  In this paper, we consider decode-and-forward (DF) relay beamforming with
imperfect channel state information (CSI), cooperative artificial noise (AN)
injection, and finite-alphabet input in the presence of an user and $J$
non-colluding eavesdroppers. The communication between the source and the user
is aided by a multiple-input-multiple-output (MIMO) DF relay. We use the fact
that a wiretap code consists of two parts: i) common message (non-secret), and
ii) secret message. The source transmits two independent messages: i) common
message (non-secret), and ii) secret message. The common message is transmitted
at a fixed rate $R_{0}$, and it is intended for the user. The secret message is
also intended for the user but it should be kept secret from the $J$
eavesdroppers. The source and the MIMO DF relay operate under individual power
constraints. In order to improve the secrecy rate, the MIMO relay also injects
artificial noise. The CSI on all the links are assumed to be imperfect and CSI
errors are assumed to be norm bounded. In order to maximize the worst case
secrecy rate, we maximize the worst case link information rate to the user
subject to: i) the individual power constraints on the source and the MIMO
relay, and ii) the best case link information rates to $J$ eavesdroppers be
less than or equal to $R_{0}$ in order to support a fixed common message rate
$R_{0}$. Numerical results showing the effect of perfect/imperfect CSI,
presence/absence of AN with finite-alphabet input on the secrecy rate are
presented.
