| Summary: | We investigate the physical-layer security problems of two-way relay networks in this paper. The whole network consists of two legitimate users and one eavesdropper, where the legal users need the help of some intermediate nodes to exchange information. We propose joint jamming and relaying mechanism, where a jamming node is selected from all intermediate nodes, to create interference upon eavesdropper, while other nodes are used to relay data for two users. All intermediate nodes are subjected to per-node power constraints. Based on null-space beamforming, we propose two different transmission schemes when the number of relay nodes is large enough. One scheme is designed to maximize the secrecy sum rate (MSSR) of the network, and the other is designed to maximize the minimum secrecy rate (MMSR) between two users. In the MMSR scheme, a series of semi-definite programming problems with a rank-1 constraint needs to be solved. We prove that this rank-1 constraint does not affect the solutions of original problems. In addition, we propose two modified secure transmission schemes when the number of relay nodes is less abundant. In short, no matter how many intermediate nodes in the communication networks, one can find suitable transmission schemes to ensure communication security.
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