Title: Secrecy Outage Analysis for Downlink Transmissions in the Presence of Randomly Located Eavesdroppers,
Abstract: We analyze the secrecy outage probability in the downlink for wireless
networks with spatially (Poisson) distributed eavesdroppers (EDs) under the
assumption that the base station employs transmit antenna selection (TAS) to
enhance secrecy performance. We compare the cases where the receiving user
equipment (UE) operates in half-duplex (HD) mode and full-duplex (FD) mode. In
the latter case, the UE simultaneously receives the intended downlink message
and transmits a jamming signal to strengthen secrecy. We investigate two models
of (semi)passive eavesdropping: (1) EDs act independently and (2) EDs collude
to intercept the transmitted message. For both of these models, we obtain
expressions for the secrecy outage probability in the downlink for HD and FD UE
operation. The expressions for HD systems have very accurate approximate or
exact forms in terms of elementary and/or special functions for all path loss
exponents. Those related to the FD systems have exact integral forms for
general path loss exponents, while exact closed forms are given for specific
exponents. A closed-form approximation is also derived for the FD case with
colluding EDs. The resulting analysis shows that the reduction in the secrecy
outage probability is logarithmic in the number of antennas used for TAS and
identifies conditions under which HD operation should be used instead of FD
jamming at the UE. These performance trends and exact relations between system
parameters can be used to develop adaptive power allocation and duplex
operation methods in practice. Examples of such techniques are alluded to
herein.