On Secure NOMA Systems with Transmit Antenna Selection Schemes

Abstract

This paper investigates the secrecy performance of a two-user downlink non-orthogonal multiple access systems. Both single-input and single-output and multiple-input and singleoutput systems with different transmit antenna selection (TAS) strategies are considered. Depending on whether the base station has the global channel state information of both the main and wiretap channels, the exact closed-form expressions for the secrecy outage probability (SOP) with suboptimal antenna selection and optimal antenna selection schemes are obtained and compared with the traditional space-time transmission scheme. To obtain further insights, the asymptotic analysis of the SOP in high average channel power gains regime is presented and it is found that the secrecy diversity order for all the TAS schemes with fixed power allocation is zero. Furthermore, an effective power allocation scheme is proposed to obtain the nonzero diversity order with all the TAS schemes. Monte-Carlo simulations are performed to verify the proposed analytical results.

Full text

Received July 5, 2017, accepted August 1, 2017, date of publication August 9, 2017, date of current version September 19, 2017.
Digital Object Identifier 10.1109/ACCESS.2017.2737330
On Secure NOMA Systems With Transmit
Antenna Selection Schemes
HONGJIANG LEI1,2, (Member, IEEE), JIANMING ZHANG1, KI-HONG PARK2, (Member, IEEE),
PENG XU1, IMRAN SHAFIQUE ANSARI3, (Member, IEEE), GAOFENG PAN4, (Member, IEEE),
BASEL ALOMAIR5, (Member, IEEE), AND MOHAMED-SLIM ALOUINI2, (Fellow, IEEE)
1Chongqing Key Lab of Mobile Communications Technology, Chongqing University of Posts and Telecommunications, Chongqing 400065, China
2Computer, Electrical, and Mathematical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900,
Saudi Arabia
3Department of Electrical and Computer Engineering, Texas A&M University at Qatar, Education City, Doha 23874, Qatar
4School of Computing and Communications, Lancaster University, Lancashire LA1 4WA, U.K.
5National Center for Cybersecurity Technology, King Abdulaziz City for Science and Technology, Riyadh 11442, Saudi Arabia
Corresponding author: Hongjiang Lei (leihj@cqupt.edu.cn)
This work was supported in part by the National Natural Science Foundation of China under Grant 61471076, in part by the Chinese
Scholarship Council under Grant 201607845004, in part by the Program for Changjiang Scholars and Innovative Research Team in
University under Grant IRT_16R72, in part by the special fund for the Key Lab of Chongqing Municipal Education Commission, in part by
the Project of Fundamental and Frontier Research Plan of Chongqing under Grant cstc2015jcyjBX0085 and Grant cstc2017jcyjAX0204,
in part by the Scientific and Technological Research Program of Chongqing Municipal Education Commission under Grant KJ1600413 and
Grant KJ1704088, and in part by the Qatar National Research Fund (a member of Qatar Foundation) under Grant NPRP7-125-2-061.
ABSTRACT This paper investigates the secrecy performance of a two-user downlink non-orthogonal
multiple access systems. Both single-input and single-output and multiple-input and single-output systems
with different transmit antenna selection (TAS) strategies are considered. Depending on whether the base
station has the global channel state information of both the main and wiretap channels, the exact closed-form
expressions for the secrecy outage probability (SOP) with suboptimal antenna selection and optimal antenna
selection schemes are obtained and compared with the traditional space-time transmission scheme. To obtain
further insights, the asymptotic analysis of the SOP in high average channel power gains regime is presented
and it is found that the secrecy diversity order for all the TAS schemes with fixed power allocation is zero.
Furthermore, an effective power allocation scheme is proposed to obtain the non-zero diversity order with
all the TAS schemes. Monte Carlo simulations are performed to verify the proposed analytical results.
INDEX TERMS Non-orthogonal multiple access, physical layer security, transmit antenna selection, secrecy
outage probability.
I. INTRODUCTION
A. BACKGROUND AND RELATED WORKS
Recently, non-orthogonal multiple access (NOMA) has been
accepted as a potential technology for the fifth genera-
tion (5G) mobile networks in face of the explosive growth of
the mobile traffic demands [1]–[4]. As opposed to the con-
ventional orthogonal multiple access (OMA) technologies
(e.g. time/frequency/code division multiple access), NOMA
can substantially improve the spectral efficiency by accom-
modating multiple users simultaneously via power domain
multiplexing. For example, in a downlink NOMA system that
consists of a near user (that has high channel gain), a far
user (that has low channel gain), and a base station. The
base station transmits signals to both users simultaneously
using superposition coding [5] and more transmission power
is allocated to the far user. Under NOMA scheme, the near
user decodes the signal to the far user first and then decodes
its signal after subtracting the decoded signal to the far user
by adopting successive interference cancellation (SIC). The
signal to the far user can be decoded without significant inter-
ference from the signal to the near user, which is weak. The
outage probability and ergodic capacity of NOMA system
were studied in [6] when the users are randomly distributed
in the vicinity of the base station, and confirmed that the
performance of NOMA was significantly superior to the tra-
ditional OMA when the power allocation scheme was used.
NOMA can also make the radio resources allocation more
flexible, as well as can improve the user’s fairness by employ-
ing appropriate resource allocation schemes [7]–[11]. As a
promising candidate for 5G wireless networks, the integration
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H. Lei et al.: On Secure NOMA Systems With TAS Schemes
of NOMA and conventional OMA paradigms also plays an
indispensable role in avoiding strong co-channel interference
caused by serving multiple users at the same time, frequency,
and spreading code [12], [13].
Recently, multiple-antenna technology has been uti-
lized in NOMA systems to improve the system
performance [14]-[18]. It was demonstrated that the sum
rate of a multiple-input multiple-output (MIMO) NOMA
system is strictly larger than that of a MIMO OMA system
in [15]. The outage performance of NOMA system with
a multiple-antenna energy harvesting relay was analyzed
in [16]. Although the performance can potentially scale up
with the number of antennas, the improvement comes at
the price of expensive RF chains at the terminal. To avoid
the high hardware costs while preserving the diversity and
throughput benefits from multiple antennas, transmit antenna
selection (TAS) technique has been recognized as an effec-
tive solution [16]–[18]. Due to the efficiency and flexibility,
NOMA can also be combined with many other wireless
technologies to enhance the system performance, such as
cooperative communication [19], [20], full duplex [21], cog-
nitive radio (CR) [22], millimeter wave [23], and visible light
communication [24], etc.
Physical layer security has taken one of the hottest spots
in both information security and wireless communications
as it can realize the secrecy communication by utilizing the
randomness and time-varying nature of the wireless chan-
nels without any encryption algorithm [25]. Zhang et al.
studied the security performance of single-input-single-
output (SISO) NOMA system, and confirmed that the secrecy
sum rate performance of NOMA outperforms the one of the
conventional OMA [26]. Qin et al. studied the physical layer
security of NOMA systems in large-scale networks wherein
both NOMA users and eavesdroppers are spatially deployed
at randomly location [27], and new exact and asymptotic
expressions for the secrecy outage probability (SOP) were
derived. Furthermore, the secrecy performance of multiple-
antenna NOMA with artificial noise was investigated and
the exact and asymptotic expressions for SOP were derived
in [28]. Depending on whether the base station has the global
channel state information (CSI) of both the main and wire-
tap channels, Zhu et al. proposed optimal antenna selec-
tion (OAS) and suboptimal antenna selection (SAS) schemes
to enhance the secrecy performance of a MIMO system
in [29], which was compared with the traditional space-
time transmission (STT) scheme. The closed-form expres-
sions for the exact and asymptotic SOP of an underlay
MIMO system were derived in [30]. The obtained results
showed that both SAS and OAS schemes can significantly
enhance the secrecy performance. Now we are wondering
about the secrecy performance of these TAS schemes in
NOMA system.
B. MOTIVATION AND CONTRIBUTIONS
Based on the open literature and to the best of the authors’
knowledge, it is still an open issue to study the secrecy
performance of multiple-input single-output (MISO) NOMA
systems with TAS schemes. The main contributions of our
work are listed as follows:
1) We investigate the secrecy outage performance of
MISO NOMA system consisting of one base station
with multiple antennas, two legitimate receivers, and
an eavesdropper. We investigate the secrecy outage
performance of OAS and SAS schemes for MISO
NOMA system and compare them with the STT
scheme. The closed-form expressions for the exact and
asymptotic SOP are derived. Moreover, the accuracy
of the analytical results are validated via Monte-Carlo
simulations. The results show that the SOP for the
far user with fixed power allocation scheme deteri-
orates as the transmit power surpasses some thresh-
old and then reaches a floor as the interference from
the near user increases while increasing the transmit
power.
2) To obtain further insights, the asymptotic analysis of
SOP is conducted and the secrecy diversity order for
different TAS schemes is derived when the average
signal-to-noise ratio (SNR) of the main channel tends
to infinity. The results show that the secrecy diversity
order for all the TAS schemes with fixed power alloca-
tion are zero.
3) Moreover, an effective power allocation scheme is pro-
posed to obtain non-zero diversity order under all the
TAS schemes. Simulations and numerical results are
given to validate the accuracy of all the analytical
results.
The rest of this paper is organized as follows. In Section II,
the two-user NOMA system model is described. The
performance analysis of SOP in a SISO NOMA sys-
tem is carried out in Section III. In Sections IV and V
the exact and asymptotic SOP in TAS-based MISO
NOMA system are investigated. An effective power allo-
cation method is proposed in Section VI and the secrecy
diversity order for all the cases with proposed power
allocation scheme is derived. Section VII presents and dis-
cusses the numerical results. Finally, we conclude the paper
in Section VIII.
II. SYSTEM MODEL
In this work, we consider a downlink NOMA system that
includes a base station (S), two legitimate users D1(the near
user) and D2(the far user), and an eavesdropper (E), as shown
in Fig. 1.1Both the legitimate and illegitimate receivers are
equipped with a single antenna. It is assumed that the fad-
ing coefficients between each antenna at Sand destinations
1Although only two users were considered in this works, the results can be
easily extended to the downlink NOMA system with N(N>2)users. For
example, the hybrid multiple access scheme proposed in [13] and [14], where
NOMA scheme is implemented among the users within each group, while
the conventional OMA scheme is utilized for inter-group multiple access.
In these scenarios, in order to decrease the complexity of message detection,
a user pairing technique can be used to ensure that only two users share a
specific orthogonal resource slot [28].
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FIGURE 1. System model consisting of a transmitting source (S) equipped
with multiple antennas, two legitimate receivers (D1and D2), and an
illegitimate eavesdropper (E).
(including D1,D2, and E) experience independent Rayleigh
fading.
Under NOMA scheme, the signal received at Dmcan be
expressed as
yDm=hDmpa1PSx1+pa2PSx2+nm,(1)
where m= {1,2},hDmdenotes the channel coefficient
between Sand Dm,amrepresents the power allocation coeffi-
cients, a1+a2=1, PSdenotes the transmit power at S, and xm
represents the messages to Dm.nmis complex additive white
Gaussian noise (AWGN) with zero mean and variance σ2
m,
For simplicity, we assume σ2
1=σ2
2=σ2.
In this work, it is assumed that users are not ordered by
their channel conditions.2Under NOMA scheme [6], D1can
detect x1by using SIC and x1will be regarded as interference
when D2decodes its received signal. Therefore, the received
instantaneous signal-to-interference-noise ratio (SINR) of the
m-th user can be given as [6]
γDm=






