5462 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 66, NO. 6, JUNE 2017
Complexity-Aware Relay Selection for 5G Large-Scale
Secure Two-Way Relay Systems
Chensi Zhang, Member, IEEE, Jianhua Ge, Jing Li, Member, IEEE,
Fengkui Gong, Member, IEEE, and Haiyang Ding, Member, IEEE
Abstract—This paper focuses on the complexity issue of relay selection
for fifth-generation (5G) large-scale secure two-way amplify-and-forward
(TWR-AF) relay systems with massive relays and eavesdroppers. A novel
and distributed relay selection criterion [i.e., low-complexity relay selec-
tion criterion (LRSC)] is proposed to maximize the overall secrecy per-
formance. Utilizing LRSC, the calculations of sources’ received signal-
to-noise ratios (SNRs), the channel estimation, and the knowledge of the
relay–eavesdropper links are not required. Particularly, a relay is selected
based on the received power of relays and the average knowledge of the
source–eavesdropper links. These low-complexity properties make LRSC
more attractive for 5G large-scale networks. Moreover, the secrecy outage
probability and the impact of t he eavesdropper density on the secrecy per-
formance are presented, and some useful insights are obtained. Simulation
results highlight the effectiveness of our relay selection design.
Index Terms—Amplify-and-forward (AF), distributed relay selection,
low-complexity, physical-layer security, two-way relaying (TWR).
I. INTRODUCTION
As a promising way to extend the transmission coverage and im-
prove the system capacity in the fifth-generation (5G) multitier cel-
lular wireless networks, relay-assisted cooperative transmission has
been well studied by the research community [1]. Two-way relaying
with amplify-and-forward (TWR-AF) strategy is of particular interest
[2]–[4] because of its high spectrum efficiency, low complexity, and
potential application to peer-to-peer networks. For TWR-AF, the ap-
plication of relay selection has been widely explored to improve the
system performance, e.g., sum data rate [5] or outage performance
[6], but until recently, many more authors have employed the relay
selection approach to secure cooperative networks and shown consid-
erable secrecy performance gains [7], [8]. However, most of the existing
works are carried out for one-way relaying systems [9] or TWR with
decode-and-forward (DF) strategy [10]. Relay selection schemes for
physical-layer security in TWR-AF have not yet been well investigated.
Unlike one-way relaying, bidirectional communications are performed
Manuscript received June 11, 2016; revised September 9, 2016 and October
21, 2016; accepted October 22, 2016. Date of publication October 25, 2016; date
of current version June 16, 2017. This work was supported by the National High
Technology Research and Development Program (863 Program) of China under
Grant 2014AA01A704; the National Natural Science Foundation of China under
Grant 61501347, Grant 61372067, and Grant 61301135; the Project Funded by
the China Postdoctoral Science Foundation (2015M580816); the Fundamental
Research Funds for the Central Universities (JB160111); the “111” project
(B08038); and the Open research fund of State Key Laboratory of Integrated
Service Networks (ISN15-05). The review of this paper was coordinated by Dr.
R.-D. Souza.
C. Zhang, J. Ge, J. Li, and F. Gong are with the State Key Laboratory of In-
tegrated Service Networks, Xidian University, Xi’an 710071, China (e-mail:
cszhang@xidian.edu.cn; jhge@xidian.edu.cn; jli@xidian.edu.cn; fkgong@
xidian.edu.cn).
H. Ding is with the State Key Laboratory of Integrated Service Networks,
Xidian University, Xi’an 710071, China, and also with Xi’an Communication
Institute, Xi’an 710106, China (e-mail: dinghy2003@hotmail.com).
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/TVT.2016.2621126
simultaneously over the same channel in TWR-AF, making the relay
selection design a challenging job.
In the wide body of TWR-AF literature studies, there appear sev-
eral papers dealing with the issue of relay selection for physical-layer
security [11]–[13]. In [11], r elay and jamming node selection criteria
were developed to increase the intercept probability. Specifically, three
nodes were selected, where the first node played the role of conven-
tional relay node and the rest two nodes acted as jammers. In [12],
several joint relay and jammer selection schemes were proposed for
TWR-AF to increase the system secrecy rate. However, it was also
pointed out that the cooperative jamming approach may not be effec-
tive and even degrades the system secrecy performance. Based on these
two studies, game-theoretic power control approach was discussed in
[13] to optimize the benefits of legitimate nodes.
Motivations: 1) The relay selection and jamming schemes in [12]
and [13] are centralized. In the relay (jammer) selection process, the
global channel state information (CSI) and complicated calculations
are required for the centralized control node to make the decisions.
2) In TWR-AF, the received signal at the eavesdropper in each time
slot is an overlapped counterpart of the two sources’ signals. One
source’s signal acts as the jamming noise if the eavesdropper tries to
decode the other source’s signal, and vice versa. Thus, unlike two-way
DF relaying systems, the two sources in TWR-AF both play the role of
jammer.
1
Particularly, it has been shown in [12] that the secrecy capacity
is not necessarily increased with the help of a jammer. Therefore,
it deserves to design a new low-complexity relay selection scheme
without a jamming process to deal with the issue of massive relays and
eavesdroppers.
With aforementioned motivations, this paper proposes a low-
complexity distributed relay selection criterion for large-scale TWR-
AF with massive relays and eavesdroppers. The main goal of our relay
selection design is to minimize the secrecy outage probability and,
thereby, to optimize the overall physical-layer security performance.
Furthermore, the secrecy outage probability and the impact of the
eavesdropper density on the secrecy performance are presented. It has
been shown that the massive number of eavesdroppers would cause
relatively big performance loss, even though the number of relays also
scales up.
II. SYSTEM MODEL
We consider a secure two-way relay systems with two sources A
and B, K legitimate (or friendly) relay nodes, and M passive eaves-
droppers. All the nodes have single antenna. The friendly node R
k
(k ∈{1, 2,...,K}) helps to relay the signal transmitted by the source,
while the eavesdropper E
m
(m ∈{1, 2,...,M}) tries to intercept
the information during the transmit time slots. All the relays operate
in half-duplex mode and use amplify-and-forward protocol. All the
channels are subject to Rayleigh fading. The channel gain between
node i and j is denoted by h
ij
,wherei = j, i, j ∈{A, B, R
k
,E
m
},
k ∈{1, 2,...,K},andm ∈{1, 2,...,M}. The transmit power of A,
B, and relays is denoted by P
A
, P
B
,andP
R
, respectively.
In the first time slot, both A and B transmit their signals to
the relays, simultaneously. Then, one optimal relay is selected and
1
It is worth noting that the eavesdroppers could try a joint decoding of A
and B. Therefore, the implication of the joint decoding at eavesdropper in the
system performance is also one important work to pursue, and we will present
the results in our future publications.
0018-9545 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications
standards/publications/rights/index.html for more information.