  The feasibility of physical-layer-based security approaches for wireless
communications in the presence of one or more eavesdroppers is hampered by
channel conditions. In this paper, cooperation is investigated as an approach
to overcome this problem and improve the performance of secure communications.
In particular, a decode-and-forward (DF) based cooperative protocol is
considered, and the objective is to design the system for secrecy capacity
maximization or transmit power minimization. System design for the DF-based
cooperative protocol is first studied by assuming the availability of global
channel state information (CSI). For the case of one eavesdropper, an iterative
scheme is proposed to obtain the optimal solution for the problem of transmit
power minimization. For the case of multiple eavesdroppers, the problem of
secrecy capacity maximization or transmit power minimization is in general
intractable. Suboptimal system design is proposed by adding an additional
constraint, i.e., the complete nulling of signals at all eavesdroppers, which
yields simple closed-form solutions for the aforementioned two problems. Then,
the impact of imperfect CSI of eavesdroppers on system design is studied, in
which the ergodic secrecy capacity is of interest.