a1ρhD12,m=1
hD22a2
hD22a1+1
ρ
,m=2,(2)
where ρ=PSσ2sigifies the transmit SNR where PSis the
transmit power at the S.
In [27] and [28], the authors assumed that the eavesdropper
has enough powerful capabilities to detect multiuser data and
extracts the signal to Dmwhen it eavesdrops Dm. It is a
pessimistic assumption since the multi-user decoding ability
of Eis overestimated.
In this work, we assume Ecan eliminate
the signal to D2by SIC when it eavesdrops D1and
treats the message to D1as noise when it eavesdrops D2.3
2In these scenarios, users are categorized by their quality of service
requirements. Similar assumptions can be found in [32], and [33], in which
user 1 was assumed to be served opportunistically and user 2 was assumed
to be served for small packet transmission.
3For the wiretap scenarios with multiple users, it is assumed that the m−1
users’ messages have already been decoded before the eavesdropper tries to
decode the mth user’s message. Similar assumptions can be found in [26].
Obviously, this assumption overestimates the eavesdropper’s capability and
is pessimistic, then our results will be a lower bound of practical cases.
Thus, we have
γEm=




a1ρ|hE|2,m=1
a2|hE|2
a1|hE|2+1
ρ
,m=2,(3)
where hEis the channel coefficient and γEmis the SINR
when it eavesdrops the transmitted signal to receiver
Dm(m=1,2), respectively. The cumulative distribution
function (CDF) of γkm(k∈{D,E},m=1,2)can be
written as
Fγk1(γ)=1−e−γ
λk1a1ρ,(4)
Fγk2(γ)=




1−ϕλk2, γ , γ < a2
a1
1, γ ≥a2
a1
,(5)
respectively, where ϕ(ξ , x)=e−x
ξρ(a2−a1x),λkmrepresents the
average channel power gain. In this work, it is assumed that
λE1=λE2=λEfor simplification.
The probability density function (PDF) of
γkm(k∈{D,E},m=1,2)can be obtained as
fγk1(γ)=1
λk1a1ρe−γ
λk1a1ρ,(6)
fγk2(γ)=




