| 1. |
- Borst, Sem, et al.
(författare)
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Interacting queues with server selection and coordinated scheduling - Application to cellular data networks
- 2009
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Ingår i: Annals of Operations Research. - : Springer Science and Business Media LLC. - 0254-5330 .- 1572-9338. ; 170:1, s. 59-78
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Tidskriftsartikel (refereegranskat)abstract
- We consider a system of parallel servers handling users of various classes, whose service rates depend not only on user classes, but also on the set of active servers. We investigate the stability under two types of allocation strategies: (i) server assignment where the users are assigned to servers based on rates, load, and other considerations, and (ii) coordinated scheduling where the activity states of servers are coordinated. We show how the model may be applied to evaluate the downlink capacity of wireless data networks. Specifically, we examine the potential gains in wireless capacity from the two types of resource allocation strategies.
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| 2. |
- Broström, Peter, et al.
(författare)
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Multiobjective design of survivable IP networks
- 2006
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Ingår i: Annals of Operations Research. - : Springer Science and Business Media LLC. - 0254-5330 .- 1572-9338. ; 147:1, s. 235-253
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Tidskriftsartikel (refereegranskat)abstract
- Modern communication networks often use Internet Protocol routing and the intra-domain protocol OSPF (Open Shortest Path First). The routers in such a network calculate the shortest path to each destination and send the traffic on these paths, using load balancing. The issue of survivability, i.e. the question of how much traffic the network will be able to accommodate if components fail, is increasingly important. We consider the problem of designing a survivable IP network, which also requires determining the routing of the traffic. This is done by choosing the weights used for the shortest path calculations.
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| 3. |
- Carlsson, Fredrik, et al.
(författare)
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Using eigenstructure of the Hessian to reduce the dimension of the intensity modulated radiation therapy optimization problem
- 2006
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Ingår i: Annals of Operations Research. - : Springer Science and Business Media LLC. - 0254-5330 .- 1572-9338. ; 148:1, s. 81-94
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Tidskriftsartikel (refereegranskat)abstract
- Optimization is of vital importance when performing intensity modulated radiation therapy to treat cancer tumors. The optimization problem is typically large-scale with a nonlinear objective function and bounds on the variables, and we solve it using a quasi-Newton sequential quadratic programming method. This study investigates the effect on the optimal solution, and hence treatment outcome, when solving an approximate optimization problem of lower dimension. Through a spectral decompostion, eigenvectors and eigenvalues of an approximation to the Hessian are computed. An approximate optimization problem of reduced dimension is formulated by introducing eigenvector weights as optimization parameters, where only eigenvectors corresponding to large eigenvalues are included. The approach is evaluated on a clinical prostate case. Compared to bixel weight optimization, eigenvector weight optimization with few parameters results in faster initial decline in the objective function, but with inferior final solution. Another approach, which combines eigenvector weights and bixel weights as variables, gives lower final objective values than what bixel weight optimization does. However, this advantage comes at the expense of the pre-computational time for the spectral decomposition.
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| 4. |
- Dam, Hai Huyen, et al.
(författare)
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Spreading Code Design Using a Global Optimization Method
- 2005
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Ingår i: Annals of Operations Research. - : Springer. - 0254-5330 .- 1572-9338. ; 133:1-4, s. 249-264
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Tidskriftsartikel (refereegranskat)abstract
- The performance of a code division multiple access system depends on the correlation properties of the employed spreading code. Low cross-correlation values between spreading sequences are desired to suppress multiple access interference and to improve bit error performance. An auto-correlation function with a distinct peak enables proper synchronization and suppresses intersymbol interference. However, these requirements contradict each other and a trade-off needs to be established. In this paper, a global two dimensional optimization method is proposed to minimize the out-of-phase average mean-square aperiodic auto-correlation with average mean-square aperiodic cross-correlation being allowed to lie within a fixed region. This approach is applied to design sets of complex spreading sequences. A design example is presented to illustrate the relation between various correlation characteristics. The correlations of the obtained sets are compared with correlations of other known sequences.
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| 5. |
- Eklund, Patrik, 1958-, et al.
(författare)
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A consensus model of political decision-making
- 2008
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Ingår i: Annals of Operations Research. - : Springer. - 0254-5330 .- 1572-9338. ; 158:1, s. 5-20
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- In this paper, a model of political consensus is introduced. Parties try to reach consensus in forming a government. A government is defined as a pair consisting of a winning coalition and a policy supported by this coalition, where a policy consists of policies on given issues. A party evaluates all governments the party belongs to with respect to some criteria. We allow the criteria to be of unequal importance to a party. These criteria concern winning coalitions and policy issues. Parties may be advised to adjust their preferences, i.e., to change their evaluation concerning some government(s) or/and the importance of the criteria, in order to obtain a better political consensus.
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| 6. |
- Gabteni, S., et al.
(författare)
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Combining column generation and constraint programming to solve the tail assignment problem
- 2009
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Ingår i: Annals of Operations Research. - : Springer Science and Business Media LLC. - 1572-9338 .- 0254-5330. ; 171:1, s. 61-76
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Tidskriftsartikel (refereegranskat)abstract
- Within the area of short term airline operational planning, Tail Assignment is the problem of assigning flight legs to individual identified aircraft while satisfying all operational constraints, and optimizing some objective function. In this article, we propose that Tail Assignment should be solved as part of both the short and the long term airline planning. We further present a hybrid column generation and constraint programming solution approach. This approach can be used to quickly produce solutions for operations management, and also to produce close-to-optimal solutions for long and mid term planning scenarios. We present computational results which illustrate the practical usefulness of the approach.
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| 7. |
- Lindström, Erik
(författare)
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Estimating parameters in diffusion processes using an approximate maximum likelihood approach
- 2007
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Ingår i: Annals of Operations Research. - : Springer Science and Business Media LLC. - 1572-9338 .- 0254-5330. ; 151:1, s. 269-288
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Tidskriftsartikel (refereegranskat)abstract
- We present an approximate Maximum Likelihood estimator for univariate Ito stochastic differential equations driven by Brownian motion, based on numerical calculation of the likelihood function. The transition probability density of a stochastic differential equation is given by the Kolmogorov forward equation, known as the Fokker-Planck equation. This partial differential equation can only be solved analytically for a limited number of models, which is the reason for applying numerical methods based on higher order finite differences. The approximate likelihood converges to the true likelihood, both theoretically and in our simulations, implying that the estimator has many nice properties. The estimator is evaluated on simulated data from the Cox-Ingersoll-Ross model and a non-linear extension of the Chan-Karolyi-Longstaff-Sanders model. The estimates are similar to the Maximum Likelihood estimates when these can be calculated and converge to the true Maximum Likelihood estimates as the accuracy of the numerical scheme is increased. The estimator is also compared to two benchmarks; a simulation-based estimator and a Crank-Nicholson scheme applied to the Fokker-Planck equation, and the proposed estimator is still competitive.
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