  This paper considers a cooperative cognitive radio network with two primary
users (PUs) and two secondary users (SUs) that enables two-way communications
of primary and secondary systems in conjunction with non-linear energy
harvesting based simultaneous wireless information and power transfer (SWIPT).
With the considered network, SUs are able to realize their communications over
the licensed spectrum while extending relay assistance to the PUs. The overall
bidirectional end-to-end transmission takes place in four phases, which include
both energy harvesting (EH) and information transfer. A non-linear energy
harvester with a hybrid SWIPT scheme is adopted in which both power-splitting
and time-switching EH techniques are used. The SUs aid in relay cooperation by
performing an amplify-and-forward operation, whereas selection combining
technique is adopted at the PUs to extract the intended signal from multiple
received signals broadcasted by the SUs. Accurate outage probability
expressions for the primary and secondary links are derived under the
Nakagami-$m$ fading environment. Further, the system behavior is analyzed with
respect to achievable system throughput and energy efficiency. Since the
performance of the considered system is strongly affected by the spectrum
sharing factor and hybrid SWIPT parameters, particle swarm optimization is
implemented to optimize the system parameters so as to maximize the system
throughput and energy efficiency. Simulation results are provided to
corroborate the performance analysis and give useful insights into the system
behavior concerning various system/channel parameters.
