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Communications Letters
1
Anti-Eavesdropping Scheme Based on Quadrature
Spatial Modulation
Zhijie Huang, Zhenzhen Gao, Li Sun
Abstract—To resist the passive eavesdropping while reserving
the same hardware cost and receiver structure of the legitimate
receiver, a secure quadrature spatial modulation (QSM) scheme
is proposed in this letter. Instead of the information symbol,
a mixture of the information symbol and artificial noise is
transmitted through the activated transmit antennas. The mixed
signals are designed so that the artificial noise is cancelled at
the legitimate receiver but cause random interference to the
eavesdropper. The secrecy rate is analyzed and the optimal power
allocation between the artificial noise and the information signal
is investigated. The influence of the number of the eavesdropper’s
antennas is discussed. Simulation results show that, for multiple-
input-single-output systems, the proposed secure QSM scheme
achieves the best secrecy performance among the existing similar
schemes by using less radio frequency chains. For multiple-input-
multiple-output systems, the secure QSM scheme performs better
than the existing secure SM scheme does under the same power
constraint and transmission rate.
Index Terms—Quadrature spatial modulation, physical layer
security, artificial noise, secrecy rate.
I. INTRODUCTION
Due to the requirement of inter-antenna synchronization and
the maintenance of multiple radio-frequency chains, multiple-
antenna operations face challenges of complexity and the cost
of hardware. To deal with these issues, spatial modulation
(SM) has been proposed [1], [2]. The system’s performance
is improved by SM with reducing receiver complexity and
single radio frequency (RF) chain [1]. Due to the advantages
mentioned above, the application of SM will be very exten-
sive in wireless communication, such as visible light, green
communication, and the distributed wireless network.
The majority of the existing works focus on some aspects
of the SM systems such as reliability, efficiency, or robust
design [3]–[5]. Due to the open wireless medium of wire-
less networks, security issue is a critical concern. Although
cryptographic algorithms in the upper layers can handle the
security problem, there are difficulties and vulnerabilities in
secret key distribution and management in wireless communi-
cations. Therefore, physical layer (PHY) security techniques
have been used to further enhance the secrecy in wireless
Manuscript received July 1, 2016; revised August 8, 2016; accepted Novem-
ber 24, 2016. The work was supported in part by the National Natural Science
Foundation of China under Grant No. 61302067 and 61431011, Natural
Science Basic Research Plan in Shaanxi Province of China under Grant
No. 2016JQ6028, and Specialized Research Fund for the Doctoral Program
of Higher Education under Grant No.20130201120020. The associate editor
coordinating the review of this manuscript and approving it for publication
was Prof. Ertugrul Basar. (Corresponding author: Zhenzhen Gao.)
The authors are with the Department of Information and Communications
Engineering, Xi’an Jiaotong University, Shaanxi 710049 P. R. China (e-mail:
huang.jason@stu.xjtu.edu.cn; {zhenzhen.gao, lisun}@mail.xjtu.edu.cn).
communications [6]–[8]. In [9], secrecy mutual information is
analyzed for SM based multiple-input-single-output (MISO)
systems, and a precoder is designed when the eavesdropper’s
channel state information (CSI) is known. However in practice,
it is difficult for the legitimate nodes to acquire the eaves-
dropper’s CSI if the eavesdropper is passive. A precoding
aided SM (PSM) scheme for multiple-input-multiple-output
(MIMO) systems is proposed in [10] where information is
conveyed partially by the indices of receive antennas. An
active eavesdropper is assumed in [11] so that the CSI is
used by the legitimate transmitter to design the precoder for
secrecy. When the eavesdropper’s CSI is unavailable, artificial
noise (AN) is widely exploited to improve the secrecy [7],
[12]–[14]. The authors of [14] generalize the PSM to a
multiuser downlink scenario. With precoding vectors and the
AN elaborately designed, the scheme in [14] has the capability
to resist a multiple antenna eavesdropper. In [12], the PSM
scheme is generalized to a secret PSM (SPSM) scheme. A
time-varying precoder is designed for the SPSM so that it
can retain all the PSMs advantages at the desired receiver
while producing time varying interference to the eavesdropper.
A secure SM scheme which scatters AN isotropically in the
null space of the legitimate channel is proposed in [13]. Extra
activated transmit antennas are required to transmit AN by the
secure SM scheme [13].
The goal of this letter is to design a PHY secure scheme to
resist eavesdropping from an unknown eavesdropper without
extra hardware cost. Quadrature spatial modulation (QSM)
[15], a promising variant of SM that exploits index modulation
more effectively, has been proved to have a good performance
in point-to-point MIMO systems [15] and the cooperative
systems [16]. In this letter, QSM is exploited to design the anti-
eavesdropping scheme. By taking advantage of the structure
of QSM, mixed signals which include the information symbol
and AN are designed so that no extra antenna needs to be
activated. The designed AN can be cancelled at the legitimate
receiver so that no changes need to be made to the legitimate
receiver. Due to the independent fading of the legitimate link
and eavesdropping link, the eavesdropper will suffer from the
random interference of AN. Furthermore, we analyze the se-
crecy rate of the proposed scheme and investigate the optimal
power allocation between the information signal and AN. The
influence of the number of the eavesdropper’s antennas is also
discussed.
Notation: ⌊x⌋
2
means the closest power of 2 less than or
equal to x, E(·) denotes the expectation operation. ℜ(x) and
ℑ(x) stand for the real part and the imaginary part of x,
C
m×n
represents the complex space of m ×n dimensions. I
K