| 171. |
- Nordebo, Sven, 1963-, et al.
(författare)
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A priori modeling for gradient based inverse scattering algorithms
- 2009
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Ingår i: Progress in Electromagnetics Research B. - 1937-6472. ; 16, s. 407-432
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a Fisher information based Bayesian approach to analysis and design of the regularization and preconditioning parameters used with gradient based inverse scattering algorithms. In particular, a one-dimensional inverse problem is considered where the permittivity and conductivity profiles are unknown and the input data consist of the scattered field over a certain bandwidth. A priori parameter modeling is considered with linear, exponential and arctangential parameter scalings and robust preconditioners are obtained by choosing the related scaling parameters based on a Fisher information analysis of the known background. The Bayesian approach and a principal parameter (singular value) analysis of the stochastic Cramer-Rao bound provide a natural interpretation of the regularization that is necessary to achieve stable inversion, as well as an indicator to predict the feasibility of achieving successful reconstruction in a given problem set-up. In particular, the Tikhonov regularization scheme is put into a Bayesian estimation framework. A time-domain least-squares inversion algorithm is employed which is based on a quasi-Newton algorithm together with an FDTD-electromagnetic solver. Numerical examples are included to illustrate and verify the analysis.
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| 172. |
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| 173. |
- Nordebo, Sven, et al.
(författare)
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A Quasi-Static Electromagnetic Analysis for Experiments with Strong Permanent Magnets
- 2014
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Ingår i: Progress in Electromagnetics Research B. - 1937-6472. ; 61, s. 1-16
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Tidskriftsartikel (refereegranskat)abstract
- An electromagnetic analysis is presented for experiments with strong permanent disc magnets. The analysis is based on the well known experiment that demonstrates the effect of circulating eddy currents by dropping a strong magnet through a vertically placed metal cylinder and observing how the magnet is slowly falling through the cylinder with a constant velocity. This experiment is quite spectacular with a super strong neodymium magnet and a thick metal cylinder made of copper or aluminum. A rigorous theory for this experiment is provided based on the quasi-static approximation of the Maxwell equations, an infinitely long cylinder (no edge effects) and a homogeneous magnetization of the disc magnet. The results are useful for teachers and students in electromagnetics who wish to obtain a deeper insight into the analysis and experiments regarding this phenomenon, or with industrial applications such as the grading and calibration of strong permanent magnets or with measurements of the conductivity of various metals, etc.. Several experiments and numerical computations are included to validate and to illustrate the theory.
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| 174. |
- Nordebo, Sven, et al.
(författare)
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A Semi-definite Programming Approach to Spatial Decorrelation of Independently Polarized Signals
- 2007
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Ingår i: Wireless Communications & Mobile Computing. - : Wiley. - 1530-8669 .- 1530-8677. ; 7:1, s. 91-101
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Tidskriftsartikel (refereegranskat)abstract
- A MIMO channel spatial decorrelation scheme based on semi-definite programming is introduced. As a particular application example, the paper addresses the potential gain of using multiple antennas and MIMO-OFDM techniques in order to increase the bandwidth efficiency in satellite communication systems. In particular, we consider the increase in channel capacity that is possible by exploiting satellite and polarization diversity. A fundamental case is studied with three satellite branches, and where each transmit/receive antenna unit consists of six elemental electric and magnetic dipoles yielding six distinguishable parallel polarization channels per frequency. The numerical examples show that capacity increases linearly on a logarithmic signal-to-noise ratio scale where the constant of proportionality is the number of active parallel channels. In this respect, the simultaneous use of triple electric and triple magnetic dipoles has the potential to triple the capacity of an antenna system based on antenna units of single dipoles. Copyright (c) 2006 John Wiley & Sons, Ltd.
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| 175. |
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| 176. |
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| 177. |
- Nordebo, Sven, et al.
