library in Signal Processing: Volume 3 Array and Statistical Signal Processing. - 9780124115972 ; , s. 463-502 Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract The purpose of this chapter is to give some background material on array signal processing, which serves as a more detailed introduction to the remaining chapters. The ideal data model is introduced and its properties are explored, with a special emphasis on the array response. The general concepts of beamforming and direction-of-arrival estimation are introduced, and exemplified by some well-known techniques. Although the focus is on traditional applications involving an array of coherent sensors, we also present some extensions to non-coherent and/or distributed sensors.
35.	WINGES, JOHAN, 1987, et al. (författare) A Microwave Measurement System for Measurement of Dielectric Properties 2014 Ingår i: AntennEMB, 11-12 March, Göteborg. Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract We present a prototype of a microwave measurement system for the estimation of dispersive dielectric properties. The measurement system consists of a metal cavity that features a measurement region in the center formed by the intersection of six rectangular waveguides. The waveguides are terminated by adapters connected to coaxial cables. We exploit the frequency band 2.7-5.1 GHz and a connected network analyzer and switch allows for automatic measurements of the 6-by-6 scattering matrix S.We estimate the complex effective permittivity of densely packed MCC (micro-crystalline-cellulose) pellets over frequency for different moisture contents. This material is used in pharmacutical processes.
36.	Cerullo, Livia, 1982, et al. (författare) Microwave Measurements for Metal Vessels 2013 Ingår i: 7th European Conference on Antennas and Propagation, EuCAP 2013, Gothenburg, Sweden, 8-12 April 2013. - 2164-3342. - 9781467321877 ; , s. 3869 - 3873 Konferensbidrag (refereegranskat)abstract We present two different measurement techniques intended for closed metal vessels, where the objective is to measure the permittivity inside the metal vessel. This problem is relevant for many applications found in e.g. process industry. The first approach exploits the measurement of resonance frequencies, where the metal vessel is used as a microwave resonator. In the second approach, we let the boundary of the metal vessel be equipped with aperture antennas, where the aperture antennas are implemented in terms of rectangular waveguides. The waveguide apertures loads the cavity significantly and we exploit the scattering matrix parameters for the solution of the inverse problem.
37.	Mecklenbräuker, Christoph F., et al. (författare) Sequential Bayesian Sparse Signal Reconstruction Using Array Data 2013 Ingår i: IEEE Transactions on Signal Processing. - : Institute of Electrical and Electronics Engineers (IEEE). - 1941-0476 .- 1053-587X. ; 61:24, s. 6344-6354 Tidskriftsartikel (refereegranskat)abstract In this paper, the sequential reconstruction of source waveforms under a sparsity constraint is considered from a Bayesian perspective. Let the wave field, which is observed by a sensor array, be caused by a spatially-sparse set of sources. A spatially weighted Laplace-like prior is assumed for the source field and the corresponding weighted Least Absolute Shrinkage and Selection Operator (LASSO) cost function is derived. After the weighted LASSO solution has been calculated as the maximum a posteriori estimate at time step, the posterior distribution of the source amplitudes is analytically approximated. The weighting of the Laplace-like prior for time step is then fitted to the approximated posterior distribution. This results in a sequential update for the LASSO weights. Thus, a sequence of weighted LASSO problems is solved for estimating the temporal evolution of a sparse source field. The method is evaluated numerically using a uniform linear array in simulations and applied to data which were acquired from a towed horizontal array during the long range acoustic communications experiment.
