Improving the physical-layer security of wireless two-way relaying via analog network coding

In this paper, we address the physical-layer security of two-way (bi-directional) transmissions of two terminals with the help of multiple relay nodes in the presence of an eavesdropper, where each node in the network is only equipped with single antenna. The secrecy sum rate is used as the metric of security, which is the rate difference between the legitimate information exchange between the terminals and the information leakage to the eavesdropper. We propose two-phase analog network coding (ANC) and power allocation scheme for the relay nodes and terminals to enhance the security of the data exchange. Three different schemes are proposed. The first scheme is the optimal ANC to achieve the maximum secrecy sum rate, i.e. secrecy capacity, which is found to be mathematically difficult to solve. Two suboptimal schemes are proposed with and without the instantaneous channel state information (CSI) of the eavesdropper, respectively. The solutions of both schemes are shown to be unique and globally optimum, which are solved by convex optimization techniques. Numerical evaluation results of the obtained secrecy sum rate and transmit power are provided and analyzed to show the properties and efficiency of the proposed ANCs.