  In this paper, we address the problem of optimal power allocation at the
relay in two-hop secure communications. In order to solve the challenging issue
of short-distance interception in secure communications, the benefit of
large-scale MIMO (LS-MIMO) relaying techniques is exploited to improve the
secrecy performance significantly, even in the case without eavesdropper
channel state information (CSI). The focus of this paper is on the analysis and
design of optimal power allocation for the relay, so as to maximize the secrecy
outage capacity. We reveal the condition that the secrecy outage capacity is
positive, prove that there is one and only one optimal power, and present an
optimal power allocation scheme. Moreover, the asymptotic characteristics of
the secrecy outage capacity is carried out to provide some clear insights for
secrecy performance optimization. Finally, simulation results validate the
effectiveness of the proposed scheme.
