| 1. |
- Abrahamsson, Richard, et al.
(författare)
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Enhanced covariance matrix estimators in adaptive beamforming
- 2007
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Ingår i: 2007 IEEE International Conference on Acoustics, Speech, and Signal Processing, Vol II, Pts 1-3. ; , s. 969-972
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Konferensbidrag (refereegranskat)abstract
- In this paper a number of covariance matrix estimators suggested in the literature are compared in terms of their performance in the context of array signal processing. More specifically they are applied in adaptive beamforming which is known to be sensitive to errors in the covariance matrix estimate and where often only a limited amount of data is available for estimation. As many covariance matrix estimators have the form of diagonal loading or eigenvalue adjustments of the sample covariance matrix and as they sometimes offer robustness to array imperfections and finite sample error, they are compared to a recent robustified adaptive Capon beamforming (RCB) method which also has a diagonal loading interpretation. Some of the covariance estimators show a significant improvement over the sample covariance matrix and in some cases they match the performance of the RCB even when a priori knowledge, which is not available in practice, is used for choosing the user parameter of RCB.
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| 2. |
- Axell, Erik, 1980-
(författare)
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Topics in Spectrum Sensing for Cognitive Radio
- 2009
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cognitive radio is a new concept of reusing licensed spectrum in an unlicensed manner. Cognitive radio is motivated by recent measurements of spectrum utilization, showing unused resources in frequency, time and space. The spectrum must be sensed to detect primary user signals, in order to allow cognitive radios in a primary system. In this thesis we study some topics in spectrum sensing for cognitive radio.The fundamental problem of spectrum sensing is to discriminate samples that contain only noise from samples that contain a very weak signal embedded in noise. We derive detectors that exploit known structures of the signal, for the cases of an OFDM modulated signal and an orthogonal space-time block coded signal. We derive optimal detectors, in the Neyman-Pearson sense, for a few different cases when all parameters are known. Moreover we study detection when the parameters, such as noise variance, are unknown. We propose solutions the problem of unknown parameters.We also study system aspects of cognitive radio. More specifically, we investigate spectrum reuse of geographical spectrum holes in a frequency planned primary network. System performance is measured in terms of the achievable rate for the cognitive radio system. Simulation results show that a substantial sum-rate could be achieved if the cognitive radios communicate over small distances. However, the spectrum hole gets saturated quite fast, due to interference caused by the cognitive radios.
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| 3. |
- Baldemair, Robert, et al.
(författare)
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Evolving Wireless Communications
- 2013
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Ingår i: IEEE Vehicular Technology Magazine. - : Institute of Electrical and Electronics Engineers (IEEE). - 1556-6072 .- 1556-6080. ; 8:1, s. 24-30
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Tidskriftsartikel (refereegranskat)abstract
- The wireless-access networks of today will have to evolve in several ways in order to address the challenges and expectations of the future. New technology components will be introduced as part of the evolution of current wireless-access technologies, such as high-speed packet access (HSPA) and long-term evolution (LTE). However, additional components may also constitute future new wireless-access technologies, which may complement the evolved technologies. Examples of such new technology components are new ways of accessing spectrum and substantially higher frequency ranges, the introduction of massive antenna configurations, direct device-to-device communication, and ultradense deployments.
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| 4. |
- Larsson, Erik G., et al.
(författare)
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Adaptive equalization for frequency-selective channels of unknown length
- 2004
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Ingår i: IEEE Global Telecommunications Conference, 2004. Vol. 2. - 0780387945 ; , s. 1076-1080
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Konferensbidrag (refereegranskat)abstract
- This paper presents a new method for adaptive equalization of communication channels with intersymbol interference, for which the impulse response has an unknown length. Our approach is based on a parameterization of the impulse response as a Gaussian mixture model, whose parameters are estimated from a training sequence. It is shown that the new method can outperform conventional adaptive equalizers using training to estimate the channel.
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| 5. |
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| 6. |
- Larsson, Erik G, et al.
(författare)
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Adaptive equalization for frequency-selective channels of unknown length
- 2005
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Ingår i: IEEE Transactions on Vehicular Technology. - 0018-9545 .- 1939-9359. ; 54:2, s. 568-579
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Tidskriftsartikel (refereegranskat)abstract
- This paper studies adaptive equalization for time-dispersive communication channels whose impulse responses have unknown lengths. This problem is important, because an adaptive equalizer designed for an incorrect channel length is suboptimal: it often estimates an unnecessarily large number of parameters. Some solutions to this problem exist (e.g., attempting to estimate the ``channel length'', and then switching between different equalizers); however, these are suboptimal owing to the difficulty of correctly identifying the channel length, and the risk associated with an incorrect estimation of this length. Indeed, to determine the channel length is effectively a model order selection problem, for which no optimal solution is known.We propose a novel, systematic approach to the problem under study, which circumvents the estimation of the channel length. The key idea is to model the channel impulse response via a mixture Gaussian model, which has one component for each possible channel length. The parameters of the mixture model are estimated from a received pilot sequence. We derive the optimal receiver associated with this mixture model, along with some computationally efficient approximations of it. We also devise a receiver, consisting of a bank of soft-output Viterbi algorithms (SOVAs), that can deliver soft decisions. Via numerical simulations, we show that our new method can significantly outperform conventional adaptive Viterbi equalizers that use a fixed or an estimated channel length.
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| 7. |
- Larsson, Erik G., et al.
(författare)
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Linear regression with a sparse parameter vector
- 2006
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Ingår i: Conference Record of the 31st International Conference on Acoustics, Speech, and Signal Processing (ICASSP). - 142440469X ; , s. 309-
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Konferensbidrag (refereegranskat)abstract
- We consider linear regression under a model where the parameter vector is known to be sparse. Using a Bayesian framework, we derive a computationally efficient approximation to the minimum mean-square error (MMSE) estimate of the parameter vector. The performance of the so-obtained estimate is illustrated via numerical examples.
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| 8. |
- Larsson, Erik G., et al.
(författare)
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Linear Regression With a Sparse Parameter Vector
- 2007
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Ingår i: IEEE Transactions on Signal Processing. - 1053-587X .- 1941-0476. ; 55:2, s. 451-460
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Tidskriftsartikel (refereegranskat)abstract
- We consider linear regression under a model where the parameter vector is known to be sparse. Using a Bayesian framework, we derive the minimum mean-square error (MMSE) estimate of the parameter vector, and a computationally efficient approximation of it. We also derive an empirical-Bayesian version of the estimator, which does not need any a priori information, nor does it need the selection of any user parameters. As a byproduct, we obtain a powerful model (``basis'') selection tool for sparse models. The performance and robustness of our new estimators are illustrated via numerical examples.
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| 9. |
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| 10. |
- Popovsk, Petar, et al.
(författare)
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EU FP7 INFSO-ICT-317669 METIS, D1.1 Scenarios, requirements and KPIs for 5G mobile and wireless system
- 2013
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Bok (övrigt vetenskapligt/konstnärligt)abstract
- This deliverable introduces generic scenarios based on fundamental challenges, and the specific problem description of test cases that will be relevant for beyond future radio access. Specific characteristics of each scenario and each test case include the key assumptions regarding requirements and key performance indicators. In order not to constrain the potential solutions, the requirements are specified from an end-user perspective. The deliverable will not only serve as the guideline for the technical work and system concept design in METIS, but also can serve in external research communities to help to harmonize the work towards the future radio access system including the new generation system of 5G
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