Secure Transmission using Decode-and-Forward Protocol for Underlay Cognitive Radio Networks Nhu Tri Do Dept. of Electronic & Computer Engineering in Graduate School, Hongik University Republic of Korea dotrinhu@gmai1.com Abstrct- In this paper, we study the physical layer security for underlay cognitive radio network in the presence of an eaves dropper that tries to intercept the secondary transmission. Specifcally, a transmission at each hop uses a same frequency band of a primary user as long as the interference that caused by the secondary transmission at a primary receiver does not excess a maximum tolerable interference level. We investigate the secrecy outage probability (SOP) for a cognitive decode-and forward (DF) relay network. Moreover, we drive a closed-form expression of the SOP of the considered system. Simulation results validate our theoretical results under three distinct cases as follows: the signal-to-noise ratio (SNR) of the main link between a secondary transmitter and a corresponding receiver is, respectively, smaller, equal to, or larger than the SNR of the eavesdropper link between the same secondary transmitter and the eavesdropper. Keywords-secrecy outage probabilit; physical lyer securit; underlay; cognitive radio; decode-and-forward I. INTRODUCTION Cognitive radio (CR) is emerging as an effective mean to solve the spectrum scarcity problem by exploiting the existing wireless spectrum opportunistically. The basic idea of CR is to allow the secondary users (SUs), which are the unlicensed users, to share the licensed spectrum bands that are assigned to the primary users (PUs) [1]. In order to access and use such a licensed spectrum, the SUs typically have to detect whether the PU is present or absent on that spectrum by using spectrum sensing functionality [2]. On the other hand, in the underlay paradigm, the SUs are allowed to simultaneously access the licensed spectrum with the PUs as long as the interference caused by the secondary transmission that does not excess a maximum tolerable interference level which is defned by the primary receiver [3]. Therefore, the direct secondary transmis sion in underlay cognitive radio network can be limited to short range communication [3]. With the ability of extending the coverage of wireless networks, multihop relay communications have been recog nized as an effcient way to overcome such a problem in underlay cognitive radio networks (CRNs) [4]. In networks where the direct transmission between source node and destination node is not available, relay transmission is utilized. Specifcally, relays receive the message from source and forward it to destination using relay protocols such as amplify- 978-1-4799-8993-5/15/$31.00 m015 IEEE 914 Beongku An* *Dept. of Computer & Information Communications Engineering, Hongik University Republic of Korea beongku@hongik.ac.kr and-forward (AF) or decode-and-forward (DF) protocols. Due to the broadcast nature of wireless transmissions, muItihop relay communications are vulnerable to eavesdropping attacks. Physical layer security [5] has been developed as a promising mechanism that prevents the eavesdropper from intercepting transmission messages on the wireless links. Physical layer security (PLS) exploits physical characteristics such as random and noisy nature of the wireless channels [6]. Secrecy capacity and secrecy outage probability (SOP) are the two main performance metrics in PLS. Assume that there is an eavesdropper that observes the transmission of a pair of nodes, the secrecy capacity is the difference between the channel capacity of the main link, which is the link from source to destination, and that of the eavesdropper link, which is the link from source to eavesdropper [7]. In addition, the secrecy outage probability is the probability of the event that the secrecy capacity drops below a predefned target secrecy rate [8]. The physical layer security using relay protocols in underlay cognitive radio networks has been studied in [9-11]. The authors in [9] investigate the existence constraints under which the secrecy capacity is achievable in a dual-hop AF relaying system. In [10], a relay selection scheme that chooses a trusted decode-and-forward relay to assist the secondary transmission is proposed to enhance the PLS for underlay CRNs. In addition, another selection scheme is proposed in [11] to improve PLS for a cooperative DF underlay CRNs under scenario of feedback delay (outdated channel state information). However, the aforementioned works consider schemes in which the direct link between the source and destination is available. In this paper, we evaluate the secrecy outage probability of a dual-hop decode-and-forward underlay cognitive relay network in which we assume that there is no direct link from source to destination. This assumption has been mentioned in several related works [7-8], [12-13] while the authors in these works have studied PLS without considering scenario of underlay cognitive radio. In our considered system, the secre cy outage event is separately taken into account on each hop of the system. We further assume that the eavesdropper cannot do joint coding. This assumption may be reasonable since the randomized code-books can be used at each hop [14] to confuse the eavesdropper. The main contribution of the paper ICUFN 2015