I. Introduction
Secure and energy-saving communication receives a large amount of attention in current research on 5G communications technology because wireless devices are widely used and battery capacity is usually limited. Traditional methods to secure communication focus on cryptography techniques at higher layers, while prolonging battery life relies on evolution of the battery materials. However, there are two promising techniques, namely, physical layer security (PLS) and EH, that secures communications from an information theoretical perspective and enable devices to recharge their battery within the wireless signal coverage.