  We consider a Gaussian MISO wiretap channel, where a multi-antenna source
communicates with a single-antenna destination in the presence of a
single-antenna eavesdropper. The communication is assisted by multi-antenna
helpers that act as jammers to the eavesdropper. Each helper independently
transmits noise which lies in the null space of the channel to the destination,
thus creates no interference to the destination. Under the assumption that
there is eavesdropper channel uncertainty, we derive the optimal covariance
matrix for the source signal so that the secrecy rate is maximized subject to
probability of outage and power constraints. Assuming that the eavesdropper
channels follow zero-mean Gaussian model with known covariances, we derive the
outage probability in a closed form. Simulation results in support of the
analysis are provided.
