IEEE COMMUNICATIONS LETTERS, VOL. 21, NO. 6, JUNE 2017 1329
RSS-Based Localization in WSNs Using Gaussian Mixture
Model via Semidefinite Relaxation
Yueyue Zhang, Student Member, IEEE, Song Xing, Member, IEEE, Yaping Zhu, Student Member, IEEE,
Feng Yan, Member, IEEE, and Lianfeng Shen, Senior Member, IEEE
Abstract—Energy-based source localization methods are
normally developed according to the channel path-loss models
in which the noise is generally assumed to follow Gaussian
distributions. In this letter, we represent the practical additive
noise by the Gaussian mixture model, and develop a localization
algorithm based on the received signal strength to achieve a max-
imum likelihood location estimator. By using Jensen’s inequality
and semidefinite relaxation, the initially proposed nonlinear
and nonconvex estimator is relaxed into a convex optimization
problem, which can be efficiently solved to obtain the glob-
ally optimal solution. Besides, the corresponding Cramer–Rao
lower bound is derived for performance comparison. Simulation
and experimental results show a substantial performance gain
achieved by our proposed localization algorithm in wireless
sensor networks.
Index Terms— Received signal strength (RSS), Gaussian
mixture model (GMM), semidefinite relaxation.
I. INTRODUCTION
W
IRELESS source localization has attracted
considerable attentions over the past decades. Among
different localization methods, energy-based localization
via received signal strength (RSS) or received signal
strength difference (RSSD) enables a simple implementation
compared to other conventional technologies such as time
of arrival (TOA) [1], time difference of arrival (TDOA) [2],
and angle of arrival (AOA) [3]. Recent progress has made it
practical to realize the energy-based localization in various
networks including wireless sensor networks (WSNs) [4],
wireless local area network (WLAN) [5], and vehicular
ad-hoc networks (VANETs).
Nevertheless, RSS-based localization to achieve maximum
likelihood (ML) estimator of the coordinates of target nodes
leads to a nonlinear and nonconvex optimization problem.
Several methods have been proposed to address this issue.
In [6], for example, a weighed least squares (WLS) formu-
lation has been derived to jointly estimate the sensor node
location and the transmit power, based on the unscented trans-
formation (UT). In [4], a proposed estimator approximates the
Manuscript received January 10, 2017; accepted January 30, 2017. Date
of publication February 8, 2017; date of current version June 8, 2017.
This work was supported by the National Natural Science Foundation of
China (No. 61471164, 61601122), the Innovation Project of Jiangsu Province
(KYLX16 0222), and the Research Fund of National Mobile Communications
Research Laboratory (NCRL), Southeast University (SEU) (No. 2016B02).
The associate editor coordinating the review of this letter and approving it for
publication was L. Mucchi. (Corresponding author: Lianfeng Shen.)
Y. Zhang, Y. Zhu, F. Yan, and L. Shen are with the National
Mobile Communications Research Laboratory, Southeast University,
Nanjing 210096, China (e-mail: douleyue@seu.edu.cn; xyzzyp@seu.edu.cn;
feng.yan@seu.edu.cn; lfshen@seu.edu.cn).
S. Xing is with the Department of Information Systems, California
State University, Los Angeles, CA 90032 USA (e-mail: sxing@exchange.
calstatela.edu).
Digital Object Identifier 10.1109/LCOMM.2017.2666157
ML estimate for low noise level first, and then is relaxed by
applying the efficient convex relaxations that are based on the
second-order cone programming (SOCP). And, a semidefinite
programming (SDP) estimator has been designed specifically
for the RSS-based localization problems [7].
Additionally, it is worth noting that traditional localiza-
tion algorithm designs in WSNs are based on the classical
channel path-loss model with the noise generally modeled by
Gaussian distribution. However, in reality the noise does not
always follow Gaussian distribution due to the heterogeneity
of multiple sources [8]. Hence, the conventional Gaussian
model cannot properly represents the measured noise, leading
to improper localization algorithms in WSNs. To the best of
our knowledge, no study has utilized a practical channel path-
loss model with non-Gaussian noise in the node localization
algorithms in WSNs.
Motivated by above observations, in this letter we proposed
an improved RSS-based node localization algorithm, called
Gaussian mixture-semidefinite programming (GM-SDP) esti-
mator, to achieve the ML estimation of the node positions
in WSNs. Specifically, the noise is represented by a non-
Gaussian model followed by an empirical parameter estimate
of the noise. Then we formulate the localization algorithm
as an optimization problem to achieve ML performance. The
subsequent nonlinear and nonconvex optimization problem is
solved by semidefinite relaxation to obtain the suboptimal
solution. Finally, both simulation and experimental results
illustrate the performance gain of our proposed GM-SDP
algorithm over the traditional localization algorithms.
Notation: In the paper, R
n
and S
n
denote the set of n vectors
and the n ×n symmetric matrix, respectively. In addition, for
any symmetric matrix A, A 0 means that A is positive
semidefinite. ·
1
, ·
2
and ·
∞
denote the
1
,
2
and
∞
vector norms, respectively.
II. S
YSTEM MODEL AND PRELIMINARIES
Let’s denote the unknown coordinates of the jth target
node as ϕ
ϕ
ϕ
j
=[ϕ
j1
,ϕ
j2
]
T
(ϕ
ϕ
ϕ
j
∈ R
2
, j = 1, ···, M)andthe
known coordinates of the ith anchor node as α
α
α
i
=[α
i1
,α
i2
]
T
(α
α
α
i
∈ R
2
, i = 1, ···, N), where M and N are the total
number of targets and anchors, respectively. From [4] and [7],
the power received at the jth target from the ith anchor (or
vice versa) is typically modeled as
P
i, j
= P
0
− 10β log
10
d(ϕ
ϕ
ϕ
j
,α
α
α
i
)
d
0
+ n
i, j
, (1)
where P
0
denotes the transmitted power at reference
distance d
0
from the receiver, d(ϕ
ϕ
ϕ
j
,α
α
α
i
) =ϕ
ϕ
ϕ
j
−α
α
α
i
2
is the
Euclidean distance between the jth target and the ith anchor
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