# Secure Communication for Uplink Cellular Networks Assisted with Full-Duplex Device-to-Device User
## 1 Introduction  
The increasing use of wireless devices has led to explosive growth in data traffic, so wireless spectrum resources are becoming increasingly scarce [1; 2; 3]. D2D communication as an underlying network can share the cellular communication resources to realize information transmission between D2D users, which improves the utilization of spectrum resources [4; 5]. At the same time, the D2D cellular network has reliable synchronism by using the joint detection method [6]. In [7], the power control problem is studied by using a game-theoretic method based on sigmoid cost function for the underlying D2D network. Furthermore, when the distance between the CU and the base station (BS) is very long, the D2D user can act as a relay to assist the cellular link transmission. By using cooperative protocols, such as amplify and forward (AF), decode and forward (DF) and quantize and forward (QF), the cooperative D2D communication not only ensures reliable and effective information transmission [8; 9], but also reduces the performance degradation of the CUs caused by the D2D users reusing the cellular networks spectrums. Therefore, both the CUs and the D2D users can benefit from the cooperation, and some existing studies have shown that D2D-assisted cellular networks can better improve system performance. In [10], a two-way communication scheme between BS and CU in two time slots based on D2D assistance is proposed. On this basis, considering the interference caused by the D2D reuse of cellular network resources, a superposition coding scheme [11] is proposed at the D2D transmitter and the power allocated to assist the downlink transmission of the cellular is minimized. Considering multiple D2D pairs and CUs, the weighted sum energy efficiency [12] of the D2D user is maximized through bandwidth allocation, power allocation, and selection of the optimalD2D user as a relay. In addition, [PERSON] et al. propose a half-duplex (HD) D2D-assisted cooperative relay system [13] using non-orthogonal multiple access which can achieve maximum total capacity scaling. The aforementioned studies have focused on D2D cooperation in HD mode, which lose spectrum resources. The combination of FD technology and D2D communication underlaying cellular networks can effectively improve the spectrum efficiency of the system [14; 15]. However, the FD technology causes self-interference, which restricts the performance improvement of the FD wireless communication system to a certain extent. In fact, the current self-interference cancellation technology (SIC) has made a huge breakthrough [16; 17; 18], and D2D in FD mode to assist cellular transmission becomes practical. A beam-based analog SIC scheme for FD MIMO systems is proposed in [14], which can effectively eliminate self-interference. However, there is still residual self-interference after analog SIC, and digital SIC is a supplement to the previous analog cancellation. The nonlinear digital SIC [15] is achieved by pre-calibrating at the transmitter. A more general technique is studied in [16], and a framework that combines digital SIC and analog SIC is established to adaptively eliminate various interferences in dynamic environments. For D2D communication using FD technology, the feasibility of the FD function of D2D communication is proven in a heterogeneous network and a solution is obtained to reduce interference in an FD D2D network [19]. In [20], a virtual carrier scheme is proposed to solve the problem of intercarrier interference in V2V networks. Nevertheless, the inherent openness of wireless communication threatens the security of wireless networks [21; 22]. For D2D user-assisted cellular communication, since the D2D user as a relay may try to decode the signal to be forwarded, it is vital to protect the secure transmission of cellular link information. However, conventional encryption technology is difficult to implement in cooperative communication networks because of its complexity and a huge amount of calculation.