  Physical layer security is an emerging technique to improve the wireless
communication security, which is widely regarded as a complement to
cryptographic technologies. To design physical layer security techniques under
practical scenarios, the uncertainty and imperfections in the channel knowledge
need to be taken into consideration. This paper provides a survey of recent
research and development in physical layer security considering the imperfect
channel state information (CSI) at communication nodes. We first present an
overview of the main information-theoretic measures of the secrecy performance
with imperfect CSI. Then, we describe several signal processing enhancements in
secure transmission designs, such as secure on-off transmission, beamforming
with artificial noise, and secure communication assisted by relay nodes or in
cognitive radio systems. The recent studies of physical layer security in
large-scale decentralized wireless networks are also summarized. Finally, the
open problems for the on-going and future research are discussed.
