Convergence Analysis of Reweighted Sum-Product Algorithms

August, 2007
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Tanya Roosta, Martin J. Wainwright, and Shankar Sastry

Markov random fields are designed to represent structured dependencies among large collections of random variables, and are well-suited to capture the structure of real-world signals. Many fundamental tasks in signal processing (e.g., smoothing, denoising, segmentation etc.) require efficient methods for computing (approximate) marginal probabilities over subsets of nodes in the graph. The marginalization problem, though solvable in linear time for graphs without cycles, is computationally intractable for general graphs with cycles. This intractability motivates the use of approximate ``message-passing'' algorithms. This paper studies the convergence and stability properties of the family of \emph{reweighted sum-product algorithms}, a generalization of the widely-used sum-product or belief propagation algorithm, in which messages are adjusted with graph-dependent weights. For homogeneous models, we provide a complete characterization of the potential settings and message weightings that guarantee uniqueness of fixed points, and convergence of the updates. For more general inhomogeneous models, we derive a set of sufficient conditions that ensure convergence, and provide bounds on convergence rates. The experimental simulations on various classes of graphs validate our theoretical results.

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