Secure communications can be impeded by eavesdroppers in conventional relaysystems. This paper proposes cooperative jamming strategies for two-hop relaynetworks where the eavesdropper can wiretap the relay channels in both hops. Inthese approaches, the normally inactive nodes in the relay network can be usedas cooperative jamming sources to confuse the eavesdropper. Linear precodingschemes are investigated for two scenarios where single or multiple datastreams are transmitted via a decode-and-forward (DF) relay, under theassumption that global channel state information (CSI) is available. For thecase of single data stream transmission, we derive closed-form jammingbeamformers and the corresponding optimal power allocation. Generalizedsingular value decomposition (GSVD)-based secure relaying schemes are proposedfor the transmission of multiple data streams. The optimal power allocation isfound for the GSVD relaying scheme via geometric programming. Based on thisresult, a GSVD-based cooperative jamming scheme is proposed that showssignificant improvement in terms of secrecy rate compared to the approachwithout jamming. Furthermore, the case involving an eavesdropper with unknownCSI is also investigated in this paper. Simulation results show that thesecrecy rate is dramatically increased when inactive nodes in the relay networkparticipate in cooperative jamming.
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