Secure OFDM transmission based on multiple relay selection and cooperation

In cooperative communications, spatial channel diversity introduced by scattered locations of cooperating nodes can be exploited to enhance transmission security. In this paper, we propose a secure cooperative orthogonal frequency division multiplexing (OFDM) transmission scheme with the help of multiple relay nodes. For each relay, the wireless channels related to the destination and the eavesdropper are independent of each other, leading to different achievable rates over each subcarrier. In the proposed scheme, relay nodes with high rate differences between the intended receiver and the eavesdropper are selected for transmission. Different cooperation strategies ranging from single-relay selection to per-subcarrier-based multiple-relay selection are studied and compared. As a result of relay selection, each cooperative relay node only forwards a subset of subcarriers that exhibits high capacity differences, thereby enhancing transmission security of the whole system. Secrecy outage probabilities for the proposed scheme with different relay selection strategies are formulated. Numerical simulation results demonstrate the validity of the scheme for security enhancement.