  Optimal power allocation (PA) strategies can make a significant rate
improvement in secure spatial modulation (SM). Due to the lack of secrecy rate
(SR) closed-form expression in secure SM networks, it is hard to optimize the
PA factor. In this paper, two PA strategies are proposed: gradient descent, and
maximum product of signal-to-interference-plus-noise ratio (SINR) and
artificial-noise-to-signal-plus-noise ratio (ANSNR)(Max-P-SINR-ANSNR). The
former is an iterative method and the latter is a closed-form solution.
Compared to the former, the latter is of low-complexity. Simulation results
show that the proposed two PA methods can approximately achieve the same SR
performance as exhaustive search method and perform far better than three fixed
PA ones. With extremely low complexity, the SR performance of the proposed
Max-P-SINR-ANSNR performs slightly better and worse than that of the proposed
GD in the low to medium, and high signal-to-noise ratio regions, respectively.
