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Sökning: WFRF:(Johansson Mikael) > (2005-2009) > Johansson Mikael

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1.
  • Johansson, Björn, et al. (författare)
  • Distributed Model Predictive Consensus
  • 2006
  • Ingår i: Proceedings of the 17th International Symposium on Mathematical Theory of Networks and Systems. ; , s. 2438-2444
  • Konferensbidrag (refereegranskat)abstract
    • In this paper we consider the problem of designing a distributed control strategy such that a linear combination of the states of a number of vehicles coincide at a given time. The vehicles are described by linear difference equations and are subject to convex input constraints. It is demonstrated how primal decomposition techniques and incremental subgradient methods allow us to find a solution in which each vehicle performs individual planning of its trajectory and exchanges critical information with neighbors only. We explore various communication, computation, and control structures, and demonstrate the performance of the algorithms by numerical examples.
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2.
  • Johansson, Björn, et al. (författare)
  • On decentralized negotiation of optimal consensus
  • 2008
  • Ingår i: Automatica. - : Elsevier BV. - 0005-1098 .- 1873-2836. ; 44:4, s. 1175-1179
  • Tidskriftsartikel (refereegranskat)abstract
    • A consensus problem consists of finding a distributed control strategy that brings the state or output of a group of agents to a common value, a consensus point. In this paper, we propose a negotiation algorithm that computes an optimal consensus point for agents modeled as linear control systems subject to convex input constraints and linear state constraints. By primal decomposition and incremental subgradient methods, it is shown that the algorithm can be implemented such that each agent exchanges only a small amount of information per iteration with its neighbors.
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3.
  • Johansson, Björn, et al. (författare)
  • Subgradient methods and consensus algorithms for solving convex optimization problems
  • 2008
  • Ingår i: Decision and Control, 2008. CDC 2008. 47th IEEE Conference on. - : IEEE. - 9781424431236 ; , s. 4185-4190
  • Konferensbidrag (refereegranskat)abstract
    • In this paper we propose a subgradient method for solving coupled optimization problems in a distributed way given restrictions on the communication topology. The iterative procedure maintains local variables at each node and relies on local subgradient updates in combination with a consensus process. The local subgradient steps are applied simultaneously as opposed to the standard sequential or cyclic procedure. We study convergence properties of the proposed scheme using results from consensus theory and approximate subgradient methods. The framework is illustrated on an optimal distributed finite-time rendezvous problem.
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6.
  • Flärdh, Oscar, et al. (författare)
  • A Control Framework for Online Error Control Adaptation in Networked Applications
  • 2006
  • Ingår i: Proceedings of IEEE Second International Symposium on Control, Communications, and Signal Processing.
  • Konferensbidrag (refereegranskat)abstract
    • For many real-time applications running over packet-switched networks, it is important to maintain delivered data quality using a limited amount of network resources. It is therefore natural to employ cost functions that allow online trade-off between the experienced application quality and the resource usage. However, minimizing such cost functions requires perfect knowledge of the network state at the transmission side, while, in general, such information is only partially available. In this paper, we introduce a new adaptive error correction algorithm that optimizes the amount of redundancy based on the available information from the application and the network. An extremum-seeking control algorithm is employed to deal with the high level of uncertainty in the network models. The validity of our approach is illustrated in simulations with varying network loads and loss correlation.
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7.
  • Flärdh, Oscar, et al. (författare)
  • A new feedback control mechanism for error correction in packet-switched networks
  • 2005
  • Ingår i: 44th IEEE Conference on Decision and Control & European Control Conference. ; , s. 488-493
  • Konferensbidrag (refereegranskat)abstract
    • Error correction mechanisms enable control and other real-time applications to be executed over unreliable packet-switched networks. By adding carefully adjusted redundancy to transmitted data at the sender, it is possible to recover lost data at the receiver and thereby improve effective throughput. We describe simple models for packet loss, which allow us to find the optimal redundancy as a function of packet loss probability. Two feedforward control mechanisms based on the packet loss probability are presented: one that is computed off-line and another one using an on-line algorithm. A drawback with these approaches is their dependency on accurate network state information and precise loss models. To cope with these issues, we propose a new feedback solution that tracks the optimum using gradient estimation. Simulations in ns-2 illustrate the behavior of the error correction schemes, demonstrating that the feedback solution outperforms the feedforward solution in presence of model errors.
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8.
  • Flärdh, Oscar, et al. (författare)
  • Analysis of a simple feedback scheme for error correction over a lossy network
  • 2007
  • Ingår i: 2007 IEEE INTERNATIONAL CONFERENCE ON NETWORKING, SENSING, AND CONTROL. - : IEEE. - 9781424410750 ; , s. 261-266
  • Konferensbidrag (refereegranskat)abstract
    • A control theoretic analysis of a simple error correction scheme for lossy packet-switched networks is presented. Based on feedback information from the error correction process in the receiver, the sender adjusts the amount of redundancy using a so called extremum-seeking controller, which do not rely on any accurate model of the network loss process. The closed-loop system is shown to converge to a limit cycle in a neighborhood of the optimal redundancy. The result are validated using packet-based simulations with data from wireless sensor network experiments.
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9.
  • Flärdh, Oscar, 1980- (författare)
  • Modelling, analysis and experimentation of a simple feedback scheme for error correction control
  • 2007
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Data networks are an important part in an increasing number of applications with real-time and reliability requirements. To meet these demands a variety of approaches have been proposed. Forward error correction, which adds redundancy to the communicated data, is one of them. However, the redundancy occupies communication bandwidth, so it is desirable to control the amount of redundancy in order to achieve high reliability without adding excessive communication delay. The main contribution of the thesis is to formulate the problem of adjusting the redundancy in a control framework, which enables the dynamic properties of error correction control to be analyzed using control theory. The trade-off between application quality and resource usage is captured by introducing an optimal control problem. Its dependence on the knowledge of the network state at the transmission side is discussed. An error correction controller that optimizes the amount of redundancy without relying on network state information is presented. This is achieved by utilizing an extremum seeking control algorithm to optimize the cost function. Models with varying complexity of the resulting feedback system are presented and analyzed. Conditions for convergence are given. Multiple-input describing function analysis is used to examine periodic solutions. The results are illustrated through computer simulations and experiments on a wireless sensor network.
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10.
  • Henriksson, Roger, et al. (författare)
  • Brain Tumors - Prognostic and Predictive Markers
  • 2009
  • Ingår i: Histological and Serological Tumor Markers and Gene Expression and Their Clinical Usefulness in Cancers. - Hauppauge : Nova Science Publishers, Inc.. - 9781607413820 ; , s. 53-75
  • Bokkapitel (refereegranskat)abstract
    • This review summarizes the status of prognostic and predictive markers in brain tumors with a focus on the most frequent tumors, gliomas. Brain tumors are a heterogeneous group of different tumors with a huge variation in outcome. Although the most common tumor, high-grade malignant glioma, still has a dismal prognosis, the last years have seen a significant improvement in the management in this tumor as well as in most other brain tumors. Age, tumor grade and KPS are still the most reliable prognostic and predictive variables available for patients with brain tumors. Although chromosome 1p/19q co-deletion and methylation status of the promoter of the MGMT gene (encoding O6-methylguanine-DNA methyl transferase) have been identified as the most promising potential predictors of response to chemotherapy in malignant gliomas, there are as yet no reliable biomarkers for tumour grading or tumour monitoring in the clinical setting.
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