  We address physical layer security in distributed antenna (DA) systems, where
eavesdroppers (Eves) can intercept the information transmitted for the intended
receiver (IR). To realize a user-centric, power-efficient and physical layer
security-addressing system, we aim at minimizing total power consumption by
jointly designing DA selection and secure precoding. Different from the
conventional artificial noise (AN)-aided secure transmission, where AN is
treated as an undesired element for the IR, we design AN such that it is
constructive to the IR while keeping destructive to the Eves. Importantly, we
investigate two practical scenarios, where the IR and Eves' channel state
information (CSI) is imperfectly obtained or the Eves' CSI is completely
unknown. To handle the CSI uncertainties, we solve the problems in
probabilistic and deterministic robust optimization respectively, both
satisfying the IR' signal-to-interference-and-ratio (SINR) requirement by use
of constructive AN and addressing security against the Eves. Simulation results
demonstrate our algorithms consume much less power compared to the centralized
antenna (CA) systems with/without antenna selection, as well as the DA systems
with conventional AN processing. Last but not least, by the proposed
algorithms, the activation of DAs closely relates to users' locations and
quality-of-service (QoS) requirements, featuring a user-centric and on-demand
structure.
