A secure space-time code for asynchronous cooperative communication systems with untrusted relays

An amplify-and-forward relay network composed of a source (S), N relays and a destination (D) is considered, where the relays are untrusted in the sense that they may eavesdrop on the transmission from S to D, that is they may decode messages of the source. As a part of the system, these untrusted relays are willing to help the communication from S to D. To prevent the relays from decoding the source message, a secure spacetime code with full diversity is designed at the source node. In this paper, no secret information is exchanged between S and D in advance. Training symbols are transmitted by S and D respectively. Based on the assumption of channel reciprocity, S and D can obtain the equivalent channels between them, which are unavailable to the relays. By exploiting the equivalent channels and random source antenna selection, random phase rotation is designed for each space-time code block at S to prevent the relays from eavesdropping. Simulation results are presented to verify the performance of the proposed secure spacetime coding scheme.