# Joint Resource Allocation in Secure OFDMA-Based Networks Taking a Base Station as a Two-Way Relay
## 1 Introduction  
Security refers to the system resistance ability facing man-made threats such as wiretaps, attacks or tampering in the process of data transmission. With the continuous development of communication technology, security has become a main factor to measure the reliability of a communication system [1]. Due to the broadcasting characteristics of a wireless channel, the mobility of the wireless terminal and the instability of transportation, wireless communication systems are facing more security threats than traditional cable communication systems. Thus, communication privacy and information security in a wireless network has become an important factor demanding more attention, especially in fields such as military and national security. The purpose of secure communication is to ensure that legitimate users can receive source point information, while it prevents eavesdroppers in wireless networks from obtaining information [2]. The physical safety design is from the viewpoint of information theory, and makes good use of the physical characteristics of a wireless channel to achieve the purpose of transmitting information safely; such design ideas have attracted more and more attention from researchers. Collaborative communication, for example, with the enrichment of physical layer transmission technology and physical security features, has been paid more and more attention [3]. In collaborative communication, the source point not only can broadcast information directly to the destination node, but can also complete information transmission to the destination node through the assistance of relay nodes, which use diversity gain to effectively overcome multipath fading and improve system performance. However, due to the broadcast feature of a wireless transmission environment,the eavesdropper not only can steal information through the link between itself and source point, but also can acquire information through the link between the relay node and eavesdropper, thus increasing the likelihood of information leakage and threatening the security of the system. How to design node collaboration solutions and resource allocation strategies to ensure secure communication to prevent eavesdropping on legal information in the physical layer has become the research focus in collaborative communication in recent years [4]. Joint relay and jammer selection for secure two-way relay networks was put forward by [PERSON] [5]. Cooperating relays were used to improve the wireless physical layer security by [PERSON] [6]. Secure resource allocation and scheduling was put forward by [PERSON] [7] in Orthogonal Frequency Division Multiple Access (OFDMA) decode-and-forward relay networks. Under the premise of ensuring secrecy rate for primary users in cognitive radio networks, power allocation, time allocation, and relay selection problem [8] were investigated. For cases when there are one or more eavesdroppers [9], a different relay cooperation scheme was put forward to ensure safe communication between the source and the destination node. In particular, a cooperative jamming scheme was presented for a two-hop relay network [10]. Considering the limited power of the relay, a kind of cooperative jamming strategy coordination strategy was put forward in which each relay sends a weighted disturbance signal in order to reduce the eavesdropper channel quality [11]. Secure communications with untrusted secondary nodes in cognitive radio networks were put forward by [PERSON] [12]. In cognitive radio networks, a kind of relay selection scheme was put forward for physical layer [13]. Diversity techniques were used to improve physical layer security in wireless communications [14]. For the Multiple-Input Multiple-Output (MIMO) channel with a multiple antenna eavesdropper, the secrecy rate optimization problem was investigated by [PERSON] [15]. In order to improve the secrecy rate, robust beamforming technology [16] was used in systems with wireless information and power transfer.