Optimal precoder designs for energy-efficiency maximization in secure MIMO systems

In this paper, precoding techniques are investigated for secure energy-efficient multiple-input multiple-output (MIMO) systems in the presence of an eavesdropper. To tackle the non-convexity of the secure energy efficiency maximization (SEEM) problem, we employ branch-and-reduce-and-bound (BRB) approach to obtain the globally optimal solution. It is observed that the BRB algorithm suffers from highly computational cost, its globally optimal solution is importantly served as a benchmark for the performance evaluation of the suboptimal algorithms. This motivates us to develop a low-complexity approach using the well-known zero-forcing (ZF) technique to cancel the wiretapped signal. In ZF based method, the SEEM problem can be transformed to a concave-convex fractional one and, then, be solved by the combination of the Dinkelbach and bisection search algorithm. Simulation results reveal that the ZF based method can converge fast and obtain a sub-optimal performance in compared with the optimal solution of the BRB method. The ZF based scheme also outperforms the conventional secrecy rate maximization precoder design in terms of the energy efficiency.