38.	Panahi, Ashkan, 1986, et al. (författare) A novel method of DOA tracking by penalized least squares 2013 Ingår i: 2013 5th IEEE International Workshop on Computational Advances in Multi-Sensor Adaptive Processing, CAMSAP 2013. - 9781467331463 ; , s. 61-64 Konferensbidrag (refereegranskat)abstract This work develops a new DOA tracking technique by proposing a novel semi-parametric method of sequential sparse recovery for a dynamic sparsity model. The proposed method iteratively provides a sequence of spatial spectrum estimates. The final process of estimating direction paths from the spectrum sequence is not considered. However, the simulation results show concentration of the spectrum around the true directions, which simplifies DOA tracking, for example, using a pattern recognition approach. We have also proved analytical results indicating consistency in terms of spectral concentration, which we omit in the interest of space and postpone to a more extensive work. The semi-parametric nature of the proposed method avoids highly complex data association and makes the method robust against crossing. The computational complexity per time sample is proportional to grid size, which can be contrasted to a single-snapshot LASSO solution that has a polynomial complexity order.
39.	Ström, Marie, 1984, et al. (författare) Robust transceiver design for wideband MIMO radar utilizing a subarray antenna structure 2013 Ingår i: Signal Processing. - : Elsevier BV. - 0165-1684. ; 93:12, s. 3541-3552 Tidskriftsartikel (refereegranskat)abstract In this paper, we investigate the possibility to suppress interference in wideband multiple-input multiple-output radar. The idea is to employ tunable filters at the transmitter and the receiver sides, and to derive filter coefficients that result in optimal transmitted signals from a system performance point of view, for a given radar scenario. The system performance is measured in signal-to-interference-and-noise ratio (SINR) at the receiver output, from which the filter properties are derived. The focus is to suppress active jamming interference, and especially deceptive jamming interference. We discuss two ways to derive the transmit and the receive filters. Each procedure utilizes two different power constraints related to the transmit filters. To incorporate imperfections in the given scenario, a robust extension to the design problem is proposed. Two different robust methods are evaluated: one that utilizes a Taylor series expansion of the SINR, and one that exploits a worst-case SINR maximization. Numerical validation illustrates the possibility to suppress interference without actually forming a spatial null in the direction towards interference, and the necessity to design transmit filters that are robust to uncertainties in the given scenario. (C) 2013 Elsevier B.V. All rights reserved.
40.	Bengtsson, Tomas, 1983, et al. (författare) Regularized Optimization for Joint Super-Resolution and High Dynamoc Range Image Reconstruction in a Percetually Uniform Domain. 2012 Ingår i: ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings. - 1520-6149. - 9781467300469 ; , s. 1097-1100 Konferensbidrag (refereegranskat)abstract This paper discusses resolution enhancement of a set of images with varying exposure durations, having a high combined dynamic range. So far, little has been said in relation to the Human Visual System when it comes to Super-Resolution and High Dynamic Range fusion, unlike the case for traditional Super-Resolution where errors are measured with respect to human perception in the pixel domain. We propose a Super-Resolution method in the Lab* domain to bridge the gap and present some image reconstruction results.
41.	Cerullo, Livia, 1982, et al. (författare) Inverse scattering for a closed cavity equipped with microwave antenna sensors 2012 Ingår i: AntennEMB, 5-8 March, Stockholm. Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract Pharmaceutical processes are often hosted by completely closed vessels that are difficult of access by means of measurement equipment. In particular, it is challenging to assess the interior or global process state with sensors placed on the vessel enclosing the process. This information about the process would allow for a better process monitoring and control. Here, a novel application of microwave sensing for process tomography is presented.
42.	Lioliou, Panagiota, 1982, et al. (författare) Bayesian Approach to Channel Estimation for AF MIMO Relaying Systems 2012 Ingår i: IEEE Journal on Selected Areas in Communications. - : Institute of Electrical and Electronics Engineers (IEEE). - 0733-8716 .- 1558-0008. ; 30:8, s. 1440-1451 Tidskriftsartikel (refereegranskat)abstract In this paper, we investigate the problem of channel estimation in amplify-and-forward multiple-input multiple-output relaying systems operating over random wireless channels. Using the Bayesian framework, novel linear minimum mean square error and expectation-maximization based maximum a posteriori channel estimation algorithms are developed, that provide the destination with full knowledge of all channel parameters involved in the transmission. Moreover, new, explicit expressions for the Bayesian Cramer-Rao bound are deduced for predicting and evaluating the channel estimation accuracy. Our simulation results demo nstrate that the incorporation of prior knowledge into the channel estimation algorithm offers significantly improved performance, especially in the low signal-to-noise ratio regime.