a2
λk2ρ(a2−a1γ)2ϕλk2, γ , γ < a2
a1
0, γ ≥a2
a1
.
(7)
III. SECRECY OUTAGE PROBABILITY ANALYSIS
OF A SISO NOMA SYSTEM
In this section, we consider a two-user NOMA system with
a single-antenna base station, as motivated by the following
reasons: 1) preparing for the performance analysis of MISO
NOMA systems; 2) setting up a benchmark for comparing
the performance of MISO NOMA systems to testify the
enhancement befitted from using the multiple antennas at S.
The instantaneous secrecy capacity of Dm(m=1,2)can
be expressed as [25]
Cs,m=log21+γDm−log21+γEm+,(8)
where [x]+=max {x,0}.
SOP is defined as the probability that the secrecy capac-
ity is less than a preset target rate. Thus, the SOP at Dm
(m=1,2)can be expressed as
Pout,Dm=Pr Cs,m≤Rs,m
=Z∞
0
FγDm(2mγ+2m−1)fγEm(γ)dγ , (9)
where Rs,mdenotes the target rate at Dmand 2m=2Rs,m≥1.
Substituting (4) and (6) into (9), and using
[34, eq. (3.381.4)], the SOP at D1can be expressed as
Pout,D1=1−λD1
21λE+λD1
e−21−1
a1ρλD1.(10)
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H. Lei et al.: On Secure NOMA Systems With TAS Schemes
Observing (10), one can deduce that the SOP at D1will be
improved when the transmit SNR (ρ) increases. Furthermore,
there exists a floor at 21λE
21λE+λD1
for Pout,D1as ρ→ ∞ since
the average secrecy capacity will reach a ceiling, which is
testified in [35].
Lemma 1: the SOP at D2is expressed as
Pout,D2=1−a2$ π
2NλEρ
N
X
n=1p1−τn2
(a2−a1ϑn)2e−Bn,(11)
where Bn=22(1+ϑn)−1
λD2ρ(a2−a1(22(1+ϑn)−1)) +ϑn
λEρ(a2−a1ϑn),Nis the
number of terms, τn=cos 2n−1
2Nπand ϑn=$(τn+1)
2.
Proof: Substituting (5) and (7) into (9), we obtain
Pout,D2
=Z$
0
FγD2(22γ+22−1)fγE2(γ)dγ+Za2
a1
$
fγE2(γ)dγ
=1−Z$
0
ϕλD2, 22γ+22−1fγE2(γ)dγ
| {z }
1
=χ
,(12)
where $=1
a122−1≤a2
a1. To the best of the
authors’ knowledge, it is very difficult to obtain the closed-
form expression of χ. Using Gaussian-Chebyshev quadrature
[36, eq. (25.4.38)], χis approximated as
χ=a2
λEρZ$
0
1
(a2−a1γ)2
×e−22γ+22−1
λD2ρ(a2−a1(22−1)−a122γ)+γ
λEρ(a2−a1γ)dγ
=a2$ π
2NλEρ
N
X
n=1p1−τn2
(a2−a1ϑn)2e−Bn.(13)
Remark 1: We rewrite Pout,D2as
Pout,D2=1−
N
X
n=1
An
ρe−Bn
ρ,(14)
where An=a2$ π
2NλE
√1−τn2
(a2−a1ϑn)2>0, Bn>0.
We obtain
dPout,D2
dρ=
N
X
n=1
An
ρ2e−Bn
ρ1−Bn
ρ.(15)
One can find that dPout,D2
dρis negative within a certain
range (ρis lower), else dPout ,D2
dρis positive. That is to say that
Pout,D2will decrease within a certain range as ρincreases,
else it will increase in the other ranges. There is an optimal
ρthat can obtain the best SOP. This is the main difference
between Pout,D1and Pout ,D2. That means increasing the trans-
mit power may degrade the SOP of two-user NOMA system.
It is assumed that Eis interested only in a specific user’s
message, but the transmitter does not know which user E
wants to wiretap. Hence the SOP for the two-user NOMA
system can be expressed as4
Pout =Pr {D1secrecy outage or D2secrecy outage}
=Pr {D1secrecy outage, Eeavesdrops D1}
+Pr {D2secrecy outage, Eeavesdrops D2}
=Pr {D1secrecy outage |when Eeavesdrops D1}
×Pr {Eeavesdrops D1}
+Pr {D2secrecy outage |when Eeavesdrops D2}
×Pr {Eeavesdrops D2}
=Pout,D1×Pr {Eeavesdrops D1}
+Pout,D2×Pr {Eeavesdrops D2}.(16)
Pr {Eeavesdrop D1}and Pr {Eeavesdrop D2}can be
analyzed based on practical scenarios. To simplify the
analysis, it is assumed that Pr {Eeavesdrop D1}=
Pr {Eeavesdrop D1}=1
2. Then the SOP for the two-user
NOMA system is expressed as
Pout =1
2Pout,D1+Pout ,D2.(17)
IV. SECRECY OUTAGE PROBABILITY ANALYSIS OF A
MISO NOMA SYSTEMS
In this section, the secrecy outage performance of a MISO
NOMA system with different TAS schemes utilized at Sis
analyzed. Firstly, we analyze the secrecy outage performance
with OAS and SAS schemes, depending on whether the
global CSI is available at the base station. To better demon-
strate the enhancement of TAS schemes, we consider the STT
scheme as a benchmark, wherein all the antennas are utilized
to send the message with equal power.
A. THE OPTIMAL ANTENNA SELECTION SCHEME
In those scenarios where the users play dual roles as legiti-
mate receivers for some signals and eavesdroppers for others,
the eavesdropper is active (e.g., in a time-division multiple-
access (TDMA) environment) such that the source node can
estimate the eavesdropper’s channel during the eavesdrop-
per’s transmissions [37], [38]. When the CSI of both the wire-
tap and main links are known at the base station, the antenna
that maximizes the secrecy capacity is optimal [29], [30].
In this subsection we will analyze the SOP of MISO NOMA
systems while considering D1or D2, respectively.
Firstly, we consider the case that the secrecy issue of D1
is more important than the one of D2, the transmit antenna
is selected based on the secrecy capacity performance of D1.
The instantaneous secrecy capacity for such scenarios can be
expressed as [30]
COAS
s,1=max
i∈NSnCiOAS
s,1o,(18)
4In [27] and [28], a different definition of the SOP for the selected user
pair was given as Pout =1−1−Pout,D11−Pout ,D1. The results with
this definition can be easily obtained with the results of our work and the
same conclusion can be observed. Here we assume that Ecan not eavesdrop
both the users simultaneously.
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H. Lei et al.: On Secure NOMA Systems With TAS Schemes
where CiOAS
s,1=hlog21+γiOAS
D1−log21+γiOAS
E1i+
is the instantaneous secrecy capacity of D1when Sis
equipped with a single antenna.
The SOP at D1in the MISO NOMA system can be
expressed as
POAS
out,D1=Pr max
i∈NSnCiOAS
s,1o≤Rs,1
=
NS
Y
i=1
Pr nCiOAS
s,1≤Rs,1o=PiOAS
out,D1NS,(19)
where PiOAS
out,D1=Pr nCiOAS
s,1≤Rs,1ois the SOP at D1when
Sis equipped with a single antenna. Obviously, we have
PiOAS
out,D1=Pout ,D1.
It is noted that selecting the optimal transmit antenna for D1
corresponds to selecting a random transmit antenna for D2,
which means the SOP at D2in this scenario can also be given
by (12).
When the transmitting antenna is selected based on D1,
the overall SOP can be expressed as
POAS,1
out =1
2Pout,D1NS+Pout ,D2.(20)
Similarly, we obtain the SOP with OAS scheme based on
D2as
POAS,2
out =1
2Pout,D1+Pout ,D2NS.(21)
B. THE SUBOPTIMAL ANTENNA SELECTION SCHEME
When the CSI of the wiretap link is not available at the base
station, the best transmit antenna is selected to maximize the
capacity of the main channel [29], [30].5We analyze the
secrecy outage performance for MISO NOMA systems while
considering D1or D2, respectively.
1) SAS SCHEME CONSIDERING D1
When D1is more important than D2for some practical rea-
sons, the transmit antenna is selected based on the capacity
of S−D1link, which means γSAS,1
D1=max
1≤i≤NSγSiD1, where
γSiD1means the received instantaneous SINR at the near user
from the i-th antenna at S. The CDF and PDF of γSiD1are
the same as (4) and (6). Then the CDF of γSAS,1
D1can be
obtained as
FSAS
γD1(γ)=FγD1(γ)NS
=
NS
X
i=0
(−1)iNS!
i!(NS−i)!e−iγ
λD1a1ρ.(22)
Note that selecting the optimal transmit antenna for D1cor-
responds to selecting a random transmit antenna for D2and E,
which means the CDF and PDF of γSAS,1
D2and γSAS,1
Emare same
5Note that selecting the strongest transmit antenna for the destination node
corresponds to selecting a random transmit antenna for E. Thus the secrecy
capacity with this scheme may not be the maximum and then this scheme is
called suboptimal antenna selection scheme.
as (5) and (7), respectively. Thus the SOP at D2in this case
is given by (12).
Substituting (7) and (22) into (9) and making use of
[34, eq. (3.351.3)], we obtain
PSAS
out,D1=Z∞
0
FSAS
γD1(21(1+γ)−1)fγE2(γ)dγ
=
NS
X
i=0
(−1)iNS!λD1
i!(NS−i)!i21λE+λD1e−i(21−1)
a1ρλD1.(23)
The SOP that maximizes the capacity of S−D1link is
obtained by substituting (12) and (23) into (17) as
PSAS,1
out =1
2PSAS
out,D1+Pout ,D2.(24)
2) SAS SCHEME CONSIDERING D2
Similarly, when transmit antenna is selected based on
the capacity of S−D2link, the CDF and PDF of
γSAS,2
D1and γSAS,2
Emare same as (5) and (7), respectively. The
SOP at D1in this case is given by (10).
The CDF of γSAS,2
D2is obtained as
FSAS
γD2(γ)=






