User power allocation and relay beamforming design for secrecy sum rate maximization in two-way relay networks

In this paper, we investigate the physical-layer security (PLS) for multiple peer-to-peer communications in a twoway relaying networks involving 2L single-antenna users, an NR-antenna relay, and an NE-antenna eavesdropper. Only a pair of peer-to-peer users demand security to exchange messages because the eavesdropper intends to wiretap them in both transmission phases. Our goal is to maximize the achievable secrecy sum rate (SSR) of specific peer-to-peer users, provided that restrictive constraints are satisfied. We endorse a null space beam forming technique at the relay to simplify the optimization problem and exclude private information leakage. The main problem is still very complicated due to coupled variables of the user transmit power with the relay beamforming matrix. Therefore, we convert the main problem into two subproblems, one in terms of the relay beamforming matrix and the other in terms of the user transmit power. To find a locally optimal solution for the relay beamforming, we turn the main problem into a difference of convex (DC) programming problem and for user power allocation, we drive the main problem into a constrained nonlinear optimization problem. Finally, simulations are performed to evaluate the performance benefits of the proposed method.