| 1. |
- Andersson, Sören, et al.
(författare)
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A Study of Adaptive Arrays for Mobile Communication Systems
- 1990
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The application of adaptive antenna techniques to increase the channel capacity in mobile radio communication is discussed. Directional sensitivity is obtained by using an antenna array at the base station, possibly both in receiving and transmitting mode. A scheme for separating several signals at the same frequency is proposed. The method is based on high-resolution direction finding following by optimal combination of the antenna outputs. Comparisons to a method based on reference signals are made. Computer simulations are carried out to test the applicability of the technique to scattering scenarios that typically arise in urban areas. The proposed scheme is found to have great potential in rejecting cochannel interference, albeit at the expense of high computational requirements.
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| 2. |
- Andersson, Sören, et al.
(författare)
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An adaptive array for mobile communication systems
- 1991
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Ingår i: IEEE Transactions on Vehicular Technology. - Toronto, Ont, Can : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9545 .- 1939-9359. ; 40:1 pt 2, s. 230-236, s. 3289-3292
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Tidskriftsartikel (refereegranskat)abstract
- The use of adaptive antenna techniques to increase the channel capacity is discussed. Directional sensitivity is obtained by using an antenna array at the base station, possibly both in receiving and transmitting modes. A scheme for separating several signals at the same frequency is proposed. The method is based on high-resolution direction-finding followed by optimal combination of the antenna outputs. Comparison with a method based on reference signals is made. Computer simulations are carried out to test the applicability of the technique to scattering scenarios that typically arise in urban areas. The proposed scheme is found to have great potential in rejecting cochannel interference, albeit at the expense of high computational requirements.
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| 3. |
- Andersson, Sören, et al.
(författare)
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Analysis of a polarized seismic wave model
- 1994
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We present a model for polarized seismic waves where the data are collected by three-component geophone receivers. The model is based on two parameters describing the polarization properties of the waveforms. These parameters are the ellipticity and the orientation angle of the polarization ellipse. The model describes longitudinal waveforms (P-waves) as well as elliptically polarized waves. For the latter waves the direction-of-propagation of the waveform is in the plane spanned by the ellipse's major and minor axes; Rayleigh waves are treated as a special case. We analyze the identifiability of the models and derive the Cramer-Rao and mean-square-angular-error (MSAE) bounds involving one or two three-component geophones.
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| 4. |
- Andersson, Sören
(författare)
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Asymptotic Analysis of Subspace Methods for Beamspace Direction-of-Arrival Estimation using Large Arrays
- 1992
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Asymptotic analysis methods for performance prediction of so-called subspace directionof -arrival estimation methods has been developed earlier, assuming that a large-enough number of array measurements, or snapshots, is collected. This paper also addresses the problem of making performance predictions, but for beamspace-based subspace methods. The novel approach in this paper assumes the number of array elements to be large, while the number of snapshots is arbitrary. The perturbation effect, due to additive sensor noise, on a certain subspace is used for establishing the asymptotic behavior of direction-of-arrival estimates. The asymptotic estimation errors for the estimators resulting from Signal Subspace Fitting methods, such as WSF, and Noise Subspace Fitting (NSF) methods, such as MUSIC and a multi-dimensional counterpart to WSF, are shown to be asymptotically unbiased and normally distributed. Provided that the array response vectors become orthogonal when the number of array elements increases, the NSF methods are shown to give consistent estimates even in the case of fully coherent emitter signals, and the WSF method is shown to be consistent for coherent emitter signals even without this assumption. Comparisons with results for Maximum-Likelihood methods yield conditions for guaranteeing efficiency of the methods. Some simulation examples are also included.
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| 5. |
- Andersson, Sören
(författare)
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Beamspace Transformation Matrix Design using Numerical Optimization
- 1992
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This paper addresses the problem of designing a data-dimension reducing transformation matrix, to be used for beamspace direction-of-arrival (DOA) estimation. The design of the transformation matrix is based on numerical optimization techniques. The criteria to be satisfied is to retain as much as possible of the achievable, optimal estimation accuracy using the non-reduced data-sizes, while also taking into account the sidelobe levels of the beampattern. Comparisons of estimation accuracy and sensitivity to out-of-sector emitters for the design methods considered herein are carried out by means of simulation examples, using the WSF-method for the DOA-estimation. In order to reduce the number of parameters to be optimized, a parametrization of the transformation matrix is made, that utilizes the properties of Householder-reflections.
