538 IEEE WIRELESS COMMUNICATIONS LETTERS, VOL. 7, NO. 4, AUGUST 2018
Outage Performance of NOMA-Based Hybrid
Satellite-Terrestrial Relay Networks
Xiaojuan Yan , Hailin Xiao, Member, IEEE, Cheng-Xiang Wang, Fellow, IEEE, and Kang An
Abstract—In this letter, we investigate the outage probabil-
ity (OP) of amplify-and-forward hybrid satellite-terrestrial relay
networks with a nonorthogonal multiple access (NOMA) scheme.
By assuming that a single antenna satellite communicates with
multiple multiantenna users simultaneously through the help of
a single antenna relay and the NOMA scheme, we first derive the
closed-form OP expressions for each NOMA user. Then, asymp-
totic OP expressions at the high signal-to-noise ratio regime are
also obtained to evaluate the achievable diversity order and cod-
ing gain. Finally, simulations are provided to the validity of
theoretical results, the superiority of introducing the NOMA
scheme in satellite-terrestrial relay networks, and the effect of
key parameters on the performance of NOMA users.
Index Terms—Hybrid satellite-terrestrial relay networks, non-
orthogonal multiple access, outage probability.
I. INTRODUCTION
H
YBRID satellite-terrestrial relay networks (HSTRNs), in
which a relaying technique is adopted to achieve the ben-
efit of spatial diversity, has been proposed as an effective way
to mitigate the masking effect and improve the reliability of
satellite communications [1]. Many efforts have been devoted
to investigate the key performance measures of HSTRNs, such
as outage probability (OP), ergodic capacity, and bit error
rate (BER) [2]–[6]. Although the performance of HSTRNs
can be significantly enhanced, it should be noted that these
enhancements via the deployment of additional relay node
were achieved with more resource consumption, i.e., time and
power. Moreover, the time division multiple access (TDMA)
scheme, which was adopted in those aforementioned works,
Manuscript received December 15, 2017; revised January 11, 2018;
accepted January 11, 2018. Date of publication January 15, 2018; date
of current version August 21, 2018. This work was supported in part by
the National Natural Science Foundation of China under Grant 61472094
and Grant 61261018, in part by the EPSRC TOUCAN Project under
Grant EP/L020009/1, in part by the EU H2020 RISE TESTBED Project
under Grant 734325, in part by the EU FP7 QUICK Project under Grant
PIRSES-GA-2013-612652, and in part by the Innovation Project of Guangxi
Graduate Education. The associate editor coordinating the review of this paper
and approving it for publication was A. Kammoun. (Corresponding author:
Hailin Xiao.)
X. Yan is with the School of Information and Communication, Guilin
University of Electronic Technology, Guilin 541004, China, and also with
the Engineering Training Center, Qinzhou University, Qinzhou 535011, China
(e-mail: yxj9609@163.com).
H. Xiao is with the College of Physics and Electronic Information
Engineering, Wenzhou University, Wenzhou 325035, China, and also with the
School of Information and Communication, Guilin University of Electronic
Technology, Guilin 541004, China (e-mail: xhl_xiaohailin@163.com).
C.-X. Wang is with the Institute of Sensors, Signals, and Systems, School
of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh
EH14 4AS, U.K. (e-mail: cheng-xiang.wang@hw.ac.uk).
K. An is with the National University of Defense Technology,
Nanjing 210007, China (e-mail: ankang@nuaa.edu.cn).
Digital Object Identifier 10.1109/LWC.2018.2793916
does restrict the improvement of resource utilization since only
one user is served at any time slot. In future satellite commu-
nication systems, high information transmission quality to a
large number of terrestrial users is required [2]. Under this
situation, other multiple access schemes should be taken into
account in future satellite communications.
Having the ability to serve multiple users simultaneously
and provide high resource efficiency, non-orthogonal multiple
access (NOMA) scheme is attracting considerable interests
and becoming a promising technology for the fifth genera-
tion (5G) networks [7]. Several works studied the superior-
ity of introducing the NOMA scheme in terrestrial cellular
networks [8]–[10]. In addition, applying the NOMA scheme to
mmWave communications was studied in [11]. An extension
of [11] to study NOMA-based mmWave massive multiple-
input multiple-output (MIMO) systems was given in [12].
Recently, the work [13] incorporated the NOMA scheme
into multibeam satellite networks to further improve the
performance of the NOMA scheme. Yan et al. [14]integrated
the NOMA scheme into cognitive satellite terrestrial networks,
so that the ergodic capacity of a cognitive network can be
increased. However, no results regarding the performance of
NOMA-based HSTRNs have been reported thus far. To fill
this gap, this letter studies the performance of NOMA-based
amplify-and-forward (AF) HATRNs. Specifically, we first
derive the end-to-end signal-to-interference-plus-noise ratios
(SINRs) for each NOMA user. Then, the exact and asymp-
totic OP expressions are derived. Finally, simulation results
are provided to show the validity of our theoretical analysis,
the superiority of introducing the NOMA scheme in HSTRNs,
and the effects of various parameters on the OP performance
of each NOMA user.
II. S
YSTEM MODEL
In this letter, we consider downlink NOMA-based AF
HSTRNs, where a land mobile satellite (S) communicates
with M terrestrial users D
i
(i = 1, 2 ···M) simultaneously
through the help of a terrestrial relay (R) and the NOMA
scheme. Each terrestrial user is equipped with N
D
i
anten-
nas, while S and R nodes are both equipped with a single
antenna. It is assumed that all the direct links between S and
D
i
are not available due to raining, fog, or other masking
effect [2]. Without loss of generality, we also assume these
users are ordered based on their effective channel gains, i.e.,
h
D
1
2
F
≤h
D
2
2
F
≤···≤h
D
M
2
F
, where h
D
i
is the N
D
i
×1
channel vector of R → D
i
link and ·
F
is the Frobenius norm
of a matrix. For simplicity, we assume only the p
th
and q
th
users (1 ≤ p < q ≤ M) are selected to form a NOMA group.
The overall communication takes place in two time phases.
During the first phase, the S node broadcasts a superposing
2162-2345
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