  In this paper, we study simultaneous wireless information and power transfer
(SWIPT) in orthogonal frequency division multiple access (OFDMA) systems with
the coexistence of information receivers (IRs) and energy receivers (ERs). The
IRs are served with best-effort secrecy data and the ERs harvest energy with
minimum required harvested power. To enhance the physical layer security for
IRs and yet satisfy energy harvesting requirements for ERs, we propose a new
frequency domain artificial noise (AN) aided transmission strategy. With the
new strategy, we study the optimal resource allocation for the weighted sum
secrecy rate maximization for IRs by power and subcarrier allocation at the
transmitter. The studied problem is shown to be a mixed integer programming
problem and thus non-convex, while we propose an efficient algorithm for
solving it based on the Lagrange duality method. To further reduce the
computational complexity, we also propose a suboptimal algorithm of lower
complexity. The simulation results illustrate the effectiveness of proposed
algorithms as compared against other heuristic schemes.
