IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 62, NO. 1, JANUARY 2013 341
Sequential Particle-Based Sum-Product Algorithm
for Distributed Inference in Wireless
Sensor Networks
Wei Li, Zhen Yang, and Haifeng Hu
Abstract—Graphical models have been widely applied in solv-
ing distributed inference problems in wireless sensor networks
(WSNs). In this paper, the factor graph (FG) is employed to model
a distributed inference problem. Using particle filtering methods, a
sequential particle-based sum-product algorithm (SPSPA) is pro-
posed for distributed inference in FGs with continuous variables
and nonlinear local functions. Importance sampling methods are
used to sample from message products, and the computational
complexity of SPSPA is thus linear in the number of particles.
The SPSPA is applied to a distributed tracking problem, and its
performance is evaluated based on the number of particles and
the measurement noise.
Index Terms—Distributed inference, factor graph (FG), particle
filtering, sum-product algorithm (SPA), target tracking.
I. INTRODUCTION
I
T IS widely recognized that distributed inference methods
developed for graphical models comprise a principled ap-
proach to information fusion in wireless sensor networks
(WSNs) [1]. With graphical inference tools, the s imilarity
between WSNs and graphical models compels researchers to
model sensor network problems using graphical models so that
the graph-based inference algorithms, including belief propaga-
tion (BP) [2], sum-product algorithm (SPA) [3], and variational
algorithms [4], can be applied to WSNs.
Manuscript received February 3, 2012; revised May 16, 2012, August 15,
2012 and September 19, 2012; accepted September 20, 2012. Date of publi-
cation October 2, 2012; date of current version January 14, 2013. This work
was supported in part by the National Basic Research Program of China
(973 Program) under Grant 2011CB302903; by the National Natural Science
Foundation of China under Grant 60971129, Grant 61271335, and Grant
61071092; by the Fundamental Research Funds for the Central Universities
under Grant 2009B31714; by the Scientific Innovation Research Program of
College Graduate in Jiangsu Province under Grant CXZZ12_0473 and Grant
CXLX11_0408; by the China Postdoctoral Science Foundation under Grant
2012M511309; by the Postdoctoral Science Foundation of Jiangsu Province
under Grant 1101125C; by the Open Research Fund of National Mobile
Communications Research Laboratory of Southeast University under Grant
2011D04; and by a project from the Priority Academic Program Development
of Jiangsu Higher Education Institutions. The review of this paper was coordi-
nated by Dr. G. Mao.
W. Li is with the Key Laboratory of Broadband Wireless Communication
and Sensor Network Technology, Ministry of Education, Nanjing University of
Posts and Telecommunications, Nanjing 210003, China, and also with the Col-
lege of Computer and Information Engineering, Hohai University, Changzhou
213022, China (e-mail: liw@hhuc.edu.cn).
Z. Yang and H. Hu are with the Key Laboratory of Broadband Wireless Com-
munication and Sensor Network Technology, Ministry of Education, Nanjing
University of Posts and Telecommunications, Nanjing 210003, China (e-mail:
yangz@njupt.edu.cn; huhf@njupt.edu.cn).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TVT.2012.2221484
A wide range of distributed inference problems has been
reformulated with graphical models, including self-localization
[5], multitarget tracking [6], and nonlinear distributed estima-
tion [7]. However, in many distributed inference problems,
graphical models often involve continuous variables and non-
linear relations between the variables. The corresponding in-
ference algorithms usually involve integral equations that are
analytically intractable.
Particle filters (PFs) [8], [9] have proven to be an effective
solution to nonlinear/non-Gaussian problems. They form the
basis for many Bayesian tracking algorithms [10]. PFs use
Monte Carlo methods [11] to represent the posterior probability
density function (pdf) by a set of random samples with associ-
ated weight and compute estimates based on these samples and
weights. Although PFs are often effective, they are specialized
to temporal inference problems whose corresponding graphs
are Markov chains. For most of the distributed inference prob-
lems in WSNs, which are modeled by more complex graphical
models, PFs appear to be not suitable to these problems.
The nonparametric BP (NBP) algorithm [12] effectively
extends PFs to general graphs by combining the information
from the neighboring nodes in graphical models. NBP asso-
ciates a regularizing kernel with each particle and computes the
message products using a local Gibbs sampling procedure. This
algorithm has been widely used in many distributed inference
problems [13], [14]. However, the use of NBP is restricted to
graphical models with pairwise local functions.
Factor graphs (FGs) [3] provide a powerful general frame-
work for developing statistical models of distributed inference
problems. Once the stochastic dependence between the vari-
ables is modeled by an FG, the SPA can be used to infer or
estimate knowledge about the unknown variables. Based on
the FG and the SPA, [15] developed a nonparametric solution
for the general nonlinear and non-Gaussian inference problems
using the Monte Carlo method. The message-passing algorithm
(MPA) proposed in [15] is superior to the NBP in the sense
that it assumes no limitations on the type of relations between
the variables. Nevertheless, similar to NBP, the Gibbs sampler
used in [15] leads to high computational complexity, which is
quadratic in the number of particles.
This paper presents a generic framework for performing
distributed inference in WSNs. An FG is employed to model
the distributed inference problems. Based on PFs, a sequen-
tial particle-based SPA (SPSPA) is proposed for the general
nonlinear/non-Gaussian inference problems in FGs. In the pro-
posed algorithm, the importance sampling methods are used to
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