NS
X
i=0
(−1)iNS!
i!(NS−i)!e−iγ
λD2ρ(a2−a1γ), γ < a2
a1
1, γ ≥a2
a1
.
(25)
Lemma 2: The SOP at D2in the case with SAS scheme
considering D2is expressed as
PSAS
out,D2=$ π a2NS!
2ρNλE
NS
X
i=0
N
X
n=1
(−1)ip1−τn2e−8
i!(NS−i)!(a2−a1ϑn)2
+e−$
λEρ(a2−a1$),(26)
where 8=i(22(1+ϑn)−1)
λD2ρ(a2−a1(22(1+ϑn)−1)) +ϑn
λEρ(a2−a1ϑn).
Proof: Substituting (7) and (25) into (9), we obtain
PSAS
out,D2=Z$
0
FSAS
γD2(22(1+γ)−1)fγE2(γ)dγ
| {z }
ISAS,2
1
+Za2
a1
$
fγE2(γ)dγ
| {z }
ISAS,2
2
.(27)
Using (7), (25) and Gaussian-Chebyshev quadrature
method, we can achieve
ISAS,2
1=
NS
X
i=0
(−1)ia2NS!
i!(NS−i)!λEρZ$
0
1
(a2−a1γ)2
×e−i(22(1+γ)−1)
λD2ρ(a2−a1(22(1+γ)−1)) −γ
λEρ(a2−a1γ)dγ
=$ π a2NS!
2ρNλE
NS
X
i=0
N
X
n=1
(−1)ip1−τn2e−8
i!(NS−i)!(a2−a1ϑn)2.(28)
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H. Lei et al.: On Secure NOMA Systems With TAS Schemes
Using (7), we can easily obtain
ISAS,2
2=e−$
λEρ(a2−a1$).(29)
The SOP in this case is obtained by substituting (10)
and (26) into (17) as
PSAS,2
out =1
2Pout,D1+PSAS
out,D2.(30)
C. THE SPACE-TIM E TRANSMISSION SCHEME
To demonstrate the enhancement of TAS scheme, in this
subsection we analyze the SOP with the traditional STT
scheme wherein all the antennas are utilized to transmit sig-
nals with power PS
NS[29], [30]. It is assumed that the selection
combination (SC) scheme is utilized at both legitimate and
illegitimate receivers, which means γSTT
km=max
1≤i≤NSγSikm.
With some simple algebraic manipulations, the CDF and PDF
of γkm(k∈{D,E},m=1,2)can be expressed as
FSTT
γk1(γ)=
NS
X
i=0
(−1)iNS!
i!(NS−i)!e−iNSγ
λk1a1ρ,(31)
FSTT
γk2(γ)=






NS
X
i=0
(−1)iNS!
i!(NS−i)!e−iNSγ
λk2ρ(a2−a1γ), γ < a2
a1
1, γ ≥a2
a1
,
(32)
fSTT
γk1(γ)=NS2
λk1a1ρ
NS−1
X
i=0
(−1)i(NS−1)!
i!(NS−1−i)!e−(i+1)NSγ
λk1a1ρ,(33)
fSTT
γk2(γ)=

















a2NS2
λk2ρ(a2−a1γ)2
NS−1
X
i=0
(−1)i
i!
×(NS−1)!
(NS−1−i)!e−(i+1)NSγ
λk2ρ(a2−a1γ), γ < a2
a1
0, γ ≥a2
a1
,
(34)
respectively.
Substituting (31) and (33) into (9) and making use of
[34, eq. (3.351.3)], we obtain
PSTT
out,D1=Z∞
0
FSTT
γD1(21γ+21−1)fSTT
γE1(γ)dγ
=NS2
λEa1ρ
NS
X
i=0
NS−1
X
j=0
(−1)i+jNS!
i!(NS−i)!
(NS−1)!
j!(NS−1−j)!
×e−iNS(21−1)
λD1a1ρZ∞
0
e−i21λE+(j+1)λD1NSγ
λD1λEa1ρdγ
=
NS
X
i=0
NS−1
X
j=0
(−1)i+jNS!
i!(NS−i)!
(NS−1)!
j!(NS−1−j)!
×NSλD1
i21λE+λD1(j+1)e−iNS(21−1)
λD1a1ρ.(35)
Lemma 3: The SOP at D2in the case with STT scheme is
expressed as
PSTT
out,D2=a2$ π NS2
2ρNλE
NS
X
i=0
NS−1
X
j=0
N
X
n=1
ψp1−τn2e−9
(a2−a1ϑn)2
+1−
NS
X
i=0
(−1)iNS!
i!(NS−i)!e−i$NS
λEρ(a2−a1$),(36)
where
ψ=(−1)j+iNS!
i!(NS−i)!
(NS−1)!
j!(NS−1−j)!
and
9=iNS(22ϑn+22−1)
λD2ρ(a2−a1(22ϑn+22−1)) +(j+1)NSϑn
λEρ(a2−a1ϑn).
Proof: Substituting (32) and (34) into (9), we obtain
PSTT
out,D2=Z$
0
FSTT
γD2(22γ+22−1)fSTT
γE2(γ)dγ
| {z }
ISTT
1
+Za2
a1
$
fSTT
γE2(γ)dγ
| {z }
ISTT
2
.(37)
Utilizing (32), (34), and Gaussian-Chebyshev quadrature
method, we obtain
ISTT
1=a2NS2
ρλE
NS
X
i=0
NS−1
X
j=0
ψZ$
0
1
(a2−a1γ)2
×e−iNS(22γ+22−1)
λD2ρ(a2−a1(22γ+22−1)) −(j+1)NSγ
λEρ(a2−a1γ)dγ
=a2$ π NS2
2ρNλE
NS
X
i=0
NS−1
X
j=0
N
X
n=1
ψp1−τn2e−9
(a2−a1ϑn)2.(38)
Using (32), we obtain
ISTT
2=1−
NS
X
i=0
(−1)iNS!
i!(NS−i)!e−i$NS
λEρ(a2−a1$).(39)
Finally, substituting (35) and (36) into (17), the SOP of
MISO NOMA system with STT scheme is obtained as
PSTT
out =1
2PSTT
out,D1+PSTT
out,D2.(40)
V. ASYMPTOTIC SECRECY OUTAGE
PROBABILITY ANALYSIS
In order to get more insights, we analyze the secrecy outage
performance in the high SINR regime, which is mathemati-
cally described as λD2→ ∞ and λD1=β λD2(β > 1). The
secrecy diversity order is expressed as [30]
Gd,m= − lim
λD2→∞
ln P∞
out,Dm
ln λD2
,(41)
where P∞
out,Dmdenotes the asymptotic SOP.
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H. Lei et al.: On Secure NOMA Systems With TAS Schemes
A. SISO NOMA SYSTEMS
Using (4), (5), and ex=
k
P
m=0
xm
m!+Oxk, we obtain F∞
γD1(γ)
and F∞
γD2(γ)as
F∞
γD1(γ)=γ
βλD2a1ρ,(42)
F∞
γD2(γ)=




