Cooperation strategies for secrecy in MIMO relay networks with unknown eavesdropper CSI

We investigate secure communications for a four-node relay-eavesdropper channel with multiple data stream transmission, assuming that the eavesdropper´s channel state information (ECSI) is unavailable. Our approach is to allocate part of the global transmit power to satisfy a fixed target rate for the relay link, and then use the remaining power for jamming the eavesdropper. Two cooperative jamming strategies are investigated. In the first, jamming signals are only broadcast by the terminals that are also transmitting data. In the second scheme, the normally inactive nodes in the relay network are used as cooperative jamming sources. Algorithms are proposed for allocating the transmit power and the number of signal dimensions that carry data. Simulation results show that, under a global transmit power constraint, the secrecy rate is dramatically increased when the normally inactive nodes in the relay network provide jamming support for the relayed signals.