  This paper studies the use of multi-antenna harvest-and-jam (HJ) helpers in a
multi-antenna amplify-and-forward (AF) relay wiretap channel assuming that the
direct link between the source and destination is broken. Our objective is to
maximize the secrecy rate at the destination subject to the transmit power
constraints of the AF relay and the HJ helpers. In the case of perfect channel
state information (CSI), the joint optimization of the artificial noise (AN)
covariance matrix for cooperative jamming and the AF beamforming matrix is
studied using semi-definite relaxation (SDR) which is tight, while suboptimal
solutions are also devised with lower complexity. For the imperfect CSI case,
we provide the equivalent reformulation of the worst-case robust optimization
to maximize the minimum achievable secrecy rate. Inspired by the optimal
solution to the case of perfect CSI, a suboptimal robust scheme is proposed
striking a good tradeoff between complexity and performance. Finally, numerical
results for various settings are provided to evaluate the proposed schemes.
