Date Added: Nov 2012
A wireless relay channel is considered, consisting of a multi-antenna source, a single-antenna destination, a single antenna eavesdropper and a set of multi-antenna relays (helpers) that act as jammers to the eavesdropper. Each helper knows only its own link to the receiver and independently transmits jamming noise, lying in the null space of its own link to the destination, thus causes no interference to the destination. The source, knowing the main channel explicitly and having statistical information on the eavesdropper channel, designs the input covariance matrix so that the secrecy rate is maximized subject to an outage probability constraint and a sum power constraint. The authors show that the optimal input covariance matrix has rank one.