  A Gaussian multiple-input multiple-output (MIMO) wiretap channel model is
considered. The input is a two-antenna transmitter, while the outputs are the
legitimate receiver and an eavesdropper, both equipped with multiple antennas.
All channels are assumed to be known. The problem of obtaining the optimal
input covariance matrix that achieves secrecy capacity subject to a power
constraint is addressed, and a closed-form expression for the secrecy capacity
is obtained.