γ
λD2ρ(a2−a1γ)γ < a2
a1
1, γ ≥a2
a1
,
(43)
respectively.
Substituting (6) and (42) into (9) and making use
of (3.381.4) of [34], we obtain the asymptotic SOP at D1as
P∞
out,D1=Z∞
0
F∞
γD1(21(1+γ)−1)fγE1(γ)dγ
=21−1+21λEa1ρ
βλD2a1ρ.(44)
Similarly, making use of Gaussian-Chebyshev quadrature
method, the asymptotic SOP at D2is obtained as
P∞
out,D2=Z$
0
F∞
γD2(22γ+22−1)fγE2(γ)dγ
+Za2
a1
$
fγE2(γ)dγ
=e−$
λEρ(a2−a1$)+a2
λD2λEρ2
×Z$
0
(22γ+22−1)e−γ
λEρ(a2−a1γ)dγ
(a2−a1(22(1+γ)−1)) (a2−a1γ)2
=e−$
λEρ(a2−a1$)+a2$ π
2NλD2λEρ2
×
N
X
n=1
(22(1 +ϑn)−1)p1−τn2e−ϑn
λEρ(a2−a1ϑn)
(a2−a1(22(1+ϑn)−1)) (a2−a1ϑn)2.
(45)
Remark 2: Based on (44) and (45), one can easily obtain
the secrecy diversity order of the near and far users as 1 and 0,
respectively. This is because P∞
out,D2will become a constant
when λD2→ ∞. Furthermore, this means that increasing
the transmit power does not have any impact on the secrecy
performance of D2.
Finally, the overall SOP is obtained by substitut-
ing (44) and (45) into (17). The secrecy diversity order of
SISO NOMA system is easily obtained as GSISO
d=0 since
the secrecy diversity order of two-user NOMA is determined
by the one that has small diversity order [28].
B. MISO NOMA SYSTEMS WITH SAS SCHEME
When λD2→ ∞ and λD1=βλD2, we have
F∞,SAS
γD1(γ)=γNS
βλD2a1ρNS,(46)
F∞,SAS
γD2(γ)=






γNS
λD2ρ(a2−a1γ)NSγ < a2
a1
1, γ ≥a2
a1
.
(47)
By substituting (6) and (46) into (9) and using
of [34, eq. (3.351.3)], we obtain
P∞,SAS
out,D1=Z∞
0
F∞,SAS
γD1(21(1+γ)−1)fγE1(γ)dγ
=NS!
λEβλD2NS(a1ρ)NS+1
×
NS
X
k=0
2k
1(21−1)NS−k
k!(NS−k)!Z∞
0
γke−γ
λEa1ρdγ
=NS!
βNSλD2
NS
NS
X
k=0
2k
1(21−1)NS−kλEk
(NS−k)!(a1ρ)NS−k.(48)
Substituting (7) and (47) into (27) and utilizing
Gaussian-Chebyshev quadrature method, we obtain
P∞,SAS
out,D2=Z∞
0
F∞,SAS
γD2(22(1+γ)−1)fγE2(γ)dγ
=ISAS,2
2+Z$
0
a2e−γ
λEρ(a2−a1γ)
λEρ(a2−a1γ)2
×(22(1+γ)−1)NSdγ
λD2ρ(a2−a1(22(1+γ)−1))NS
=ISAS,2
2+41
N
X
n=1
e−ϑn
λEρ(a2−a1ϑn)
×p1−τn2(22(1+ϑn)−1)NS
(1−a122(1+ϑn))NS(a2−a1ϑn)2,(49)
where 41=$ π a2
2NλEρNS+1λD2
NS.
Substituting (48) and (49) into (24) and (30), respectively,
the asymptotic SOP for MISO NOMA system with SAS
scheme is obtained. The secrecy diversity order for SAS
scheme while considering D1or D2is NSand 0, respectively.
One can easily obtain the diversity order of two-user MISO
NOMA system for SAS scheme considering D1and D2as
GSAS,1
d=GSAS,2
d=0.(50)
C. MISO NOMA SYSTEMS WITH OAS SCHEME
Substituting (44) and (45) into (20) and (21), respec-
tively, the asymptotic SOP for MISO NOMA system
with OAS schemes while considering a single user is
obtained, respectively. The secrecy diversity order of two-
user MISO NOMA system for these two cases is obtained as
GOAS,1
d=GOAS,2
d=0.
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D. MISO NOMA SYSTEMS WITH STT SCHEME
When λD2→ ∞ and λD1=βλD2, we have
F∞,STT
γD1(γ)=NSNSγNS
βλD2a1ρNS,(51)
F∞,STT
γD2(γ)=






