Ensuring equal outage performance for down-link secondary users in full/half duplex cognitive NOMA systems

This study considers an underlay cognitive radio (CR) system, where the secondary network employs cooperative non-orthogonal multiple access (NOMA) to transfer information for downlink secondary users (SUs). Here the near user is enabled to act as a full duplex/half duplex (FD/HD) decode-and-forward (DF) relay for the far user. We derive analytical expressions for the outage probabilities experienced by the SUs in the considered FD/HD - CR-NOMA network. The maximum transmit power constraint of the secondary nodes and the tolerable interference power constraints at the primary receiver are considered for the analysis. We provide numerical and simulation results to identify the impact of key system parameters on the outage performance of SUs. Further, we provide insights on the selection of key system parameters such as NOMA power allocation coefficient for ensuring that both the SUs experience equal outage performance. Finally, we derive an analytical expression for the system outage probability of the FD-CR-NOMA network. We determine the optimal power allocation (OPA) factor at the secondary transmitter that minimises the system outage probability. We establish that OPA can reduce the system outage significantly as compared to the default choice of power allocation factor. The analytical results are validated by extensive simulation studies.