Secrecy energy efficiency optimization for downlink two-user OFDMA networks with SWIPT

Simultaneous wireless information and power transfer (SWIPT), as a promising technology by which mobile devices are designed by both information decoding and energy harvesting at the same time, has recently drawn significant interests. However, in multiuser scenario, a challenging physical layer security issue arises as the confident messages sent to the intended information receivers may be eavesdropped by other receivers. To address this problem, in this paper, secrecy energy efficiency maximization (SEEM) problem is investigated for two-user orthogonal frequency-division multiple access networks with SWIPT. Specifically, a power splitting scheme is adopted for each user, which would split received radio frequency signals into two independent parts for information decoding and energy harvesting, respectively. Taking both the individual secrecy quality of service requirement and the individual harvested energy constraint into consideration, the SEEM problem is formulated as a nonlinear fractional programming problem. Moreover, according to Dinkelbach's method, a power allocation and power splitting (PAPS) algorithm is proposed by solving the equivalent convex optimization problem. Finally, numerical results verify the impact of static circuit power consumption, maximum transmit power, power split ratio, subcarrier bandwidth, and path loss on secrecy energy efficiency of our PAPS algorithm.