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| 6. |
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| 7. |
- Andersson, Sören
(författare)
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HIV-1 and HIV-2 infections in Guinea-Bissau, West Africa : studies of immune responses, prevailing viruses and epidemiological trends
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis encompasses immunological, virological and epidemiological studies of HIV-1 and HIV-2 infections in Guinea-Bissau. We have established a robust and reliable diagnostic strategy based on a combination of enzyme-linked immunosorbent assays (ELISAs) and rapid simple tests. Evaluations showed that the strategy had a high capacity to discriminate between HIV-1 and HIV-2 and a high concordance with polymerase chain reaction (PCR) testing. Epidemiological studies including annual sentinel surveillance of pregnant women and a prospective cohort study of police officers revealed gradually increasing prevalence rates but relatively stable incidence rates for HIV-1, and significantly decreasing prevalence and incidence rates for HIV-2 over a seven- to ten-year period. A study of a possible protective effect of HIV-2 infection against subsequent HIV-1 infection could not confirm a previous report by others of such an effect. The prevailing virus variants were characterized and it was demonstrated that a majority of the HIV-1 variants circulating in Guinea-Bissau and neighbouring countries are subtype A/G recombinants. Comparative viral load measurements revealed that the plasma viral load was significantly lower in HIV-2-infected as compared to HIV-1-infected individuals and that the viral setpoint after primary infection was 28-fold lower for HIV-2 as compared to HIV-1. The plasma viral load was inversely correlated to CD4+T-lymphocyte levels for both HIV-1 and HIV-2. HIV-1 and HIV-2 dually infected individuals carried virus of the same genetic subtypes as the singly infected persons but the viral load measurements were less predictable without apparent correlation with CD4+ T-lymphocyte levels. Assays for HIV-2-specific cell-mediated immune responses and mitogen-induced ßchemokine production established in conjunction with HIV-2 monkey vaccine studies at the Swedish Institute for Infectious Disease Control (SMI) were subsequently applied for human investigations in Guinea-Bissau. In monkeys, lymphocyte proliferative responses and CTL were readily detectable after HIV-2 infection and in a proportion of animals immunized with an HIV-2 recombinant canarypox virus vaccine alone or in combination with HIV-2 gp125 or V3 peptide boosters. Four out of ten primeboost immunized monkeys but none of four monkeys given the HIV-2 recombinant vaccine alone was protected against intravenous challenge with live HIV-2. However, no clear correlation between the cell-mediated immune responses and protection against live HIV-2 challenge was observed. Studies of HIV-2 discordant couples in Guinea-Bissau showed that HIV-2 infected as well as HIV-2 exposed uninfected individuals had increased HIV-2 specific T-lymphocyte proliferative responses compared to control subjects. The exposed uninfected individuals had reactivity to HIV-2 V3 peptides while the HIV-2-infected persons mainly reacted to HIV-2 whole viral lysate. Quantification of mitogen-induced CD8+ T-cell production of ß-chemokines (RANTES, MIP-1[alpha], MIP-1ß, which are known to suppress HIV replication, showed increased production in HIV-2-infected and HIV-2-exposed uninfected individuals as well as in HIV-seronegative outpatients as compared to seronegative healthy controls. The demonstrated immune responses associated with HIV-2 infection and exposure to HIV-2 could be important for resistance to this infection and control of disease progression. These investigations have given some new insight into the characteristics of HIV-1 and HIV-2 infections and provide a basis for continued studies of these infections in Guinea-Bissau.
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| 8. |
- Andersson, Sören
(författare)
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On Dimension Reduction in Sensor Array Signal Processing
- 1992
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During the last decades, sensor array signal processing has been a very active research area. More recently, relations between many of the proposed methods has been examined. The problem of assessing the estimation accuracy of these methods has also been addressed. Realworld applications of these techniques involves spatial distribution of several sensors to be used for collecting measurements of interesting emitted waveforms. From the measurements, detection and localization as well as estimation of the emitted waveforms can be accomplished. Common examples of applications are radar (electromagnetic waveforms) and sonar (acoustical underwater waveforms).Another aspect of array processing that recently has been addressed in the literature is that of dimension reduction, where the data vectors collected at the sensor outputs are reduced in size. This reduction is employed mainly in order to lower the amount of computations necessary for obtaining the parameter-estimates of interest; hut some other improvcments has also been observed. These include, e.g., lower sensitivity to sensor noise correlations and, for some estimation methods, higher resolution capability.In this thesis, it is demonstrated how to make the dimension reduction in an optimal fashion, where the optimality is with respect to estimation accuracy. More precisely, an expression to be satisfied by a transformation matrix acting on the sensor outputs is derived , that preserves the optimally achievable estimation accuracy (the Cramer-Rao bound) also in the reduced space. A transformation matrix design method that tries to reduce some unwanted properties of the optimal transformation is also outlined and examined. This method is based on numerical optimization of a particular performance mea.sure, motivated by the insight obtained in the process of finding the optimal transformation.l\foreover, an asymptotic analysis is performed, using the reduced data vectors, that examines the estimation accuracy of several estimation methods when a !arge number of sensor elements is used. This analysis is valid for a fairly general transformation matrix, and the methods considered are the Weighted Subspace Fitting (WSF) and Noise Subspace Fitting (NSF) methods, including MUSIC. By employing the optimal transformation matrix, the WSF method is shown to to be efficient, i.e., to attain the Cramer-Rao bound. An examination of the estimation accuracy, compared to that optimally attainable, is performed for the case when the transformation matrix differs from the optimal one. Finally, an application is studied, considering the potential use of sensor arrays in mobile communication systems.