43.	Lioliou, Panagiota, 1982, et al. (författare) Channel Tracking for AF MIMO Relaying Systems 2012 Ingår i: IEEE Vehicular Technology Conference. - 1550-2252. - 9781467322171 ; , s. 1-5 Konferensbidrag (refereegranskat)abstract In this paper, we consider the problem of channelestimation in multiple-input multiple-output (MIMO) amplifyand-forward (AF) relaying systems operating over time varyingchannels. Only data at the receiving end are assumed availablefor the estimation. By employing a first-order autoregressive(AR) model for characterizing the time-varying nature of thechannels to be estimated, we derive an expectation-maximization(EM) Kalman filter (KF) that utilizes the received signal at thedestination to track the individual channel links. The extendedKF algorithm is also derived and compared to the proposedEM-based KF. Our simulation results show that the proposedEM-based KF offers better estimation performance with lesscomplexity when compared to the EKF algorithm.
44.	Lioliou, Panagiota, 1982, et al. (författare) Efficient Channel Estimation Techniques for Amplify and Forward Relaying Systems 2012 Ingår i: IEEE Transactions on Communications. - 0090-6778 .- 1558-0857. ; 60:11, s. 3150-3155 Tidskriftsartikel (refereegranskat)abstract In this paper, we present a channel estimation scheme for Amplify-and-Forward (AF) relaying systems, using measurements at the destination only. A Least-Squares (LS) based channel estimation algorithm is developed, that provides the destination with full knowledge of all channel responses involved in the transmission. To investigate the algorithm performance, the Cramer-Rao lower bound (CRB) is analytically computed and compared with the asymptotic covariance of the proposed estimator. Since the existing estimator does not reach the CRB, we also propose and analyze an improved algorithm by taking into account the noise characteristics via weighted LS (WLS). The improved algorithm is asymptotically efficient, since it attains the CRB as the SNR tends to infinity.
45.	Panahi, Ashkan, 1986, et al. (författare) A robust ℓ1 penalized DOA estimator 2012 Ingår i: Conference Record - Asilomar Conference on Signals, Systems and Computers. - 1058-6393. - 9781467350518 ; , s. 2013-2017 Konferensbidrag (refereegranskat)abstract The SPS-LASSO has recently been introduced as a solution to the problem of regularization parameter selection in the complex-valued LASSO problem. Still, the dependence on the grid size and the polynomial time of performing convex optimization technique in each iteration, in addition to the deficiencies in the low noise regime, confines its performance for Direction of Arrival (DOA) estimation. This work presents methods to apply LASSO without grid size limitation and with less complexity. As we show by simulations, the proposed methods loose a negligible performance compared to the Maximum Likelihood (ML) estimator, which needs a combinatorial search We also show by simulations that compared to practical implementations of ML, the proposed techniques are less sensitive to the source power difference.
46.	Panahi, Ashkan, 1986, et al. (författare) Fast Candidate Points Selection in the LASSO Path 2012 Ingår i: IEEE Signal Processing Letters. - 1070-9908 .- 1558-2361. ; 19:2, s. 79-82 Tidskriftsartikel (refereegranskat)abstract The LASSO sparse regression method has recently received attention in a variety of applications from image compression techniques to parameter estimation problems. This paper addresses the problem of regularization parameter selection in this method in a general case of complex-valued regressors and bases. Generally, this parameter controls the degree of sparsity or equivalently, the estimated model order. However, with the same sparsity/model order, the smallest regularization parameter is desired. We relate such points to the nonsmooth points in the path of LASSO solutions and give an analytical expression for them. Then, we introduce a numerically fast method of approximating the desired points by a recursive algorithm. The procedure decreases the necessary number of solutions of the LASSO problem dramatically, which is an important issue due to the polynomial computational cost of the convex optimization techniques. We illustrate our method in the context of DOA estimation.
