A comprehensive review on energy harvesting MAC protocols in WSNs: Challenges and tradeoffs

Nowadays, wireless sensor networks (WSNs) are broadly used to set up distributed monitoring infrastructures in self-healing, self-configuring, and self-managing systems. They are composed by many elementary devices (or motes) equipped with basic sensing, computing, and communications capabilities, which interact on a collaborative basis to sense a target environment and report collected data to one or more sinks. WSNs are expected to be operational for very long periods of time, even if each mote cannot bring large energy storage units. Accordingly, Energy Harvesting mechanisms can greatly magnify the expected lifetime of WSNs. Over the years, Energy Harvesting-Wireless Sensor Networks (EH-WSN) have been thoroughly studied by the scientific and industrial communities to bridge the gap from the vision to the reality. A critical facet of EH-WSN lies in the interplay between EH techniques and MAC protocols. In fact, while EH technologies feed motes with energy, the MAC layer is responsible for a significant quota of spent energy because of message transmission/reception and channel sensing operations. In addition, the energy brought by EH technologies is not easily predictable in advance because of time-varying nature: this makes the design of the MAC protocol even more challenging. To draw a comprehensive review of the state of the art on this subject, the present manuscript first provides a detailed analysis on existing energy harvesting systems for WSNs; then it extensively illustrates pros and cons of key MAC protocols for EH-WSNs with a special focus on: fundamental techniques, evaluation approaches, and key performance indicators. Finally, it summarizes lessons learned, provides design guidelines for MAC protocols in EH-WSNs, and outlooks the impact on Internet of Things.