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  • Fhager, Andreas, et al. (författare)
  • Image Reconstruction in Microwave Tomography Using a Dielectric Debye Model
  • 2012
  • Ingår i: IEEE Transactions on Biomedical Engineering. - 0018-9294. ; 59:1, s. 156-166
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper, quantitative dielectric image reconstruction based on broadband microwave measurements is investigated. A time-domain-based algorithm is derived where Debye model parameters are reconstructed in order to take into account the strong dispersive behavior found in biological tissue. The algorithm is tested with experimental and numerical data in order to verify the algorithm and to investigate improvements in the reconstructed image resulting from the improved description of the dielectric properties of the tissue when using broadband data. The comparison is made in relation to the more commonly used conductivity model. For the evaluation, two examples were considered, the first was a lossy saline solution and the second was less lossy tap water. Both liquids are strongly dispersive and used as a background medium in the imaging examples. The results show that the Debye model algorithm is of most importance in the tap water for a bandwidth of more than 1.5 GHz. Also the saline solution exhibits a dispersive behavior but since the losses restrict the useful bandwidth, the Debye model is of less significance even if somewhat larger and stronger artifacts can be seen in the conductivity model reconstructions
  • Johansson, Sven, et al. (författare)
  • A Novel Multiple-Reference Algorithm for Active Control of Propeller-Induced Noise in Aircraft Cabins
  • 2000
  • Rapport (övrigt vetenskapligt)abstract
    • The cabin noise inside propeller aircraft is essentially dominated by strong tonal components at harmonics of the blade passage frequency of the propellers. In order to achieve an efficient reduction of such a periodic low frequency noise, it is advisable to use an active noise control system based on adaptive narrowband feedforward techniques. The feedforward controller presented in this paper exploits narrowband assumptions by using complex-valued filtering and complex modeling of control paths. This paper introduces a multiple reference controller based on the novel actuator-individual normalized Filtered-X Least-Mean-Squares (FX LMS) algorithm. This algorithm combines low computational complexity with high performance. The algorithm is of the LMS-type. However, owing to the novel normalization of the algorithm it can also be regarded as a Newton-type algorithm. A comparison between the actuator-individual normalized FX LMS algorithm and the ordinary normalized FX LMS algorithm is presented. The results demonstrate better performance in terms of convergence rate and tracking properties when the Newton-like actuator-individual normalized FX LMS algorithm is used as compared with the conventional normalized LMS algorithm. The evaluation was performed using noise signals recorded inside the cabin of a twin engine propeller aircraft during flight. The paper also discusses variants of the actuator-individual normalized FX LMS algorithm.
  • Johansson, Sven, et al. (författare)
  • A Novel Multiple-Reference, Multiple-Channel, Normalized Filtered-X LMS Algorithm for Active Control of Propeller-Induced Noise in Aircraft Cabins
  • 1998
  • Konferensbidrag (refereegranskat)abstract
    • The dominating cabin noise in propeller aircraft consists essentially of strong tonal components at harmonics of the Blade Passage Frequency (BPF) of the propellers. In order to efficiently reduce such low frequency periodic noise, it is advisable to employ an Active Noise Control (ANC) system based on a feedforward controller. This paper presents a set of normalized complex Filtered-X Least-Mean-Square (FX LMS) algorithms. By using different variants of normalization factors the convergence rate, the tracking performance and the steady-state noise attenuation can be improved. The algorithms presented are based either on a single normalization factor for the whole control system (global normalized FX LMS algorithm), or several individual normalization factors (reference-individual FX LMS algorithm) or the novel actuator-individual FX LMS algorithm. The evaluation is performed on noise recorded during flight in the cabin of a Dornier 328, a twin-engine propeller aircraft.
  • Johansson, Sven, et al. (författare)
  • Comparison of Multiple- and Single-Reference MIMO Active Noise Control Approaches Using Data Measured in a Dornier 328 Aircraft
  • 2000
  • Ingår i: International Journal of Acoustics and Vibration. - Int. Inst. of Acoustics and Vibration (IIAV). - 1027-5851. ; 5:4, s. 77-88
  • Tidskriftsartikel (refereegranskat)abstract
    • In many applications of noise control, the greatest annoyance is caused by periodic low frequency noise. Successful reduction of such noise can often be achieved by using an active noise control system with narrowband feedforward control. If several noise sources contribute to the sound field, a multiple-reference control system is usually required. This type of system allows the reference signals from each noise source to be processed individually within the controller, thereby enabling individual control of the sound field from each noise source. The present paper addresses the problem of controlling noise from two sources that are more or less synchronized. A typical application is the control of propeller-generated noise within a twin propeller aircraft. To find out whether a multiple-reference controller is necessary, or if a single-reference controller is sufficient, the performance of a single- versus twin-reference control algorithm is evaluated in a comparative study. The study is performed as a computer simulation (off-line evaluation) using real-life data recorded in a Dornier 328 under different flight conditions. The results demonstrate that the twin-reference controller performs better than the single-reference whenever there is a slight deviation in the rotational speed of the two propellers. The paper also treats the generation of reference signals. The approach presented is based on a fixed sampling rate and uses a sliding FFT filtering technique.
