Optimal resource allocation in OFDM decode-and-forward relay systems with SWIET

Employing energy harvesting (EH) at a relay node with simultaneous wireless information and energy transfer (SWIET) is promising to prolong the lifetime of energy-constrained relay systems. In this paper, we investigate the optimal resource allocation in an orthogonal-frequency-division multiplexing (OFDM) decode-and-forward (DF) relay system that implements SWIET at the source-to-relay link in order to maximize the end-to-end achievable rate. We first propose an optimal energy-transfer power allocation (EPA) policy which can utilize the frequency diversity provided by OFDM modulation. We then validate that the ordered-SNR subcarrier pairing is globally optimum. To optimize EH time and information-transmission power allocation (IPA), we formulate the involved problem as a non-convex programming problem, and then transform it into a quasi-convex optimization problem. By solving this quasi-convex optimization problem with the bisection search method, we propose an algorithm which can jointly optimize the EH time and IPA. By analytical analysis, we validate that our proposed resource allocation scheme has much lower computational complexity than the peer studies in the literature. Finally, our simulations demonstrate the optimality of our proposed resource allocation scheme.