  In this paper, collaborative use of relays to form a beamforming system and
provide physical-layer security is investigated. In particular,
amplify-and-forward (AF) relay beamforming designs under total and individual
relay power constraints are studied with the goal of maximizing the secrecy
rates when perfect channel state information (CSI) is available. In the AF
scheme, not having analytical solutions for the optimal beamforming design
under both total and individual power constraints, an iterative algorithm is
proposed to numerically obtain the optimal beamforming structure and maximize
the secrecy rates. Robust beamforming designs in the presence of imperfect CSI
are investigated for decode-and-forward (DF) based relay beamforming, and
optimization frameworks are provided.