NSNSγNS
λD2ρ(a2−a1γ)NS, γ < a2
a1
1, γ ≥a2
a1
.
(52)
Utilizing (33), (51), and [34, eq. (3.351.3)], we obtain
P∞,STT
out,D1=Z∞
0
F∞,STT
γD1(21(1 +γ)−1)fSTT
γE1(γ)dγ
=NSNS+2
λEa1ρβλD2a1ρNS
NS−1
X
i=0
NS
X
k=0Z∞
0
γke−(i+1)NSγ
λEa1ρdγ
×(−1)i(NS−1)!
i!(NS−1−i)!
NS!(21−1)NS−k21k
k!(NS−k)!
=(NS−1)!NS!
βNSλD2
NS
NS−1
X
i=0
NS
X
k=0
(−1)iNSNS−k+1
i!(NS−1−i)!
×(21−1)NS−k(21λE)k0(k+1)
k!(NS−k)!(i+1)k+1(a1ρ)NS−k.(53)
Using (34), (52), and Gaussian-Chebyshev quadrature
method, we obtain
P∞,STT
out,D2
=Z∞
0
F∞,STT
γD2(22(1+γ)−1)fSTT
γE2(γ)dγ
=ISTT
2+a2NSNS+2
λEρNS+1λD2
NS
NS−1
X
j=0
(−1)j(NS−1)!
j!(NS−1−j)!
×Z∞
0
(22(1+γ)−1)NSe−NS(j+1)γ
λEρ(a2−a1γ)dγ
λD2ρ(a2−a1(22(1+γ)−1))NS(a2−a1γ)2
=ISTT
2+42
NS−1
X
j=0
N
X
n=1
(−1)j
j!(NS−1−j)!
×p1−τn2(22ϑn+22−1)NSe−NS(j+1)ϑn
λEρ(a2−a1ϑn)
(a2−a1(22ϑn+22−1))NS(a2−a1ϑn)2,(54)
where 42=a2$ π NSNS+2(NS−1)!
2NλEρNS+1λD2
NS.
Substituting (53) and (54) into (40), the asymptotic SOP
for MISO NOMA system with STT scheme is obtained. One
can obtain the secrecy diversity order of the near and far
users in this case as NSand 0, respectively. That is to say,
the diversity order of two-user MISO NOMA system with
STT scheme is 0.
Remark 3: Observing the obtained analytical results above,
one can deduce that the diversity order of all the TAS schemes
is zero. This is because the diversity order of the far user is
zero with the fixed power allocation scheme, which deter-
mines the diversity order of two-user NOMA system.
VI. A POWER ALLOCATION METHOD TO ACHIEVE
NON-ZERO DIVERSITY ORDER
In this subsection, a new variable power allocation method is
proposed to achieve non-zero diversity order.
Let a2
a1=αλD2
ϕ(0< ϕ < 1) (α > 0). Considering a2+
a1=1, we can obtain







