  We propose and study the cooperative MIMO architecture to enable and to
improve the secrecy transmissions between clusters of mobile devices in the
presence of an eavesdropper with certain location constraint. The cooperative
MIMO system in this paper (referred to as Reconfigurable Distributed MIMO) is
formed by temporarily activating clusters of nearby trusted mobile devices,
with each cluster being centrally coordinated by its corresponding cluster
head. We assume that the transmitters apply a practical eigen-direction
precoding scheme to transmit the confidential signal and artificial noise,
while the eavesdropper can be located in multiple possible locations in the
proximity. We first obtain the expression of the secrecy rate, where the
required average mutual information between the transmitters and the
eavesdropper is characterized by closed-form approximations. The proposed
approximations are especially useful in the secrecy rate maximization, where
the original non-convex problem can be solved by the successive convex
approximations. Numerical results show that the secrecy rate can be
significantly improved by leveraging the location constraint of the
eavesdropper, compared to the existing result. We also demonstrate that the
secrecy rate can be further improved by increasing the cluster size.
