  Due to the broadcast nature of radio propagation, the wireless transmission
can be readily overheard by unauthorized users for interception purposes and is
thus highly vulnerable to eavesdropping attacks. To this end, physical-layer
security is emerging as a promising paradigm to protect the wireless
communications against eavesdropping attacks by exploiting the physical
characteristics of wireless channels. This article is focused on the
investigation of diversity techniques to improve the physical-layer security,
differing from the conventional artificial noise generation and beamforming
techniques which typically consume additional power for generating artificial
noise and exhibit high implementation complexity for beamformer design. We
present several diversity approaches to improve the wireless physical-layer
security, including the multiple-input multiple-output (MIMO), multiuser
diversity, and cooperative diversity. To illustrate the security improvement
through diversity, we propose a case study of exploiting cooperative relays to
assist the signal transmission from source to destination while defending
against eavesdropping attacks. We evaluate the security performance of
cooperative relay transmission in Rayleigh fading environments in terms of
secrecy capacity and intercept probability. It is shown that as the number of
relays increases, the secrecy capacity and intercept probability of the
cooperative relay transmission both improve significantly, implying the
advantage of exploiting cooperative diversity to improve the physical-layer
security against eavesdropping attacks.
