An Optimal/Fixed Power Allocation Schemes for Overlay CR-NOMA System with Untrusted Users

In order to resolve the security issues in 5G and future wireless networks, physical layer security (PLS) and cognitive radio (CR) technologies can be used with non-orthogonal multiple access (NOMA) to greatly improve spectrum efficiency (SE). However, privacy of information is a risk due to resource sharing in CR or NOMA. Protecting sensitive information in CR-inspired NOMA (CR-NOMA) transmissions is possible by utilizing PLS features, whilst NOMA makes use of channel property differences for bandwidth effectiveness. The objective of this work is to examine the level of secrecy achieved in a 5G overlay downlink CR-NOMA network when untrusted users encounter a Rayleigh fading channel. The analysis takes place in both imperfect and perfect successive interference cancellation (SIC) configurations. Additionally, we derive mathematical expressions for the secrecy outage probability (SOP) and, more significantly, the effective secrecy throughput (EST) of the proposed system in order to further improve the performance of users in both fading and non-fading scenarios. In addition, this work offers an optimal power allocation (PA) scheme that primarily considers the specific quality of service (QoS) requirements of two users. Subsequently, it determines the most favorable values for the PA factors in an effort to further improve the performance of secrecy. Furthermore, under perfect/imperfect SIC modes, this work presents the performance comparison between two untrusted users and the derived analytical closed-form expressions were verified via Monte Carlo models employing various channel characteristics.