a1=1
1+αλD2
ϕ
a2=αλD2
ϕ
1+αλD2
ϕ.
(55)
A. SISO NOMA SYSTEMS
Utilizing (5), (55), and ex=
k
P
m=0
xm
m!+Oxk, we obtain
F∞
γD2(γ)as
F∞
γD2(γ)=γ
a2ρλD2
.(56)
When λD2→ ∞, the instantaneous SINR of the
eavesdropper eavesdropping the transmitted signal at the
receiver D2can be given as
γE2=|hE|2a2
|hE|2a1+1
ρ≈a2ρ|hE|2.(57)
Based on (57), the PDF of the eavesdropper applicable to
the new power allocation method can written as
fnon
γE2(γ)=1
a2ρλE
e−γ
a2ρλE.(58)
Substituting (55) into (44), we obtain the asymptotic SOP
at D1as
P∞
out,D1=1+αλD2
ϕ(21−1)+ρλE21
βλD2ρ.(59)
Substituting (56) and (58) into (9) and making use of
[34, eq. (3.381.4)], we obtain
P∞
out,D2=22a2ρ λE+22−1
a2ρλD2
.(60)
From (59) and (60), one can easily obtain the diversity
order for two users with the proposed power allocation
scheme is 1 −ϕand 1, respectively. This means the diversity
order of the far user increases by decreasing the diversity
order of the near user. Thus, the secrecy diversity order of
two-user NOMA is improved by
GSISO
d=1−ϕ. (61)
Remark 4: The power allocation scheme proposed in this
paper is able to recover the diversity order by balancing the
diversity gain of the near user and the diversity gain of the far
user based on the channel gains.
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H. Lei et al.: On Secure NOMA Systems With TAS Schemes
FIGURE 2. SOP for various PSwith λD1=15 dB, λD2=10 dB, λE= −5 dB, a1=0.1, and a2=0.9. (a) SOP of a SISO NOMA
system. (b) SOP for D1. (c) SOP for D2. (d) SOP for the two-user NOMA system.
B. MISO NOMA SYSTEMS WITH SAS SCHEME
Utilizing (55) into (25), and ex=
k
P
m=0
xm
m!+Oxk, we obtain
F∞,SAS
γD2(γ)=γNS
a2ρλD2NS.(62)
Substituting (55) into (48), we obtain the asymptotic SOP
at D1as
P∞,SAS
out,D1=
NS
X
k=0
NS!2k
1(21−1)NS−kλEk1+αλD2
ϕNS−k
(NS−k)!βNSλD2
NS(ρ)NS−k.
(63)
Substituting (62) and (58) into (9) and making use of [34,
eq. (3.381.4)], we obtain
P∞,SAS
out,D2=NS!
λD2ρa2NSλEρa2
×
NS
X
k=0
(22−1)NS−k22k
k!(NS−k)!Z∞
0
γke−γ
λEρa2dγ
=NS!
λD2
NS
NS
X
k=0
(22−1)NS−k22kλEk
(NS−k)!(ρa2)NS−k.(64)
Based on (63), we easily obtain the diversity order for D1
with SAS scheme considering D1as (1−ϕ)NS. Similarly,
the diversity order for D2with SAS scheme considering D2
is obtained as NSfrom (64). Note that the SOP for D2with
SAS scheme considering D1is the same as D2in SISO
scenario, and the SOP for D1with SAS scheme considering
D2is the same as D1in SISO scenario. Thus, the diversity
order of the two-user MISO NOMA system with SAS scheme
under the proposed power allocation scheme is
GSAS,1
d=min {(1−ϕ)NS,1},(65)
GSAS,2
d=1−ϕ, (66)
respectively.
C. MISO NOMA SYSTEMS WITH OAS SCHEME
Based on (19), we can easily obtain the diversity order for the
two users with OAS scheme scenario as (1−ϕ)NSand NS,
respectively. Note that the SOP for D2with OAS scheme
considering D1is the same as D2in SISO scenario, and the
SOP for D1with OAS scheme considering D2is the same
as D1in SISO scenario. Finally, we obtain the diversity order
for the two-user MISO NOMA system with OAS scheme
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H. Lei et al.: On Secure NOMA Systems With TAS Schemes
FIGURE 3. SOP for various λD2with λE=5 dB, NS=3, a1=0.1, a2=0.9, β=1.2, and PS=10 dB. (a) SOP of a SISO NOMA
system. (b) SOP for D1. (c) SOP for D2. (d) SOP for the two-user NOMA system.
under the proposed power allocation scheme as
GOAS,1
d=min {(1−ϕ)NS,1},(67)
GOAS,2
d=1−ϕ, (68)
Remark 5: It should be noted that the SAS/OAS consider-
ing the near user is more beneficial in terms of diversity gain,
which means G1
d>G2
d.
D. MISO NOMA SYSTEMS WITH STT SCHEME
By substituting (55) into (32), and ex=
k
P
m=0
xm
m!+Oxk,
we obtain
F∞,STT
γD2(γ)=NSNSγNS
λD2ρa2NS.(69)
Similarly, the fSTT,non
γk2(γ)can be rewritten as
fSTT,non
γk2(γ)=NS2
λEρa2
NS−1
X
i=0
(−1)i(NS−1)!
i!(NS−1−i)!e−(i+1)NSγ
λEρa2.
(70)
Substituting (55) into (53), we obtain
P∞,STT
out,D1=
NS−1
X
i=0
NS
X
k=0
(−1)i(NS−1)!NS!NSNS−k+1
i!(NS−1−i)!(NS−k)!
×(21−1)NS−k(21λE)k1+αλD2
ϕNS−k
(i+1)k+1βNSλD2
NSρNS−k.
(71)
On substituting (69) and (70) into (9) and making use
of [34, eq. (3.381.4)], we obtain
P∞,STT
out,D2=NSNS+2
λD2ρa2NSλEρa2
NS
X
k=0
NS−1
X
i=0
NS!(22−1)NS−k
k!(NS−k)!
×(−1)i(NS−1)!22k
i!(NS−1−i)!Z∞
0
γke−(i+1)NSγ
λEρa2dγ
=NS!(NS−1)!
λD2ρa2NS
NS
X
k=0
NS−1
X
i=0
(−1)i(λEρa222)k
(NS−k)!i!
×NSNS−k+1(22−1)NS−k
(NS−1−i)!(i+1)k+1.(72)
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H. Lei et al.: On Secure NOMA Systems With TAS Schemes
FIGURE 4. SOP for various NSwith λD1=10 dB, λD2=8 dB, λE=0 dB, a1=0.1, a2=0.9, and PS=5 dB. (a) SOP for D1.
(b) SOP for D2. (c) SOP for the two-user NOMA system.
Similarly, the diversity order for the two users with STT
scheme is easily obtained as (1−ϕ)NSand NS, respectively.
Finally we obtain the diversity order of MISO NOMA system
with STT scheme as
GSTT
d=(1−ϕ)NS.(73)
VII. NUMERICAL RESULTS AND DISCUSSIONS
In this section, the proposed analytical models are verified
via Monte-Carlo simulation. The main parameters utilized
in the simulations are set as Rs,1=Rs,2=0.1 bit/s/Hz,
σ2=1. In all the figures, ‘SAS1’ and ‘SAS2’ means the SAS
scheme based on D1or D2, respectively. Similarly ‘OAS1’
and ‘OAS2’ means the OAS scheme based on D1or D2,
respectively.
Fig. 2 depicts the SOP versus PS, where
Figs. 2(a), 2(b), 2(c), and 2(d) plot the SOP for a SISO NOMA
system, the SOP for D1,D2, and the two-user NOMA system
with different TAS schemes, respectively. One can observe
from Fig. 2(a) that the SOP at D1is improved until it reaches
a floor. The reason is that the secrecy capacity tends to a
constant when PSbecomes large [35]. An interesting result
is found that the SOP at D2degrades before it reaches a
ceil because the interference from D1increases while PS
increasing, which testifies the result in Remark 1. Further,
the SOP at D2is superior to that at D1in the low transmit
power region although the channel quality of S-D1is better
than that of S-D2. This is because physical layer security
utilizes the randomness of the wireless channel. However,
the SOP at D2gets worse in the high transmit power region
until it reaches a constant due to increasing interference
from D1. The SOP of the NOMA system deteriorates due
to the increase of the SOP at D2. Figs. 2(b) and 2(c) depict
the SOP for D1and D2with different TAS schemes ver-
sus PS. One can observe that multiple antennas can improve
the secrecy outage performance and the SOPs of SAS and
OAS schemes outperform the STT scheme for both users.
In Fig. 2(d), it can be observed that the SOP for NOMA
system with STT outperforms those with SAS and OAS
schemes. This is because in the SAS1 and OAS1 schemes
only D1is considered in selecting transmit antenna. Since
one transmit antenna is selected randomly for D2, the SOP
for D2with SAS1 and OAS1 schemes is the same as the case
with a single antenna at S.
Fig. 3 depicts the SOP versus λD2. One can observe that
the asymptotic curves tightly approximate the exact ones as
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H. Lei et al.: On Secure NOMA Systems With TAS Schemes
FIGURE 5. SOP with the proposed power allocation scheme for various λD2with λE=3 dB, NS=3, β=2, α=1.5, ϕ=0.9, and
PS=10dB. (a) SOP of a SISO NOMA system. (b) SOP for D1. (c) SOP for D2. (d) SOP for the two-user NOMA system.
λD2increases. The SOP of NOMA system is determined by
the farther user and hence the SOP of SISO NOMA system
reaches a constant when λD2tends to infinity. Thus the diver-
sity order of SISO NOMA system is zero. Moreover, similar
results can be observed from Figs. 3(c) and 3(d).
Figs. 4 depicts the SOP for MISO NOMA system with
different TAS schemes versus NS. It is observed from
Figs. 4(a) and 4(b) that multiple antennas can improve the
secrecy outage performance of each user when OAS scheme
is considered in selecting antenna. From Fig. (c), it is found
that the SOP of the NOMA system with SAS scheme is not
affected by the number of the antennas when NS>3 whereas
the NOMA system with STT scheme is effected. This means
that increasing the antenna number hardly affects the secrecy
performance of the NOMA system with TAS scheme but
improves with the STT scheme.
Figs. 5 plots the SOP for SISO and MISO NOMA systems
with the proposed power allocation scheme. We can observe
that the non-zero diversity order is obtained for all the cases.
We see in Figs. 5(a), 5(b), and 5(c) that the diversity order of
NOMA system can be improved by decreasing the diversity
order of the farther user. Fig. 5(d) demonstrates all the cases
with the proposed power allocation scheme. We can observe
that in SAS2 and OAS2 schemes, since P∞,SAS
out,D2and P∞,OAS
out,D2
is very small (can be found in Fig. 5(c)), then the SOP of D1in
SAS2 and OAS2 are dominant part of the SOP for the NOMA
system (the SOP of D1in SAS2 and OAS2 are same as ones in
SISO in Fig. 5(b)). So there is almost no difference between
the SOP of SISO, SAS2, and OAS2 schemes. Similarly, from
Fig. 5(c), one can observe that P∞,SAS
out,D1and P∞,OAS
out,D1is small
(Fig. 5(b)) and the SOP of D2in SAS1 and OAS1 schemes
are dominant part of the SOP for the NOMA system (the
SOP of D2in SAS1 and OAS1 schemes are same as the ones
in SISO in Fig. 5(c)). Furthermore, we can observe that the
SOP with SAS1 and OAS1 schemes outperform STT scheme
scenario with the proposed power allocation method in the
high SNR regime.
VIII. CONCLUSION
In this work, the secrecy outage performance of two-user
SISO and MISO NOMA systems with different TAS schemes
was investigated. The exact and approximated closed-
form expressions of the SOP for different TAS schemes
were derived and compared with the one of traditional
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H. Lei et al.: On Secure NOMA Systems With TAS Schemes
STT scheme. The proposed analytical results were verified
via Monte-Carlo simulations. The results demonstrate that
the diversity order is zero when the fixed power allocation
scheme is utilized. Subsequently, an effective power allo-
cation strategy has been proposed and the diversity order
achieved by the proposed power allocation scheme was
analyzed.
ACKNOWLEDGMENT
The authors would like to thank the Editor and anonymous
reviewers for their helpful comments and suggestions in
improving the quality of this paper.
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HONGJIANG LEI (M’17) received the B.Sc.
degree in mechanical and electrical engineering
from the Shenyang Institute of Aeronautical Engi-
neering, Shenyang, China, in 1998, the M.Sc.
degree in computer application technology from
Southwest Jiaotong University, Chengdu, China,
in 2004, and the Ph.D. degree in instrument sci-
ence and technology from Chongqing University,
Chongqing, China, in 2015, respectively. In 2004,
he joined the School of Communication and Infor-
mation Engineering, Chongqing University of Posts and Telecommunica-
tions, Chongqing, China, where he is currently an Associate Professor.
Since 2016, he has been with the Computer, Electrical, and Mathematical
Science and Engineering Division, King Abdullah University of Science and
Technology, Saudi Arabia, where he is currently a Post-Doctoral Research
Fellow supported by the Chinese Scholarship Council. His research inter-
est spans special topics in communications theory and signal processing,
including physical layer security and cognitive radio networks. He is a TPC
Member of IEEE Globecom’17. He has also served as a reviewer for major
international journals, e.g., IEEE TVT, IEEE TCOM, IEEE TWC, IEEE
TIFS, and IEEE CL.
JIANMING ZHANG received the B.Sc. degree
in electrical engineering from the Hebei Univer-
sity of Architecture, Zhangjiakou, China, in 2015.
He is currently pursuing the M.Sc. degree in infor-
mation and communication engineering with the
Chongqing University of Posts and Telecommuni-
cations, Chongqing, China. His research interests
include cognitive radio networks, physical layer
security, and non-orthogonal multiple access.
KI-HONG PARK (S’06–M’11) received the B.Sc.
degree in electrical, electronic, and radio engineer-
ing from Korea University, Seoul, South Korea,
in 2005, and the M.S. and Ph.D. degrees from
the School of Electrical Engineering, Korea Uni-
versity, Seoul, South Korea, in 2011. Since 2011,
he has been a Post-Doctoral Fellow of electrical
engineering with the Division of Physical Science
and Engineering, King Abdullah University of Sci-
ence and Technology, Thuwal, Saudi Arabia. His
research interests are broad in communication theory and its application
to the design and performance evaluation of wireless communication sys-
tems and networks. On-going research includes the application to MIMO
diversity/beamforming systems, cooperative relaying systems, physical layer
secrecy, and optical wireless communications.
PENG XU received the B.Sc. and Ph.D. degrees in
electronic and information engineering from the
University of Science and Technology of China,
Anhui, China, in 2009 and 2014, respectively.
Since 2014, he has been a Post-Doctoral
Researcher with the Department of Electronic
Engineering and Information Science, Univer-
sity of Science and Technology of China, Hefei,
China. Since 2016, he has been with the School
of Communication and Information Engineering,
Chongqing University of Posts and Telecommunications, Chongqing, China.
His current research interests include cooperative communications, infor-
mation theory, information-theoretic secrecy, and 5G networks. He received
IEEE Wireless Communications Letters Exemplary Reviewer 2015.
IMRAN SHAFIQUE ANSARI (S’07–M’15)
received the B.Sc. degree in computer engineer-
ing (Hons.) from the King Fahd University of
Petroleum and Minerals in 2009 and the M.Sc.
and Ph.D. degrees from the King Abdullah Uni-
versity of Science and Technology in 2010 and
2015, respectively. He is currently a Post-Doctoral
Research Associate with Texas A&M University
at Qatar. In 2009, he was a Visiting Scholar
with Michigan State University, East Lansing, MI,
USA, and from 2010 to 2010, he was a Research Intern with Carleton
University, Ottawa, ON, Canada.
He has authored/co-authored more than 50 journal and conference pub-
lications. He has co-organized the GRASNET’2016 workshop in con-
junction with IEEE WCNC’2016 and co-organizing the second edition
GRASNET’2017 workshop in conjunction with IEEE WCNC’2017. His
current research interests include free-space optics, channel modeling/signal
propagation issues, relay/multihop communications, physical layer secrecy
issues, full duplex systems, and diversity reception techniques among others.
GAOFENG PAN (M’12) received the B.Sc. degree
in communication engineering from Zhengzhou
University, Zhengzhou, China, in 2005, and the
Ph.D. degree in communication and informa-
tion systems from Southwest Jiaotong University,
Chengdu, China, in 2011. He was with The Ohio
State University, Columbus, OH, USA, from 2009
to 2011, as a joint-trained Ph.D. student under the
supervision of Prof. E. Ekici. In 2012, he joined the
School of Electronic and Information Engineering,
Southwest University, Chongqing, China, where he is currently an Associate
Professor. Since 2016, he has also been with the School of Computing and
Communications, Lancaster University, Lancaster, U.K., where he currently
holds a postdoctoral position under the supervision of Prof. Z. Ding. His
research interest spans special topics in communications theory, signal pro-
cessing, and protocol design, including secure communications, CR commu-
nications, and MAC protocols. He is a TPC Member of IEEE Globecom’16
Workshop on Wireless Energy Harvesting Communication Networks. He has
also served as a reviewer for major international journals, such as the IEEE
TCOM, the IEEE TWC, the IEEE TSP, the IEEE TVT, the IEEE CL, and the
IEEE WCL.
VOLUME 5, 2017 17463

