Toward self-sustainable cooperative relays: state of the art and the future

Cooperative relaying has been recognized as a promising technique to exploit spatial diversity gain using simple hardware, but it may barely fall in the class of green communications because extra power is consumed for information relaying. Recent advances in energy harvesting devices have paved the way for self-sustainable relays that power themselves by replenishing ambient energy without wires. This article examines the current progress of EH relays with special emphasis on wireless power transfer through RF signals that carry both information and energy at the same time. In this context, information relaying and EH are two tightly coupled processes, and practical constraints of EH devices pose new challenges to self-sustainable relays. This article addresses these issues and points out future research directions.