A survey of energy harvesting communications: models and offline optimal policies

As people pay more attention to environmental protection and energy conservation issues, energy consumption in communications have become a hot research field. In wireless communications networks such as wireless sensor networks, traditional battery-operated devices or nodes have a short lifetime and die after the batteries are depleted, and replacing the batteries may be very costly and sometimes will be impossible. Therefore, energy harvesting (EH) communications have become a good means to solve this problem. EH communications mean the nodes can continue working by harvesting ambient energy. EH communications are different from the traditional battery-operated communications, so we need new models and optimal transmission policies to maximize the throughput. In this article we review different methods of harvesting the ambient energy in EH communications and the models of EH communications. We focus on offline optimal policies, then compare different policies and classify them into certain types. Finally, we propose several open research challenges and directions for future work.