47.	Seifi, Nima, 1981, et al. (författare) Impact of base station antenna tilt on the performance of network MIMO systems 2012 Ingår i: IEEE Vehicular Technology Conference. - 1550-2252. - 9781467309905 ; , s. Art. no. 6239994- Konferensbidrag (refereegranskat)abstract We study the downlink of a multicell MIMO system where clusters of multi-antenna base stations jointly serve multiple single-antenna users, commonly referred to as a network MIMO system. Most of the previous studies on network MIMO have only considered the azimuth pattern of the antenna, while ignoring the elevation pattern. In this paper, we consider both the azimuth and the elevation patterns and investigate the impact of the elevation angle tuning parameter, denoted as the antenna tilt, on the performance of such systems. Using system simulations, it is shown that the promised performance gains of network MIMO systems over conventional non-coordinated systems, crucially depend on the choice of the right tilt setting including the tilt type, i.e., mechanical or electrical, and the tilt angle. In particular, for tilt angles smaller than the optimum, network MIMO with intra-site coordination performs almost as well as the conventional system; while for tilt angles larger than the optimum, the performance of network MIMO with intra-site is similar to that of network MIMO with inter-site coordination.
48.	Seifi, Nima, 1981, et al. (författare) Throughput Optimization for MISO Interference Channels via Coordinated User-Specific Tilting 2012 Ingår i: IEEE Communications Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 1558-2558 .- 1089-7798. ; 16:8, s. 1248-1251 Tidskriftsartikel (refereegranskat)abstract In this letter, we propose a novel framework to enhance the throughput in multiple-input single-output (MISO) mutually interfering channels via selecting the tilt angles at all base stations (BSs) in a coordinated fashion. In the proposed framework, multiple BSs adjust their tilt angles jointly to maximize the sum throughput of the scheduled users, denoted as coordinated user-specific tilting. Assuming the availability of location information of the scheduled users at all BSs, accurate analytical expression for user ergodic rate is provided, which enables a decentralized deployment of the proposed framework at each BS. Simulation results show that the proposed coordinated user-specific tilting scheme outperforms the conventional schemes employing a fixed tilt angle at each BS.
49.	Cerullo, Livia, 1982, et al. (författare) Concurrent estimation of space and frequency variation for dielectrics: a microwave tomography system for process sensing applications 2011 Ingår i: Proceedings of the 9th International Conference on Electromagnetic Wave Interaction with Water and Moist Substances, Kansas City, MO USA, May 31 - June 3, 2011. - 9781629934556 ; , s. 177-184 Konferensbidrag (refereegranskat)abstract In many powder processing operations, the effective complex permittivity of the material under processing yields important information concerning the state of the process. This complex permittivity varies in general with respect to both space and frequency. For an accurate estimation of the process material parameters, it is important to be able to incorporate these dependencies in the estimation algorithm. Here, we present a rather general type of parameterization that is tailored for this type of situation. The parameterization is described in terms of a parameterization mesh, which makes it feasible to refine the computational mesh without influencing the number of degrees of freedom in the estimation problem. A test case demonstrates that our approach can yield accurate reconstructions.
50.	Johansson, Eija, 1982, et al. (författare) Interpolation based on stationary and adaptive AR(1) modeling 2011 Ingår i: ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings. - 1520-6149. - 9781457705397 ; , s. 4052-4055 Konferensbidrag (refereegranskat)abstract In this paper, we describe a minimal mean square error (MMSE) optimal interpolation filter for discrete random signals. We explicitly derive the interpolation filter for a first-order autoregressive process (AR(1)), and show that the filter depends only on the two adjacent points. The result is extended by developing an algorithm called local AR approximation (LARA), where a random signal is locally estimated as an AR(1) process. Experimental evaluation illustrates that LARA interpolation yields a lower mean square error than other common interpolation techniques, including linear, spline and local polynomial approximation (LPA).

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