Energy-efficient full-duplex wireless information and power transfer

In this paper, we study optimal resource allocation strategies focusing on the system energy efficiency with simultaneous power transfer and information decoding operation at the access point (AP). We consider a three node wireless system that consists of the AP, energy harvesting user (EHU) and non-energy harvesting user (N-EHU), and EHU opportunistically harvests energy from N-EHU and the AP. Using this model, we propose introducing an energy-bearing signal in addition to the information-bearing signal at the N-EHU, and investigate their significance to improve the overall energy efficiency. We formulate an optimization problem that maximizes the system energy efficiency by taking the harvested energy constraint at the EHU into account. Based on this, we provide analytical expressions for the optimal transmit power level when the constraint is satisfied with strict inequality by the energy-efficiency-maximizing input. In such a case, system energy efficiency improves along with the energy demand, and using only information-bearing signal is optimal. On the other hand, when the constraint needs to be satisfied with equality, including an energy-bearing signal leads to better performance. However, explicit expressions to determine power allocation strategies are not immediately available, and hence we develop an algorithm using the subgradient method to solve the problem numerically. To justify these theoretical framework, we provide simulation results. In fact, we observe that having an energy-bearing signal together with the informationbearing signal improves the system energy efficiency and leads to higher levels of harvested energy. We also demonstrate the impact of peak power on the resource allocation policies.