Optimization of relay selection and ergodic capacity in cognitive radio sensor networks with wireless energy harvesting

This paper addresses relay selection and resource allocation issues in cognitive radio sensor networks with wireless energy harvesting. We first consider a three-phase energy harvesting and information transmission protocol based on cooperative decode-and-forward relaying for a secondary system in coexistence with the primary system. In the first phase, the energy-constrained relay harvests energy through radio-frequency signals from the primary source. In the second phase, the destinations decode the primary signal. The relay uses the harvested energy to forward the primary signal and the secondary signal in the third phase. We derive the close-form upper bound of the ergodic capacity of the primary system and propose a relay selection algorithm. In particular, we calculate the critical region to ensure that the ergodic capacity of the primary system is equal or larger than that of the direct system. Finally, numerical results show that the proposed schemes achieve a satisfying performance.