  In this paper, we study the simultaneous wireless information and power
transfer (SWIPT) in downlink multiuser orthogonal frequency-division multiple
access (OFDMA) systems, where each user applies power splitting scheme to
coordinate the energy harvesting and secrecy information decoding processes.
Assuming equal power allocation across subcarriers, we formulate the problem to
maximize the aggregate harvested power of all users while satisfying secrecy
rate requirement of individual user by joint subcarrier allocation and optimal
power splitting ratio selection. Due to the NP-hardness of the problem, we
propose two suboptimal algorithms to solve the problem in the dual domain. The
first one is an iterative algorithm that optimizes subcarrier allocation and
power splitting in an alternating way. The second algorithm is based on a
two-step approach that solves the subcarrier allocation and power splitting
sequentially. The numerical results show that the proposed methods outperform
conventional methods. It is also shown that the iterative algorithm performs
close to the upper bound and the step-wise algorithm provides good tradeoffs
between performance and complexity.
