  The amount of sensitive data, which is transmitted wirelessly will increase
with future technologies. This raises many questions about secure data
transmission. Besides cryptography, information-theoretic security gained
increasing attention over the recent years. Among others, it deals with the
problem of secure data transmission on the physical layer to a legitimate
receiver (Bob) in the presence of an eavesdropper (Eve). In this work, we
investigate upper and lower bounds on the secrecy outage probability for
slowly-fading wiretap channels with an arbitrary dependency structure between
the fading channels to Bob and Eve. Both cases of absence of channel-state
information at the transmitter (CSI-T) and availability of CSI-T of only the
main channel to the legitimate receiver are considered. Furthermore, we derive
explicit expressions for the upper and lower bounds for Rayleigh fading and
compare them to the case of independent channels. The joint distribution of the
legitimate and eavesdropper channels has a tremendous impact on the achievable
secrecy outage probability. The bounds enable developing guaranteed secrecy
schemes by only measuring the marginal channel distributions.
