An Energy-Aware Retransmission Approach in SWIPT-Based Cognitive Relay Systems

In this paper, cooperative spectrum sharing is considered between a primary user (PU) and a secondary user (SU), where the off-the-grid secondary transmitter (ST) serves as a cognitive relay to forward both the received primary and secondary signals by exploiting simultaneous wireless information and power transfer (SWIPT). Based on a two-phase relaying model, power splitting is adopted by the ST for energy harvesting and information processing in the first phase. Next, the harvested energy at the ST is used to amplify-and-forward a linearly weighted combination of the primary and secondary signals, in the second phase. To enhance the reliability of the system and utilize the energy harvested more wisely, an energy-aware retransmission (EAR) scheme is proposed in the first phase to guarantee that a certain minimum amount of energy is collected by the ST before launching the second phase. Both outage and throughput performances are theoretically analyzed for the PU and the SU. Simulation results demonstrate that a win-win relationship is built between the PU and the SU under proper parameter configurations, and the proposed EAR scheme can bring additional performance gains in unfavorable conditions imposed under high rate and low power.