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| 9. |
- Andersson, Sören
(författare)
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On Optimal Dimension Reduction for Sensor Array Signal Processing
- 1991
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The computational complexity for direction-of-arrival estimation using sensor arrays increases very rapidly with the number of sensors in the array. One way to lower the amount of computations is to employ some kind of reduction of the data dimension. This is usually accomplished by employing linear transformations for mapping full dimension data into a lower dimensional space. Different approaches for selecting these transformations have been proposed. In this paper, a transformation matrix is derived that makes it possible to theoretically attain the full-dimension Cramér-Rao bound also in the reduced space. A bound on the dimension of the reduced data set is given, above which it is always possible to obtain the same accuracy for the estimates of the source localizations, using the lower-dimension data, as that achievable by using the full dimension data. Furthermore, a method is devised for designing the transformation matrix. Numerical examples, using this design method, are presented, where the achievable performance of the (optimal) Weighted Subspace Fitting method with full dimension data is compared to the performance obtained with reduced dimension data. The problem of estimating parameters of sinusoidal signals from noisy data is also addressed by a direct application of the results derived herein.
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| 10. |
- Andersson, Sören
(författare)
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On Two-Stage Schemes for Beamspace Direction-of-Arrival Estimation
- 1992
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This paper addresses performance analysis of two-stage methods for direction-of-arrival (DOA) estimation. By first finding crude initial DOA-estimates, the idea is to use these estimates for designing a transformation matrix as the first step. Then this matrix is employed for mapping the data to the lower-dimensional beamspace. Different ways for obtaining the first-stage DOA-estimates are discussed and various trade-offs that have to be considered are pointed out. The beamspace version of the Weighted Subspace Fitting (WSF) method is used for obtaining the final DOA estimates. An approximate performance bound is stated that assumes the first-stage DOA-estimates to be close to the true DOAs. For comparing the performance of some different ways to obtain the first-stage DOA-estimate, some numerical examples are included.
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| 11. |
- Andersson, Sören, et al.
(författare)
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Optimal Dimension Reduction for Array Processing : Generalized
- 1993
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This correspondence extends previously reported work [1, 2] on the problem, or rather possibility, of achieving optimality of beamspace (BS) array processing, where use is made of dimensionally reduced data vectors. The optimality here is with respect to the best possible element space (ESP) parameter estimation accuracy, i.e., the Cramér-Rao bound.
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| 12. |
- Andersson, Sören
(författare)
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Optimal Dimension Reduction for Sensor Array Signal Processing
- 1991
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Ingår i: Proceedings of the 25h Asilomar Conference on Signals, Systems and Computers. - Linköping : Linköping University. - 0818624701 ; , s. 918-922
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The computational complexity for direction-of-arrival estimation using sensor arrays increases very rapidly with the number of sensors in the array. One way to lower the amount of computations is to employ some kind of reduction of the data dimension. This is usually accomplished by employing linear transformations for mapping full-dimension data into a lower-dimensional space. In the present work, a transformation matrix is derived, that makes it possible to attain the full-dimension Cramer-Rao bound also in the reduced space. A bound on the dimension of the reduced data set is given, above which it is always possible to obtain the same accuracy for the lower-dimension estimates of the source localizations as that achievable by using the full-dimension data. Furthermore, a method is devised for designing the transformation matrix.
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| 13. |
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| 14. |
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| 15. |
- Andersson, Sören, et al.
(författare)
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Some Comparisons of Cramer-Rao Bounds for Vector Sensors and Scalar Sensor Arrays for Array Processing
- 1993
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The effect from polarization of emitted wave fronts on the parameter estimation accuracy for an array composed only of sensors sensitive to just one polarization direction has not been addressed in the literature this far. Antennas with such characteristics are, e.g., dipole (or scalar) antennas. A vector sensor, on the other hand, is a sensor whose output data consists of, for the electromagnetic case, the complete electric and magnetic fields at the sensor. This paper examines some of the effects on the Cram'er-Rao Bound for the elevation and/or azimuth angles to a single source emitting a polarized (electromagnetic) waveform. Since only one vector sensor is needed for estimation of both azimuth and elevation, it would be of interest to compare the lower parameter estimation error bound resulting from the vector sensor data model to the "ordinary" one, i.e. the data model used for scalar arrays. Such comparisons, both analytically and numerically, are herein made for an acoustic data model, as well as for an electromagnetic measurement model, for some simple scenarios and array configurations.
