  We design a new secure transmission scheme in the relay wiretap channel where
a source communicates with a destination through a decode-and-forward relay in
the presence of spatially random-distributed eavesdroppers. For the sake of
practicality, we consider a general antenna configuration in which the source,
relay, destination, and eavesdroppers are equipped with multiple antennas. In
order to confuse the eavesdroppers, we assume that both the source and the
relay transmit artificial noise signals in addition to information signals. We
first derive a closed-form expression for the transmission outage probability
and an easy-to-compute expression for the secrecy outage probability. Notably,
these expressions are valid for an arbitrary number of antennas at the source,
relay, and destination. We then derive simple yet valuable expressions for the
asymptotic transmission outage probability and the asymptotic secrecy outage
probability, which reveal the secrecy performance when the number of antennas
at the source grows sufficiently large. Using our expressions, we quantify a
practical performance metric, namely the secrecy throughput, under a secrecy
outage probability constraint. We further determine the system and channel
parameters that maximize the secrecy throughput, leading to analytical security
solutions suitable for real-world deployment.
