2088 IEEE TRANSACTIONS ON INFORMATION FORENSICS AND SECURITY, VOL. 14, NO. 8, AUGUST 2019 Distributed Secure Switch-and-Stay Combining Over Correlated Fading Channels Xiazhi Lai , Lisheng Fan , Xianfu Lei , Jin Li, Nan Yang , Member, IEEE, and George K. Karagiannidis , Fellow, IEEE Abstract— In this paper, we study decode-and-forward relaying networks in the presence of direct links, where they are used by the eavesdropper to overhear the confidential message from the source and relay. The secure data transmission can go through from either the direct or the relaying branch, and we focus on the practical communication scenarios, where the main and eavesdropper channels are correlated. Although traditional opportunistic selection techniques can choose one better branch to ensure the secure performance, it needs to continuously know the channel state information (CSI) of both branches and may result in a high branch switching rate. To overcome these limita- tions, we propose a distributed secure switch-and-stay combining (DSSSC) protocol, where only one between direct and relaying branches is activated to assist the secure data transmission, and the switching occurs when the branch cannot support the secure communication any longer. The DSSSC protocol uses either the instantaneous or the statistics of the eavesdropping CSI. For both cases, we quantify the impact of correlated fading on secure communication by deriving an analytical expression for the secrecy outage probability (SOP) as well as an asymptotic expression for the high main-to-eavesdropper ratio region. From the asymptotic SOP, we can conclude that the DSSSC can achieve the optimal secure performance of opportunistic selection with less implementation complexity, and the channel correlation can further enhance the transmission security. Index Terms— Secure transmission, correlated fading, DSSSC, secrecy outage probability. Manuscript received August 15, 2018; revised December 8, 2018; accepted December 24, 2018. Date of publication January 10, 2019; date of current version May 9, 2019. This work was supported in part by NSFC under Grant 61871139, Grant 61722203, and Grant 61801132, in part by the Guangdong Natural Science Funds for Distinguished Young Scholar under Grant 2014A030306027, in part by the Innovation Team Project of Guangdong Province University under Grant 2016KCXTD017, and in part by the Science and Technology Program of Guangzhou under Grant 201807010103. The work of X. Lei was supported in part by NSFC under Grant 61501382, in part by the Sichuan Science and Technology Program under Grant 2017HH0035, in part by the Fundamental Research Funds for the Central Universities under Grant 2682018CX27, and in part by the Open Research Fund of National Mobile Communications Research Laboratory, Southeast University, under Grant 2017D15. The associate editor coordinating the review of this manu- script and approving it for publication was Dr. Walid Saad. (Corresponding author: Lisheng Fan.) X. Lai, L. Fan, and J. Li are with the School of Computer Science, Guangzhou University, Guangzhou 510006, China (e-mail: laixzh@mail2.sysu.edu.cn; lsfan@gzhu.edu.cn; lijin@gzhu.edu.cn). X. Lei is with the Provincial Key Lab of Information Coding and Trans- mission, Southwest Jiaotong University, Chengdu 610031, China, and also with the National Mobile Communications Research Laboratory, Southeast University, Nanjing 210096, China (e-mail: xflei@home.swjtu.edu.cn). N. Yang is with the Research School of Electrical, Energy and Materi- als Engineering, The Australian National University, Canberra, ACT 2600, Australia (e-mail: nan.yang@anu.edu.au). G. K. Karagiannidis is with the Department of Electrical and Computer Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece (e-mail: geokarag@auth.gr). Digital Object Identifier 10.1109/TIFS.2019.2891932 I. I NTRODUCTION D UE to the broadcast nature of wireless channels, the transmission of confidential message may be wire- tapped by eavesdroppers in the networks, which causes the severe issue of information leakage [1]. To safeguard the secure transmission, physical-layer security (PLS) and encryp- tion algorithms should be applied, from the physical to application layer, respectively. Compared with the traditional encryption algorithms, PLS can achieve the perfect secrecy in theory and its implementation complexity is much lower [2]. Hence, it can serve as an important complement to the traditional encryption algorithms [3]. In recent years, PLS has attracted a lot of attention from the researchers in both academic and industrial fields. A. Previous Work For the PLS, the pioneering work was done by Wyner [4], where a wiretap channel model was proposed, and it is found that perfect secrecy can be achieved with prop- erly designed encoder and decoder. Then, other researchers extended Wyner’s work to fading channel environments, and studied the important secrecy performance metrics, such as secrecy data rate and secrecy outage probability (SOP) [5]–[8]. Specifically, Bloch et al. [6] investigated the secure com- munications over Rayleigh fading channels, and derived the analytical expressions of secrecy data rate and SOP. Moreover, Li and Petropulu [7] and Liu [8] investigated the secure communications over Rician fading channels, and studied the effect of Rician factor on the secrecy performance. For the smart attacker, Li et al. [9], Xiao et al. [10], and Xu et al. [11] used the the reinforcement learning to proposed efficient anti- wiretap schemes in order to maximize the secrecy data rate, and it found that the wiretap and jamming of the attacker can be efficiently suppressed. For the secondary networks, Cao et al. [12] designed an effective secure transmission scheme to optimize the network secrecy performance. In order to improve the transmission security, cooperative relaying technique can be incorporated into wireless commu- nications, where there are two fundamental relaying protocols, i.e., amplify-and-forward (AF) and decode-and-forward (DF) [13], [14]. The secrecy performance of relaying networks has been widely studied by analyzing and optimizing the network secrecy data rate and SOP [15]–[18]. An effective secure transmission scheme based on joint beamforming and time switching was proposed to maximize the secrecy data rate in 1556-6013 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. 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