Bidirectional Primary and Secondary Transmissions with Hybrid-SWIPT in Cognitive Radio Networks

Abstract

This paper considers a cooperative cognitive radio system that enables bidirectional primary and secondary transmissions along with simultaneous wireless information and power transfer (SWIPT). In the considered scheme, two secondary users (SUs) provide relay assistance to a pair of primary users (PUs), and in return, they are allowed to exploit the licensed spectrum for realizing their communications. With such a scheme, two end-to-end transmissions can be accomplished in four phases including the phase for energy harvesting (EH). Different from most previous works, a hybrid-SWIPT is adopted in this paper, whereby the SUs can adaptively utilize both EH techniques, i.e., time switching and power splitting. Both SUs perform amplify-and-forward operation to facilitate relay cooperation. On the other hand, PUs employ a selection combining technique to make use of multiple intended signal copies broadcasted from a pair of SUs. For evaluating the performance of the considered network, the exact outage probability expressions for both primary and secondary systems under Nakagami-m fading are provided. Numerical and simulation results illustrate the accuracy of derived expressions and give useful insights into the system behavior, especially with respect to the spectrum sharing factor and hybrid SWIPT parameters.