  This work studies the role of inter-user device-to-device (D2D) cooperation
for improving physical-layer secret communication in multi-user downlink
systems. It is assumed that there are out-of-band D2D channels, on each of
which a selected legitimate user transmits an amplified version of the received
downlink signal to other legitimate users. A key technical challenge for
designing such systems is that eavesdroppers can overhear downlink as well as
D2D cooperation signals. We tackle the problem of jointly optimizing the
downlink precoding, artificial noise covariance, and amplification coefficients
that maximize the minimum rate. An iterative alternating optimization algorithm
is proposed based on the matrix fractional programming. Numerical results
confirm the performance gains of the proposed D2D cooperation scheme compared
to benchmark secret communication schemes.
