  We investigate a relay network where the source can potentially utilize an
untrusted non-regenerative relay to augment its direct transmission of a
confidential message to the destination. Since the relay is untrusted, it is
desirable to protect the confidential data from it while simultaneously making
use of it to increase the reliability of the transmission. We first examine the
secrecy outage probability (SOP) of the network assuming a single antenna
relay, and calculate the exact SOP for three different schemes: direct
transmission without using the relay, conventional non-regenerative relaying,
and cooperative jamming by the destination. Subsequently, we conduct an
asymptotic analysis of the SOPs to determine the optimal policies in different
operating regimes. We then generalize to the multi-antenna relay case and
investigate the impact of the number of relay antennas on the secrecy
performance. Finally, we study a scenario where the relay has only a single RF
chain which necessitates an antenna selection scheme, and we show that unlike
the case where all antennas are used, under certain conditions the cooperative
jamming scheme with antenna selection provides a diversity advantage for the
receiver. Numerical results are presented to verify the theoretical predictions
of the preferred transmission policies.
