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Energy efficiency has become an encouragement, and more than this, arequisite for the design of next-generation wireless communications standards.In current work, a dual-hop cognitive (secondary) relaying system isconsidered, incorporating multiple amplify-and-forward relays, a rathercost-effective solution. First, the secondary relays sense the wirelesschannel, scanning for a primary network activity, and then convey their reportsto a secondary base station (SBS). Afterwards, the SBS, based on these reportsand its own estimation, decides cooperatively the presence of primarytransmission or not. In the former scenario, all the secondary nodes start toharvest energy from the transmission of primary node(s). In the latterscenario, the system initiates secondary communication via a best relayselection policy. Performance evaluation of this system is thoroughlyinvestigated, by assuming realistic channel conditions, i.e., non-identicallink-distances, Rayleigh fading, and outdated channel estimation. The detectionand outage probabilities as well as the average harvested energy are derived asnew closed-form expressions. In addition, an energy efficiency optimizationproblem is analytically formulated and solved, while a necessary condition interms of power consumption minimization for each secondary node is presented.From a green communications standpoint, it turns out that energy harvestinggreatly enhances the resources of secondary nodes, especially when primaryactivity is densely present.

Green Cognitive Relaying: Opportunistically Switching Between Data Transmission and Energy Harvesting