  This paper focuses on the design of an optimal resource allocation scheme to
maximize the energy efficiency (EE) in a simultaneous wireless information and
power transfer (SWIPT) enabled two-way decode-and-forward (DF) relay network
under a non-linear energy harvesting model. In particular, we formulate an
optimization problem by jointly optimizing the transmit powers of two source
nodes, the power-splitting (PS) ratios of the relay, and the time for the
source-relay transmission, under multiple constraints including the transmit
power constraints at sources and the minimum rate requirement. Although the
formulated problem is non-convex, an iterative algorithm is developed to obtain
the optimal resource allocation. Simulation results verify the proposed
algorithm and show that the designed resource allocation scheme is superior to
other benchmark schemes in terms of EE.