  • Johansson, Sven, et al. (författare)
  • Convergence Analysis of a Twin-Reference Complex Least-Mean-Squares Algorithm
  • 2002
  • Ingår i: IEEE transactions on speech and audio processing. - IEE/IEEE. - 1063-6676. ; 10:4, s. 213-221
  • Tidskriftsartikel (refereegranskat)abstract
    • In many noise control applications, the noise is dominated by low frequencies and generated by several independent periodic sources. In such situations the tonal noise may be suppressed by using a narrowband multiple-reference feedforward controller. The performance characteristics of the control system, e.g., the convergence behavior and noise reduction are directly related to the controller adaptation rate as well as the frequency separation of the tonal components in the noise, i.e., the beat frequency. This paper treats the convergence performance of a complex least-mean-squares (LMS) algorithm using two reference signals. An analysis of its convergence behavior is presented as well as the results from computer simulations validating the convergence behavior. The convergence of the filter weights and the decrease rate of the squared error (the learning curve) for noise control applications are also discussed.
  • Johansson, Sven, et al. (författare)
  • Evaluation of a Multiple- versus a Single-Reference MIMO ANC Algorithm on Dornier 328 Test Data Set
  • 1998
  • Konferensbidrag (refereegranskat)abstract
    • In many applications, such as in propeller aircraft, the dominating noise is periodic. Successful reduction of the periodic noise components can be achieved by using an Active Noise Control (ANC) system based on feedforward techniques. In this paper, a comparison between the performance of single--reference (single-tacho) and multiple--reference (twin-tacho) feedforward control systems is presented. The comparison is made for two different flight conditions, both with and without synchronized propellers. The evaluation results show that a multiple--reference controller provides better performance than a single--reference controller when a slight deviation exists in the propeller synchronization.
  • Nordberg, Jörgen, et al. (författare)
  • Acoustic Echo Cancellation Employing Delayless Subband Adaptive Filters
  • 1996
  • Konferensbidrag (refereegranskat)abstract
    • The use of hands-free communication in cars, computer applications and video conferencing has created a demand for high-quality acoustic echo cancellation. In these applications these acoustic channel has typically a long impulse response in the order of 100ms. Typical lengths of adaptive FIR-filters can be 500-1500 taps. In order to reduce the complexity and also to improve the convergence rate, subband processing schemes have been suggested. This paper presents an implementation of a delayless subband adaptive filter. The study shows a possible suppression of about 30 dB and also a more rapid convergence compared to a fullband LMS-filter.
  • Nordberg, Jörgen, et al. (författare)
  • Digital Beamforming as a Means for Interference Cancellation
  • 1997
  • Konferensbidrag (refereegranskat)abstract
    • The increasing demand for user capacity is one of the major challenges to network operators today. In order to increase the capacity in a mobile radio network, it is desirable to exploit the spatial domain in an efficient way. An array adds spatial domain selectivity in order to improve the Carrier-to-Interference ration (C/I). An adaptive antenna ray can further improve the Carrier-to-Interference ratio (C/I) by suppressing interfering signals. The proposed circular array consists of M elements and combines fixed beamformers with interference cancellors. The fixed beamformers use a weight matrix to form multiple beams. The radius of the circular array is assumed to be half the wavelenght of the signal. The interference cancellor suppresses undesired signals, so called jammers, leaking into the desired beam. The desired signal is filtered out by the fixed beamforming structure. Due to the sidelobes, interfering signals will also, however, be present in this beam. Two alternative strategies may be chosen to cancel these interferers; use the other fixed beam outputs as inputs to an adaptive interference canceller, or demodulate the outputs from the other fixed beams and regenerate clean signals which are used as inputs to adaptive interference cancellers. Results are presented using beamformer weights obtained from an LS minimization as well as minimax.
  • Nordberg, Jörgen, et al. (författare)
  • Digital Beamforming as a Means for Interference Cancellation
  • 1998
  • Rapport (övrigt vetenskapligt)abstract
    • Ett sätt att öka kapaciteten i dagens mobiltelefoni-system är att använda flera antenner i basstationerna d.v.s använda digital lobformning. Denna rapporten beskriver hur man med hjälp utav digital lobformning kan förbättra kvaliteten på den mottagna signalen.
  • Nordebo, Sven, et al. (författare)
  • Adaptive Beamforming
  • 1994
  • Ingår i: IEEE journal of oceanic engineering : a journal devoted to the application of electrical and electronics engineering to the oceanic environment. - New York : Institute of Electrical and Electronics Engineers, Council on Oceanic Engineering. - 0364-9059. ; 19:14, s. 583-590
  • Tidskriftsartikel (refereegranskat)abstract
    • Controlling the resolution in adaptive beamformers is often crucial. A simple method that works for both narrowband and broad-band arrays is presented. This method is based on the normalized leaky LMS algorithm in conjunction with a generalized sidelobe canceller (GSC) structure, where the GSC is designed using a spatial filtering approach. In essence, the suppression of the spatial filters and the implicit noise of the leaky LMS algorithm together determine the adaptive beamformer. Analytical expressions are given for the Wiener filters and the output spectrum versus frequency and point source location. These expressions are employed in the design specification of the spatial filters and to obtain conditions for a controlled quiescent beamformer response. Simulation results are presented to illustrate the behavior of the array.
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