| 31. |
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| 32. |
- Johansson, Björn, 1978-
(author)
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On Distributed Optimization in Networked Systems
- 2008
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Doctoral thesis (other academic/artistic)abstract
- Numerous control and decision problems in networked systems can be posed as optimization problems. Examples include the framework of network utility maximization for resource allocation in communication networks, multi-agent coordination in robotics, and collaborative estimation in wireless sensor networks (WSNs). In contrast to classical distributed optimization, which focuses on improving computational efficiency and scalability, these new applications require simple mechanisms that can operate under limited communication. In this thesis, we develop several novel mechanisms for distributed optimization under communication constraints, and apply these to several challenging engineering problems. In particular, we devise three tailored optimization algorithms relying only on nearest neighbor, also known as peer-to-peer, communication. Two of the algorithms are designed to minimize a non-smooth convex additive objective function, in which each term corresponds to a node in a network. The first method is an extension of the randomized incremental subgradient method where the update order is given by a random walk on the underlying communication graph, resulting in a randomized peer-to-peer algorithm with guaranteed convergence properties. The second method combines local subgradient iterations with consensus steps to average local update directions. The resulting optimization method can be executed in a peer-to-peer fashion and analyzed using epsilon-subgradient methods. The third algorithm is a center-free algorithm, which solves a non-smooth resource allocation problem with a separable additive convex objective function subject to a constant sum constraint. Then we consider cross-layer optimization of communication networks, and demonstrate how optimization techniques allow us to engineer protocols that mimic the operation of distributed optimization algorithms to obtain an optimal resource allocation. We describe a novel use of decomposition methods for cross-layer optimization, and present a flowchart that can be used to categorize and visualize a large part of the current literature on this topic. In addition, we devise protocols that optimize the resource allocation in frequency-division multiple access (FDMA) networks and spatial reuse time-division multiple access (TDMA) networks, respectively. Next we investigate some variants of the consensus problem for multi-robot coordination, for which it is usually standard to assume that agents should meet at the barycenter of the initial states. We propose a negotiation strategy to find an optimal meeting point in the sense that the agents' trajectories to the meeting point minimize a quadratic cost criterion. Furthermore, we also demonstrate how an augmented state vector can be used to boost the convergence rate of the standard linear distributed averaging iterations, and we present necessary and sufficient convergence conditions for a general version of these iterations. Finally, we devise a generic optimization software component for WSNs. To this end, we implement some of the most promising optimization algorithms developed by ourselves and others in our WSN testbed, and present experimental results, which show that the proposed algorithms work surprisingly well.
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| 33. |
- Johansson, Björn, et al.
(author)
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On Distributed Optimization using Peer-to-Peer Communications in Wireless Sensor Networks
- 2008
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In: Proceedings of IEEE SECON. ; , s. 497-505
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Conference paper (peer-reviewed)abstract
- We describe and evaluate a suite of distributed and computationally efficient algorithms for solving a class of convex optimization problems in wireless sensor networks. The problem class has wide applications in estimation, detection, localization, coordination and resource sharing. We focus on peer-to-peer algorithms where nodes only exchange data with their immediate neighbors, and consider three distinct alternatives: a dual-based broadcast algorithm, a novel stochastic unicast algorithm, and a linear broadcast algorithm tailored for least-squares problems. We implement the algorithms in the network simulator NS2 and present extensive simulation results for random topologies.
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| 34. |
- Johansson, Björn, et al.
(author)
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Primal and Dual Approaches to Distributed Cross-layer Optimization
- 2005
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In: IFAC Proceedings Volumes (IFAC-PapersOnline). - : Elsevier BV. - 1474-6670. ; , s. 113-118
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Conference paper (peer-reviewed)abstract
- Several approaches for cross-layer design, e.g., coordinating the traditionally separated layers in wireless networks, have been proposed. However, protocols that are close to achieving the performance bounds are still lacking. We propose three distributed algorithms for joint congestion control and resource allocation in networks with variable capacities subject to a global resource constraint. Examples include spectrum assignment in wireless networks and wavelength allocation in optical networks. For scalability, we impose the additional constraint that nodes can only negotiate and exchange resources with their neighbors. The proposed algorithms consist of two complementary approaches based on decomposition techniques, in which congestion control and resource allocations are performed on different time-scales. Two of the algorithms can be shown to converge without network delays. Copyright 2005 IFAC
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| 35. |
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| 36. |
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| 37. |
- Johansson, Jonny, et al.
(author)
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Project: EISCAT 3D
- 2007
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Other publication (pop. science, debate, etc.)
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| 38. |
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| 39. |
- Johansson, Linda, et al.
(author)
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Temperature evaluation of soft and hard PZT transducers for ultrasonic
- 2005
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In: Proceedings of µTAS 2005 Conference. ; 2, s. 1428-1430
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Conference paper (peer-reviewed)abstract
- This paper reports a comparison of soft and hard piezoceramic transducer materials used for ultrasonic particle trapping in a microfluidic bioanalytical platform. The investigation is made with the objective to obtain high acoustic forces with a minimum of temperature increase. Themperature is a critical parameter for bioassays and most often need to be kept below a certain level to allow handling of e.g. temperature sensitive proteins. The main conclusion in this paper is that it is possible to get efficieint trapping with a temperature increase of only a few degrees using a hard type III transducer material.
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| 40. |
- Johansson, Sara, 1980-, et al.
(author)
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Interactive Exploration of Ingredient Mixtures Using Multiple Coordinated Views
- 2009
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In: Proceedings Information Visualization, IV 2009. - Los Alamitos, CA, USA : IEEE Computer Society. - 9780769537337 ; , s. 210-218
-
Conference paper (peer-reviewed)abstract
- The complex nature of multivariate data sets calls forhigh interactive performance and intuitive metaphors. Aspecific type of multivariate data is where the variables sum up to a constant, here defined as multicomponent data.This application paper presents an interactive applicationfor analysis of modelled multicomponent data. The aim isto find high performance variable combinations that fulfil some requested properties. The application is basedon coordinated views that include parallel coordinates, a ternary diagram, a 2D scatter plot and a line plot. It supports numerous interaction techniques enabling fast analysisof complex patterns in multicomponent data sets. The application is developed in collaboration with researchers within the fields of statistics and chemistry. An informal usability evaluation indicates that the interactive nature ofthe application clearly facilitates the analysis process.
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