  Due to the malicious attacks in wireless networks, physical layer security
has attracted increasing concerns from both academia and industry. The research
on physical layer security mainly focuses either on the secrecy
capacity/achievable secrecy rate/capacity-equivocation region from the
perspective of information theory, or on the security designs from the
viewpoints of optimization and signal processing. Because of its importance in
security designs, the latter research direction is surveyed in a comprehensive
way in this paper. The survey begins with typical wiretap channel models to
cover common scenarios and systems. The topics on physical-layer security
designs are then summarized from resource allocation, beamforming/precoding,
and antenna/node selection and cooperation. Based on the aforementioned
schemes, the performance metrics and fundamental optimization problems are
discussed, which are generally adopted in security designs. Thereafter, the
state of the art of optimization approaches on each research topic of physical
layer security is reviewed from four categories of optimization problems, such
as secrecy rate maximization, secrecy outrage probability minimization, power
consumption minimization, and secure energy efficiency maximization.
Furthermore, the impacts of channel state information on optimization and
design are discussed. Finally, the survey concludes with the observations on
potential future directions and open challenges.
