IEEE SIGNAL PROCESSING LETTERS, VOL. 22, NO. 9, SEPTEMBER 2015 1383
Adaptive Integral Operators for Signal Separation
Xiyuan Hu, Silong Peng, and Wen-Liang Hwang, Senior Member, IEEE
Abstract—The operator-based signal separation approach uses
an adaptive operator to separate a signal into a set of additive sub-
components. In this paper, we show that differential operators and
their initial and boundary values can be exploited to derive cor-
responding integral operators. Although the differential operators
and the integral operators have the same null space, the latter are
more robust to noisy signals. Moreover, after expanding the ker-
nels of Frequency Modulated (FM) signals via eigen-decomposi-
tion, the operator-based approach with the integral operator can
be regarded as the matched filter approach that uses eigen-func-
tions as the matched filters. We then incorporate the integral op-
erator into the Null Space Pursuit (NSP) algorithm to estimate the
kernel and extract the subcomponent of a signal. To demonstrate
the robustness and efficacy of the proposed algorithm, we com-
pare it with several state-of-the-art approaches in separating mul-
tiple-component synthesized signals and real-life signals.
Index Terms—Integral equation, narrow band signal, null space
pursuit (NSP), operator-based.
I. INTRODUCTION
I
N RECENT years, several approaches [1]–[10] have
been proposed to separate a single-channel signal into
a mixture of several additive coherent subcomponents. The
method used to separate signals depends on the definition of the
subcomponents. For example, in the empirical mode decom-
position (EMD) approach [2], [10]–[12], the subcomponents
are Intrinsic Mode Functions (IMFs); in the Synchrosqueezed
Wavelet Transform (SWT) approach, the subcomponents are
IntrinsicModeTypeFunctions(IMT)[7],[8],[13];andinthe
operator-based approach [5], [6], a subcomponent is defined as
being in the null space of an operator, which is characterized
by some parameters that are estimated from the input (residual)
signal.
To improve the robustness and efficacy of the operator-based
approach, the Null Space Pursuit (NSP) algorithm was proposed
[6]. It separates a signal
into and such that is
Manuscript received April 07, 2014; revised August 10, 2014; accepted Au-
gust 22, 2014. Date of publication August 26, 2014; date of current version
February 27, 2015. This work was supported by the Natural Science Foundation
of China under Grants 61032007 and 61201375, and by the China Scholarship
Council under Grant 201304910129. The associate editor coordinating the re-
view of this manuscript and approving it for publication was Prof. Alexander
M. Powell.
X. Hu and S. Peng are with the High Technology Innovation Center (HITIC),
Institute of Automation, Chinese Academy of Sciences, Beijing, 100190,
China(e-mail: xiyuan.hu@ia.ac.cn,silong.peng@ia.ac.cn).
W.-L. Hwang is with the Institute of Information Science, Academia Sinica,
Taipei 11529, Taiwan (e-mail: whwang@iis.sinica.edu.tw)
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/LSP.2014.2352340
in the null space of an operator by minimizing the following
problem:
(1)
The first and second terms of Eq. (1) are the same as in the
operator-based approach. The parameter
in the third term of
Eq. (1) determines the amount of
to be retained in the null
space of
. The last term is the Lagrange term for the param-
eters of the operator
. Based on some assumptions, the NSP
algorithm can adaptively estimate the parameters
and and
derive the optimal solution of Eq. (1) [6].
An attractive feature of the operator-based approach is that
the operator design can be customized based on the characteris-
tics of the signal’s subcomponents. Let
be a subcomponent.
Then, any operator
with (i.e., is in the null
space of the operator) can be used in the proposed approach to
“annihilate” the subcomponent signal. For instance, to annihi-
late a frequency modulated (FM) signal
,where is
a local linear function, we can use the operator
(asdefinedin[6]).Here, is the instanta-
neous frequency (IF) of the signal. In addition, the operator
described in [14] can be used to
annihilate an amplitude modulated and frequency modulated
(AM-FM) signal
, with the parameters
( is the instantaneous bandwidth (IB))
and
( istheIF).In[5],the
general form of a differential operator is defined as
(2)
where
is a square summable sequence. For a mixture
of narrow band signals, FM or AM-FM, the proposed differ-
ential operators can separate each subcomponent successfully;
however, in some instances, particularly low SNR scenarios, the
differential operators tend to amplify the noise component when
estimating the parameters of the operator.
Also, a kind of integral operator using a simple local meanas
has been proposed in [5]. However, this
kind of integral operator can only annihilate the type of narrow
band signals that has only one frequency or a narrow range of
frequencies varying as a function of time, as defined in [15].
Therefore, we propose the following general form for an integral
operator:
(3)
where
is the parameterized integral kernel and is the
integral interval at time
.Asignal is in the null space of the
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