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| 16. |
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| 17. |
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| 18. |
- Jonsson, Erland, 1946, et al.
(författare)
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A practical dependability measure for degradable computer systems with non-exponential degradation
- 1994
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Ingår i: IFAC Symposium on Fault Detection, Supervision and Safety for Technical Processe - SAFEPROCESS 1994. ; Vol. 1, s. 231-237
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Tidskriftsartikel (refereegranskat)abstract
- This paper proposes a vectorized measure for a set of behavioural dependability attributes. The measure is based on Markov processes and is intended for practical dependability trade-offs. It describes the system performance on a number of service levels. Thus, it is possible to merge attributes such as reliability, safety and performability into one single quality. Whereas reliability describes the functional fulfillment of a system, performability reflects its ability of functional degradation. The safety attribute handles a class of failures with catastrophic consequences and can be accommodated by means of introducing two or more service levels for a failed system.Many systems exhibit time-dependent degradation rates and non–exponential lifetimes. This situation can be handled by means of applying phase–type assumptions and introducing some additional states to the system, which would allow us to remain within the universe of Markov modelling.
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| 19. |
- Jonsson, Erland, 1946, et al.
(författare)
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An Attempt to Quantitative Modelling of Behavioural Security
- 1995
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Ingår i: Proceedings of the 11th International Information Security Conference - IFIP/SEC 1995. ; addendum, s. 44-57
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Tidskriftsartikel (refereegranskat)abstract
- This paper suggests a quantitative approach to security, and specifically to a security-concept, which is regarded as an attribute of dependability together with reliability, availability and safety. We note that security is a more complex attribute of dependability than are the other three, and that it can therefore be split into preventive and behavioural aspects. We show that, in addition to availability, confidentiality could be used to denote a new type of behavioural aspect of dependability. Integrity is interpreted in terms of fault prevention, and is not directly related to system behaviour. A practical measure for behavioural dependability attributes including confidentiality is defined. Due to the dependability viewpoint of security that we take, a measure could be derived using traditional reliability methods, such as Markov modelling. The measure is meant for practical trade-offs within a class of computer systems. The measure quantifies system performance on user-specified service levels, which may be operational or failed. Certain levels may be related to confidentiality degradations or confidentiality failures. A simple Reference Monitor example is given to illustrate the use of the measure. The calculation method is then extended to handle situations with non–exponential failure rates, which is the normal case in security applications, by means of using phase–type modelling. This is illustrated by introducing malicious software, such as a Trojan Horse into the Reference Monitor.
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| 20. |
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| 21. |
- Sellgren, Ulf, et al.
(författare)
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A Contact Model for Rough Surfaces
- 1999
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Ingår i: NAFEMS World Congress, April 25-28 1999, Newport, Rhode Island,USA.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Engineering surfaces can be characterized as more or less randomly rough. Contact between engineering surfaces is thus discontinuous and the real area of contact is a small fraction of the nominal contact area. The stiffness of a rough surface layer will thus have an influence on the contact state as well as on the behavior of the surrounding system. A contact model that takes the properties of engineering surfaces into account has been developed and implemented in a commercial FE software. Results obtained with the model have been compared and verified with results from an independent numerical method. Results have shown that the height distribution of the topography has a significant influence on the contact stiffness but that the curvature of the roughness is of minor importance. The contact model that was developed for determining the apparent contact area and the distribution of the mean contact pressure could thus be based on a limited set of height parameters that describe the surface topography. By operating on the calculated apparent pressure distribution with a transformation function that is based on both height and curvature parameters, the real contact area can be estimated in a post processing step.
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| 22. |
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| 23. |
- Wikander, Jan, 1956-, et al.
(författare)
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Förfarande och anordning för beröringsfri bestämning av en kropps translations- eller vridningsläge
- 1991
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Patent (populärvet., debatt m.m.)abstract
- In a method for touch-free determination of the translatory or angular position of a body, a defined measuring path (M) on the body (K) in the direction of said translatory or angular motion of the body (1) is detected spot-wise by means of an optical sensor head (1), which emits laser light towards the measuring path and provides an output signal responsive to light reflected from said path (M), the sensor head (1) and the body (K) with the measuring path (M) being movable relative to each other. In order to provide for an exact position determination with high resolution in an inexpensive way the invention suggests that the surface profile or characteristics of the body (K) along the measuring path (M) is scanned by illumination from the sensor head (1), which consists of an optical pick-up for compact discs.
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