  In a directional modulation network, a general power iterative (GPI) based
beamforming scheme is proposed to maximize the secrecy rate (SR), where there
are two optimization variables required to be optimized. The first one is the
useful precoding vector of transmitting confidential messages to the desired
user while the second one is the artificial noise (AN) projection matrix of
forcing more AN to eavesdroppers. In such a secure network, the paramount
problem is how to design or optimize the two optimization variables by
different criteria. To maximize the SR (Max-SR), an alternatively iterative
structure (AIS) is established between the AN projection matrix and the
precoding vector for confidential messages. To choose a good initial value of
iteration process of GPI, the proposed Max-SR method can readily double its
convergence speed compared to the random choice of initial value. With only
four iterations, it may rapidly converge to its rate ceil. From simulation
results, it follows that the SR performance of the proposed AIS of GPI-based
Max-SR is much better than those of conventional leakage-based and null-space
projection methods in the medium and large signal-to-noise ratio (SNR) regions,
and its achievable SR performance gain gradually increases as SNR increases.
