Combined relay selection and secure beamforming for decode-and-forward networks with multiple eavesdroppers

In this paper, we investigate a two-stage cooperative network consisting of one source, one corresponding destination, multiple eavesdroppers, and several intermediate nodes. In the first stage, the source selects several relays from the intermediate nodes and broadcasts its message to them. In the second stage, the selected nodes operating as the decode-and-forward (DF) relays perform collaborative beamforming for the secure data transmission that prevents eavesdropping. We study the relay selection and secure beamforming problem considering both the achievable rate in the first stage and the secrecy rate constraint in the second stage, whereas the existing works leave out of the transmission in the first stage. We propose two combined relay selection and beamforming schemes. The first scheme is to maximize the secrecy rate of the relay selection and beamforming with the achievable rate constraint in the first stage. To reduce the complexity of the first scheme, the second scheme combines relay selection and a suboptimal secure beamforming scheme, in which we hypothesize a virtual eavesdropper instead of all eavesdroppers and maximize the secrecy rate according to the virtual eavesdropper. Simulation results show that the performance of our first scheme is the same as that of the exhaustive search scheme, and the second scheme significantly reduces the complexity with acceptable performance degradation.