H. Lei et al.: On Secure NOMA Systems With TAS Schemes
BASEL ALOMAIR (M’11) received the bache-
lor’s degree from King Saud University, Riyadh,
Saudi Arabia, the master’s degree from the Univer-
sity of Wisconsin-Madison, Madison, WI, USA,
and the Ph.D. degree from the University of
Washington-Seattle, Seattle, WA, USA. He is cur-
rently an Assistant Professor and the Founding
Director of the National Center for Cybersecurity
Technology, King Abdulaziz City for Science and
Technology, Riyadh, Saudi Arabia, an Affiliate
Professor, and the Co-Director of the Network Security Laboratory, Univer-
sity of Washington-Seattle, an Affiliate Professor with King Saud University,
and a cryptology consultant with various agencies.
Dr. Alomair was recognized by the IEEE Technical Committee on Fault-
Tolerant Computing and the IFIP Working Group on Dependable Computing
and Fault Tolerance (WG 10.4) with the 2010 IEEE/IFIP William Carter
Award for his significant contributions in the area of dependable computing.
He has authored/coauthored papers that received best paper awards. He was
a recipient of the 2011 Outstanding Research Award from the University
of Washington for his research in information security, the 2012 Distin-
guished Dissertation Award from the Center for Information Assurance and
Cybersecurity at the University of Washington, and the 2015 Early Career
Award in Cybersecurity by the NSA/DHS Center of Academic Excellence
in Information Assurance Research for his contributions to Modern Crypto-
graphic Systems and Visionary Leadership.
MOHAMED-SLIM ALOUINI (S’94–M’98–
SM’03–F’09) was born in Tunis, Tunisia.
He received the Ph.D. degree in Electrical Engi-
neering from the California Institute of Technol-
ogy, Pasadena, CA, USA, in 1998. He served as a
Faculty Member with the University of Minnesota,
Minneapolis, MN, USA, and then with the Texas
A&M University at Qatar, Education City, Doha,
Qatar, before joining the King Abdullah University
of Science and Technology, Thuwal, Saudi Arabia,
as a Professor of Electrical Engineering in 2009. His current research
interests include the modeling, design, and performance analysis of wireless
communication systems.
17464 VOLUME 5, 2017