(författare)
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A Systematic Approach to Robust Preconditioning for Gradient Based Inverse Scattering Algorithms
- 2008
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Ingår i: Inverse Problems. - : IOP Publishing. - 1361-6420 .- 0266-5611. ; 24:2, s. 025027-
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a systematic approach to robust preconditioning for gradient-based nonlinear inverse scattering algorithms. In particular, one- and two-dimensional inverse problems are considered where the permittivity and conductivity profiles are unknown and the input data consist of the scattered field over a certain bandwidth. A time-domain least-squares formulation is employed and the inversion algorithm is based on a conjugate gradient or quasi-Newton algorithm together with an FDTD-electromagnetic solver. A Fisher information analysis is used to estimate the Hessian of the error functional. A robust preconditioner is then obtained by incorporating a parameter scaling such that the scaled Fisher information has a unit diagonal. By improving the conditioning of the Hessian, the convergence rate of the conjugate gradient or quasi-Newton methods are improved. The preconditioner is robust in the sense that the scaling, i.e. the diagonal Fisher information, is virtually invariant to the numerical resolution and the discretization model that is employed. Numerical examples of image reconstruction are included to illustrate the efficiency of the proposed technique. © 2008 IOP Publishing Ltd.
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| 178. |
- Nordebo, Sven, et al.
(författare)
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A Well-Conditioned Quadratic Program for Unique Design of Two Dimensional Weighted Chebyshev FIR Filters
- 1996
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Konferensbidrag (refereegranskat)abstract
- The weighted Chebyshev design of two-dimensional FIR filters is in general not unique since the Haar condition is not generally satisfied. However, for a design on a discrete frequency domain, the Haar condition might be fulfilled. The question of uniqueness is, however, rather extensive to investigate. It is therefore desirable to define some simple additional constraints to the Chebyshev design in order to obtain a unique solution. The weighted Chebyshev solution of minimum Euclidean filter weight norm is always unique, and represents a sensible additional constraint since it implies minimum white noise amplification. It is shown that this unique Chebyshev solution can always be obtained by using an efficient quadratic programming formulation with a strictly convex objective function and linear constraints An example where a conventional Chebyshev solution is non-unique is discussed in the paper.
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| 179. |
- Nordebo, Sven, et al.
(författare)
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An adjoint field approach to Fisher information-based sensitivity analysis in electrical impedance tomography
- 2010
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Ingår i: Inverse Problems. - : IOP Publishing. - 1361-6420 .- 0266-5611. ; 26:12
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Tidskriftsartikel (refereegranskat)abstract
- An adjoint field approach is used to formulate a general numerical framework for Fisher information-based sensitivity analysis in electrical impedance tomography. General expressions are given for the gradients used in standard least-squares optimization, i.e. the Jacobian related to the forward problem, and it is shown that these gradient expressions are compatible with commonly used electrode models such as the shunt model and the complete electrode model. By using the adjoint field formulations together with a variational analysis, it is also shown how the computation of the Fisher information can be integrated with the gradient calculations used for optimization. It is furthermore described how the Fisher information analysis and the related sensitivity map can be used in a preconditioning strategy to obtain a well-balanced parameter sensitivity and improved performance for gradient-based quasi-Newton optimization algorithms in electrical impedance tomography. Numerical simulations as well as reconstructions based on experimental data are used to illustrate the sensitivity analysis and the performance of the improved inversion algorithm in a four-electrode measurement set-up.
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| 180. |
- Nordebo, Sven, et al.
(författare)
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Application of Infinite Dimensional Linear Programming to FIR Filter Design with Time Domain Constraints
- 1998
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Konferensbidrag (refereegranskat)abstract
- Previously the envelope-constrained filtering problem was formulated as designing an FIR filter such that the filter's L2 norm is minimized subject to the constraint that its response to a specified input pulse lies within a prescribed envelope. In this paper, we recast this filter design problem as a frequency-domain L infinity optimization problem with time-domain constraints. Motivations for solving this problem are given. Then recently developed infinite dimensional linear programming techniques are used for the design of the required FIR filter. For illustration, we apply the approach to a numerical example which deals with the design of an equalization filter for a digital transmission channel.
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